11V>JO i I U : 1 3 \ 3 ' & » 2 ^OJITVD-JO^ ^o^iwo-jo^ ^OF-CAUFORfc A-OF-CAIIFO%, *vai §\^t § ^•IIBRARYQ^ ^^1 ^JilVJ-JO^ icOF-CALIFOR^, I § I t & £ > cxf- Si I K 3 1 :t %OJI TVD-J^ ^\\E-UNIVER% § = § 3 g — I/ -yS ^ ^l]DNV-SOV I 1 1 § %^\ l $ *>~*\ IM wwm$& yo\wmnv* A. AN ELEMENTARY TEXT-BOOK OF BOTANY AN ELEMENTARY TEXT-BOOK OF BOTANY BY SYDNEY H. VINES M.A., D.Sc., F.R.S. Fellow of Magdalen College and Sherardian Professor of Botany in the University of Oxford ; Honorary Fellow of Christ 's College and formerly Reader in Botany in the University of Cambridge ; Fellow of the University of London WITH 397 ILLUSTRATIONS Honbon SWAN SONNENSCHEIN & CO Lim. NEW YORK : THE MACMILLAN COMPANY 1898 BUTLER & TANNER, THE SELWOOD PRINTING WORKS, FROME, AND LONDON. PBEFACE en THE preparation of this work was undertaken to meet a demand en which appeared to exist for a less bulky and expensive volume ^ than my Students' Text-Book of Botany. I have so far succeeded rt that this book contains about 200 pages less than the Students' g Text-Book ; but it is still about so much larger than the last issue z of Prantl's Elementary Text-Book, which it is intended to replace. However, I am convinced that it is not possible, with advantage to the student, to compress even the elementary facts and concep- tions of Botanical Science into a much smaller space than this. This book is not, however, merely an abridgment of the Students' Text-Book. More 'important than the diminution of the bulk by one quarter is the simplification which the contents have under- g* gone by the omission of certain difficult and still debatable topics, ,3 such as, for instance, the details of nuclear division, or the alter- £ nation of generations in the Thallophyta. I have also thought it desirable to follow, in the main, the classification of Phanerogams laid down in the Genera Plantarum of Bentham and Hooker. Moreover, there has been a considerable rearrangement of the matter, and the more fundamental recent discoveries— such as that of spermatozoids in the Grymnosperms — have been incorporated. Hence the contents of this book differ in various material points from those of the Students' Text-Book — a difference which I hope at some future time to render more marked by preparing an edition of the Students' Text- Book of a more advanced character and on a somewhat larger scale. A word in conclusion as to how to use this book. It is con- venient to divide — as is done here — the subject-matter of Botany into the four parts, Morphology, Anatomy, Physiology, Systematic ; but it must not be forgotten that these are all parts of one subject, different methods of studying one object, namely, the plant. Hence they must not be pursued separately, but together. For instance, the morphology of the leaf cannot be profitably studied without a knowledge of its structure and of its functions ; and it 332077 Vi PREFACE. is also important to know what is the systematic position of each of the various plants whose leaves afford the material for study. In a word, the student should not attempt to read the book straight through from the beginning as if it were a novel. On the contrary, he may begin with any one of the four parts as his main subject ; but that part must be studied in close relation with the other three parts, a procedure which is facilitated by the large number of cross-references in the text. S. H. V. July, 1898. ERRATA P. 83, line 18 from top ; for " erythrophl " read " erythrophyll." „ 161, „ 7 „ ; „ " the " (last word in the line) read " but." ., 244, „ 29 „ ; „ " Sytonemaceae " read " Scytonemacese." .. 273. „ 6 from bottom ; for " ooblastema-filaments " read " gonimo- blastic filaments." ,9 „ ; „ " arilode " read " arillode." 7 from top; for " brocteoles " read " bracteoles." CONTENTS. PART L— MORPHOLOGY. PASS 1. Introductory . 1 2. The Life-History of Plants 2 3. The Segmentation of the Body 8 4. The Symmetry of the Body and of the Members .... 4 5. The Development of the Body and of the Members ... 8 6. The Arrangement of the Lateral Members 10 7. Development of Branch-Systems 18 8. Cohesion and Adhesion 21 9. The Thallus 22 10. The Shoot 22 11. The Stem 27 12. The Leaf 28 13. The Boot 44 14. Hairs and Emergences 46 15. Reproduction . . .49 16. General Morphology of the Asexual Reproductive Organs . . 51 17. General Morphology of the Sexual Reproductive Organs . . 58 18. The Fruit 61 19. The Seed 62 PART IL— ANATOMY AND HISTOLOGY. 20. Introductory 63 CHAPTER I.— THE CELL. 21. The Structure and Form of the Cell 66 22. The Protoplasm 68 23. The Cell- Wall 72 24. Cell-Contents 78 25. Cell-Formation 83 CHAPTER II.— THE TISSUES. 26. The Connexion of the Cells . . . . . . .88 27. Intercellular Spaces 89 28. Forms of Tissue 90 29. General Morphology of the Tissue-Systems 101 30. The L'riinary Tegumentary Tissue 106 31. The Primary Ground-Tissue 110 ix CONTENTS. 32. The Stele ............ 116 33. The Primary Vascular Tissue ...... . .121 34. Histology of the Development of Secondary Members . . . 132 35. The Formation of Secondary Tissue ....... 137 36. Formation of Tissue in consequence of Injury ..... 155 PART III.— PHYSIOLOGY. 37. Introductory 157 CHAPTER I.— GENEEAL PHYSIOLOGY. 38. The Functions • '. . . . . .157 39. The External Conditions .159 40. The Functions of the Tissues . '• . '.. " . ' . . .. . 162 41. The Functions of the Members . " . . , ...... 167 CHAPTER II.— SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 42. Absorption ;. v ,~ ...-..-_. . . 177 43. Transpiration . . . . . . . ' ..7, . .179 44. Distribution of Water and other Substances . . ;»..-./. 181 45. Metabolism 185 CHAPTER III.— SPECIAL PHYSIOLOGY OF MOVEMENT. 46. Introductory 205 47. Spontaneous Movements 206 48. Induced Movements . . . . . . . . . . .211 49. Localisation of Irritability 220 50. Transmission of Stimuli 221 51. Combined Effects of Different Stimuli 222 52. Conditions of Movement . .223 53. Mechanism of the Movements 224 CHAPTER IV.— SPECIAL PHYSIOLOGY OF REPRODUCTION. 54. Introductory 227 55. Vegetative Multiplication 228 56. Spore-Reproduction 230 PART IV.— CLASSIFICATION. Introductory 233 GROUP I. THALLOPHYTA 237 Class I. Algae 237 Sub-Class I. Cyanophycese (Phycochromacese) .... 244 Sub-class II. ChlorophyceaB 246 Series I. Protococcoidese 248 Order 1. Pleurococcacese 248 2. Protococcacese . 248 CONTENTS. XI PAGE Series II. Volvocoideae 249 Order 1. Chlamydomonadaceae 249 „ 2. Volvocaceaa. . . . . . . - 249 Series III. Siphonoideae 250 Order 1. Siphonaceae 250 „ 2. Cladophoraceae 252 „ 3. Hydrodictyaceae . . . . . .253 Series IV. Confervoideae . . . . " . . . . 253 Order 1. Conjugates 254 „ 2. Ulothrichaceae 256 „ 3. Ulvace83 257 „ 4. (Edogoniaceae 257 „ 5. Coleochaetaceae 258 Series V. Charoidess 260 Order 1. Characess 260 Sub-Class III. Phaeophycese 263 Order. Diatomaceae 265 Series (a). Phaeosporeae 265 Series (b). Phaeogamse 268 Order. Fucaceae 268 Sub-Class IV. Bhodophyceae 271 Class II. Fungi 275 Sub-Class I. Schizomycetes 280 „ II. Myxomycetes 283 „ III. Phycomycetes 285 Section A. Zygomycetes 285 Order. Mucorinae 285 Section B. Oomycetes 287 Order 1. Peronosporaceae 287 „ 2. Saprolegniaceaa 289 Sub-Class IV. Ascomycetes . 290 Order 1. Gymnoasceae . 294 „ 2. Pyrenomycetes 295 „ 3. Discomycetes 296 Sub-Class V. JEcidiomycetes 298 Order 1. Uredineae 299 „ 2. Ustilagineae 300 Sub-Class VI. Basidiomycetes 301 Series I. Protobasidiomycetes 305 „ II. Autobasidiomycetes . . . . . . 305 Subsidiary Group. Lichenes 305 GEOUP II. BRYOPHYTA (MUSCINE^) Class III. Hepaticae (Liverworts) 318 Order 1. March ant iaceae 320 „ 2. Jungermanniaceae 324 „ 3. Anthocerotaceae . . 330 Xll CONTENTS. Class IV. Musci (Mosses) "332 Order 1. Sphagnaceae t '.-.;.. . . 340 Order 2. Bryinese 342 GEOUP III. PTEKIDOPHYTA (VASCULAR CEYPTOGAMS) . 346 Class V. Filicinae ; y. . . 854 Sub-Class. Eusporangiatse Homosporese. Order 1. Ophioglossacese ........ 354 „ 2. Marattiacese 355 Heterosporeae. „ 3. Isoetacese 355 Sub-Class. Leptosporangiatae Homosporeae (Filices) . t . . 358 Order 1. Hymenophyllaceae . . . r .371 „ 2. Polypodiaceae . .... . . . 371 „ 3. Cyatheaceae . . . . . . . .372 „ 4. Gleicheniaceae 372 „ 5. Schizseacese . ... . . ... .372 „ 6. Osmundaceae 372 Heterosporese (Hydropterideae) . . 373 „ 7. Salviniacese 380 „ 8. Marsileacese . • ,'' • , . . .. . 380 Class VI. Equisetinae \ , '. . . 380 Order 1. Equisetacese 380 Class VI I. Lycopodinae. 386 Sub-Class. HomosporeaB Order 1. Lycopodiacese . . . . . .386 „ 2. Psilotacese 389 Sub-Class. Heterosporese Order 3. Selaginellacese 389 PHANEROOAMIA (SPERMAPHYTA) . . .394 GEOUP IV. GYMNOSPEEM^E 419 Class VIII. Gymnospermae Order 1. Cycadacese 431 „ 2. Coniferse 432 „ 3. Gnetacese 437 GEOUP V. ANGIOSPEEMJE 438 Class IX. Monocotyledones 476 Sub-Class I. Spadiciflorse 482 COHORT I. ARALES 482 Order 1. Aracese 482 „ 2. Lemnacese 484 „ 3. Typhacese 484 COHORT II. PALMALES 484 Order 1. Palmacese 484 Sub-Class II. Glumiflorse 486 COHORT I. GLUMALES 486 Order 1. Graminacese 486 „ 2. Cyperaceae 492 CONTENTS. Xlll PAG a Sub-Class III. Petaloideee . 494 Series I. Hypogynae. Sub-Series Apocarpce. COHORT I. ALISMALES 494 Order 1. Naiadaceae .494 „ 2. Juncaginaceae 495 „ 3. Alismaceae 495 „ 4. Butoiiiaceae . . ... . • • 495 Sub-Series Syncarpce. COHORT I. LILIALES 496 Order 1. LiKacese 496 „ 2. Juncaceae 499 Series II. Epigynae. COHORT I. HYDRALES 500 Order 1. Hydrocharidaceae 500 COHORT II. DIOSCOREALES 500 Order 1. Dioscoreaceae 500 „ 2. Bromeliaceae 501 COHORT III. AMOMALES (SCITAMINEJE) 501 Order 1. Musacese 501 „ 2. Zingiberaceae 502 „ 3. Marantaceae (Cannacese) .... 502 COHORT IV. ORCHIDALES 503 Order 1. Orchidacese 503 COHORT V. NARCISSALES 507 Order 1. Amaryllidaceae 507 „ 2. Iridaceae 508 Class X. Dicotyledones 509 Sub-Class I. Monochlamydeae 514 COHORT I. URTICALES ' . .514 Order 1. Urticaceae 514 „ 2. Moraceae 515 „ 3. Cannabinaceae 515 „ 4. Ulmacese 516 COHORT II. AMENTALES 517 Order 1. Betulacse .... ... 517 „ 2. Corylaceee . .... 518 „ 3. Fagace83 520 „ 4. Juglandaceae 521 „ 5. Salicaceae 522 COHORT III. CHENOPODIALES .522 Order 1. Chenopodiaceae 522 „ 2. Polygonaceae 523 COHORT IV. ASARALES 524 Order 1. Aristolochiacese 524 COHORT V. SANTALALES 524 Order 1. Santalaceae . 524 „ 2. Loranthaceae 525 COHORT VI. EUPHORBIALES 525 Order 1. Euphorbiaceae 526 Sub-Class II. Polypetalae 527 CONTENTS. PAGE Series I. Thalamiflorse COHORT I. RANALES 527 Order 1. Kanunculacese . . . . ' ,- . .527 „ 2. Magnoliacese . . .: '• '. . .530 „ 3. Nymphseacese . . ' . ..:... .530 „ 4. Berberidacese . . . , . . .531 COHORT II. CARYOPHYLLALKS . 531 Order 1. Caryophyllacese 531 COHORT III. PARIETALES . . .'...-. .533 Order 1. Papaveracese . . ... . .533 „ 2. Fumariacese ....... 533 ., 3. Cruciferse . ... . . ' . . .534 „ 4. Cistacese ... . ... . • . .538 „ 5. Violaceae . . ... . . . .538 COHORT IV. GUTTIFERALES . . 539 Order 1. Hypericacese . . . ; . .539 COHORT V. MALVALES . . . . ; . . .539 Order 1. Tiliaceae . . .... . . -:• . .539 „ 2. Malvaceae . . . . . . .540 Series II. Disciflorse COHORT I. GERANIALES . . .< ... *, . . 541 Order 1. Geraniacese . 542 „ 2. Linacese 542 „ 3. Oxalidacese •" . .543 „ 4. Balsaminacese 543 „ 5. Eutacese 543 COHORT II. SAPINDALES 544 Order 1. Sapindacese 544 „ 2. Aceraceae 545 „ 3. Polygalaceas 545 COHORT III. CELASTRALES 546 Order 1. Celastracese 546 „ 2. Ehamnacese 546 „ 3. Ampelidacese ....... 547 Series III. Calyciflorae COHORT I. UMBELLALES 547 Order 1. Umbelliferse 548 „ 2. Araliacese 550 COHORT II. PASSIFLORALES 550 Order 1. Cucurbitaceae 551 COHORT III. MYRTALES 552 Order 1. Onagracese 552 „ 2. Lythraceaa 553 „ 3. Myrtacese 553 COHORT IV. ROSALES 554 Order 1. Rosacese 554 „ 2. Leguminosae 557 COHORT V. SAXIFRAGALES 559 Order 1. Saxifragacese 560 „ 2. Crassulaceae . .561 CONTENTS. XV PAGB Sub-Class III. Gamopetalse 562 Series I. Hypogynae COHORT I. LAMIALES 562 Order 1. Labiatse 562 COHORT II. PERSONALES 564 Order 1. Scrophulariaceae 564 „ 2. Plantaginaceae . . . . .565 „ 3. Orobanchaceae ...*... 566 „ 4. Lentibulariacese 566 COHORT III. POLEMONIALES 567 Order 1. Convolvulaceae 567 „ 2. Polemoniaceae 567 „ 3. Solanaceae 567 „ 4. Boraginaceae 569 COHORT IV. GENTIANALES 570 Order 1. Gentianacese 570 „ 2. Oleacese 570 COHORT V. PKIMULALES 571 Order 1. Primulaceae 571 „ 2. Plumbaginaceae 572 COHORT VI. ERICALES 572 Order 1. Ericaceae 572 „ 2. Pyrolaceae 573 „ 3. Vacciniaceae 573 Series II. Epigynae COHORT I. CAMPANALES 574 Order 1. Campanulaceae 574 COHORT II. RUBIALES 574 Order 1. Rubiaceae .575 „ 2. Caprifoliaceas 576 COHORT III. ASTERALES 577 Order 1. Valerianacese 577 „ 2. Dipsaceae 577 „ 3. Compositae 579 INDEX, PART I.— Morphology, Anatomy, and Physiology . . 583 „ II.— Classification and Nomenclature . 597 PART I. MORPHOLOGY. § 1. Introductory. An ordinary flowering-plant consists of a number of parts which are distinguished as roots, stems, leaves, fruits, etc. These may be considered scientifically in two ways ; either with reference to their functions in the economy of the plant, when they are regarded as the organs by which these are performed, and are the subjects of physiological study ; or, their functions being disregarded, their relative position, the place and mode of their origin, the course of their growth, and their relative size may be considered ; that is, they may be studied from a purely morphological point of view, when they are regarded merely as parts of a whole, and are designated as members. Hence the pro- vince of morphology is the study of the form of the bod}r of plants, and of the members of which it consists, including the develop- ment of the body and its members, as also the intimate structure (Anatomy and Histology) of the body and its members, in so far as structure throws light upon the morphology of any part of the body. The body of a plant, like that of an animal, consists essentially of living substance known as protoplasm. The body may consist only of protoplasm, without any investing membrane to give it a determinate form (e.g. Myxomycetes) ; or it may consist of a mass of protoplasm enclosed by a membrane (e.g. Phycomycetous Fungi and Siphonaceous Algse) ; or it may consist, as in the higher plants, of a mass 6f protoplasm segmented by partition-walls, or septa, into structural units termed cells. In all cases, however, the form and constitution of the body is determined by the proto- plasm ; for the cell-walls of which, in many cases, the body largely consists, and which give to it definiteness of form, are developed from and by the protoplasm. The study of the morphology of plants is, therefore, the study of the processes and products of the formative activity of their protoplasm ; and these are to be traced M.B. B 2 PART I.— MORPHOLOGY. [§ 2 both in the varietj^ of form presented by different plants, and in the varioias stages in the development of any one individual plant. § 2. The Life-History of Plants. The consideration of this subject is a necessary preliminary to the detailed study of Morphology. The great majority of plants are more or less poly- morphic : that is, the plant assumes, as a rule, at least two quite different forms in the course of its life. Most commonly it presents but two forms which, while they may differ more or less widely in form and structure, are essentially distinguished by the fact that the one, termed the sporophyte, has asexual reproductive organs which produce asexual reproductive cells, termed spores, each of which is capable by itself of giving rise to a new organism ; whilst the other, termed the gametophyte, has sexual reproductive organs, which produce sexual reproductive cells, termed gametes, and though each of these cells is by itself incapable of giving rise to a new organism, yet by the fusion of two of these gametes of different sex, a cell is formed which is of the nature of a spore, since from it a new organism can be developed. These two forms alternate more or less regularly in different plants, the asexually- produced spore of the sporophyte giving rise to a gametophyte ; the sexually-produced spore of the gametophyte giving rise to a sporophyte. Such a life-history presents what is known as alter- nation of generations ; that is, an alternation of a sexual with an asexual form. The alternation of generations is conspicuous in the Bryophyta and the Pteridophyta, as is fully explained in the chapters specially devoted to those groups. It also occurs in the life- history of the Phanerogams, and may be traced, more or less imperfectly, in some of the Thallophyta. But since the tracing of it in the last-named group is attended with some uncertainty, that group will be excluded from further consideration here. In the groups Bryophyta, Pteridophyta, and Phanerogamia, the two generations attain very different degrees of development. In the Bryophyta, the gametophyte is the more conspicuous generation ; it is the form to which the name attaches, and upon which the classification is mainly based ; whereas, the sporophyte is, as it were, an appendage to the gametophyte, and is generally known as the Moss-fruit. In the Pteridophyta, the sporophyte is the con- spicuous form to which the name of the plant attaches; but, though small and inconspicuous, the gametophyte is an independent organism known as the prothallus. In the Phanerogamia, as in § O. THE SEGMENTATION OF THE BODY. O the Pteridophyta, the sporophyte is the plant such as we know it, whilst the gametophyte is so much reduced that it may be regarded as an appendage upon the sporophyte. Thus, in tracing the morphology of the two generations from the Bryophyta upwards, the relations between them are gradually reversed ; so that the higher the plant is in the scale of organisation, the more conspicuous is its sporophyte, the less conspicuous 'its gametophyte. The following pages refer mainly to the morphology of the sporophyte of the higher plants, that is, of the Pteridophyta and Phanerogamia, except when the gametophyte or one of the Bryophyta or Thallophyta is especially mentioned. § 3. The Segmentation of the Body. The body of a plant may be either segmented into members, or unsegmented. The members of a segmented body may either be all similar, or they may be similar and dissimilar. Segmentation into similar mem- bers is termed branching. When the body is segmented into dissimilar members, it is said to be morphologically differentiated. When the body is morphologically undifferentiated, that is, when it is either unsegmented or segmented only into similar members (i.e. branched), it is termed a thallus. A Thallophyte is a plant having a body of this constitution and of simple structure (e.g. Yeast, Spirogyra). The primary segmentation of the body into dissimilar members consists in the differentiation of root and shoot. The Root is usually segmented, but only into similar members. It occasionally gives rise to (adventitious) shoots. The Shoot may be either unsegmented, or segmented into similar or dissimilar members. A shoot which is either unsegmented, or segmented only into similar members, is termed a thalloid shoot (e.g. Lemna, the Duckweed). A shoot which is segmented into dis- similar members consists generally of stem and leaves. The characteristics of the principal members are as follows : — The shoot bears the true (spore-producing) reproductive organs : it is generally differentiated into stem and leaf. The stem is the axial member of the shoot, and bears the leaves. The leaf is the lateral member of the shoot : it is borne upon the stem, but differs from it more or less in form. The root never bears leaves or true (spore -producing) reproduc- tive organs. The hair is an appendage which may be borne on either root, stem, or leaf. 4 PART I.— MORPHOLOGY. [8 4 The stem, leaf, and root of any one plant present the same kind of complexity of structure : the hair is of much simpler structure as a rule. § 4. The Symmetry of the Body and of the Members. Whatever the form of the body or of a member, it has three axes at right angles to each other. When these three are all equal, the body is a sphere (e.g. Volvox, Fig. 1) : when two are equal, and both longer than the third, the body or the member is a flattened circular expansion (e.g. Pediastrum and the leaf -blade of Tropseolum): when one is longer than either of the others, the body or the mem- ber is cylindrical or prismatic in form when the two shorter axes are equal (e.g. the stem generally), and of a flattened form when one of the shorter axes is longer than the other (most leaves). In most cases two opposite ends are distinguishable in the body or member, a base and an apex. The base is in all cases the end by which the body is attached to the substratum, or the members to each other, the free end being the apex. The axis or imaginary line joining the base and the apex, whether or not it be longer than the other axes, is termed the organic longitudinal axis. When the body shows no distinction of base and apex (e.g. Spirogyra), there is no organic longitudinal axis. Any section, real or imaginary, made parallel to the longitudinal axis, is a longitudinal section : it is a radial longitudinal section if it includes the longitudinal axis : it is a tangential longitudinal section if it does not include it. A section made at right angles to the longitudinal axis is a transverse section : the section of the longitudinal axis is the organic centre of the transverse section, and it commonly is also the geometrical centre of the transverse section, but occasionally the geometric and organic centres do not coincide. Thus, in transverse sections of tree-trunks, the annual rings are comparatively rarely arranged symmetrically around the geometrical centre. The longitudinal axis, then, is a line passing through the organic centres of the successive transverse sections. Two kinds of symmetry may be distinguished ; the multilateral, including the radial ; and the bilateral, including the isobilateral and the zygomorvhic. The determination of the nature of the symmetry of a body or member depends upon (a) its external form, (6) the Arrangement and form of the members which it may bear, (c) its internal structure. 1. MnlHlfit,,;,l n,nl Radial Symmetry. Absolute multilateral symmetry is only presented by a body or member which is 4. SYMMETRY. spherical and has no distinction between base and apex. For example, the body of Volvox can be divided into symmetrical halves in any plane passing through the centre (Fig. 1). The more limited form of multilateral symmetry, which, may be conveniently distinguished as radial, is that which obtains in cylindrical bodies or members. It is multilateral symmetry about the longitudinal axis. In this case the body or member can be divided in various planes along the longitudinal axis into a number of similar halves. A mushroom with a central stalk, an apple, a cylindrical tree- trunk, are radially symmetrical as regards their external form. As regards the posi- tion of the lateral members, the trunks of Pines and Spruces, with branches arising on all sides, are ra- dially symmetrical ; and, as regards the form of the lateral FIG. l.-Folro* Globafor (after Cohn ; x about 100), illustrating multilateral symmetry. members, the flowers of the Rose and of the Tulip are radial. A radial body or member can be divided by radial longitudinal sections in two or more planes, into symmetrical halves, which are to each other as an object and its image reflected in a mirror (in Fig. 2, A, the halves obtained by the sections 1-1, 2-2, 3-3, 4-4, 5-5). The possible number of such similar halves is not always the same, but it is in any case at least four. In a mushroom or a Fir-stem, there are many possible planes of symmetrical section ; but in a Tulip, the sections being taken through the longitudinal axes of the floral leaves, only three are possible ; and in an apple if they pass through the loculi of the core, only five (Fig. 2 A). The two halves are not always as exactly alike as an object and its reflected image ; this is not the case, for instance, in a Fir-trunk, because the lateral branches are not borne at the same level. The two halves are, 6 PART I.— MORPHOLOGY. [§4 however, essentially similar. When, however, a body is divisible in at least two planes into precisely similar halves, it is said to be polysym- metrical. 2. Bilateral Symmetry. A body or member is said to be bilaterally symmetrical when it presents an interior, a posterior, and two lateral surfaces ; the lateral surfaces, or flanks, being dif- ferent from the anterior and posterior. Such a body or member is divisible into two symmetrical halves, either in two planes, or in one plane only ; when it is so divisible in two planes, the halves resulting from the section in one plane are different from the halves resulting from section in the other. Bilateral members are distinguished as isobilateral or as zygo- morphic (or monosymmetrical), accordingly as they are symmetri- cally divisible in two planes or in one plane. FIG. 2.— Diagrammatic transverse sections of A an apple; B a walnut; C a peach; 1-1, 5-5, are the planes of symmetry. A with five planes of symmetry, is radially sym- metrical ; k carpel. B with two planes of symmetry, is isobilateral ; / the suture ; * the seed. C with a single plane of symmetry, is zygomorphic and dorsiventral ; R dorsal surface ; B ventral surface ; r right, and I left flank ; fc stone. a. Isobilateral Symmetry. Isobilateral symmetry is usually manifested in the external form. Thus, a walnut is at once seen to be divisible into two symmetrical halves by section, either through the suture, or at right angles to this plane (Fig. 2 B) ; so also a flattened erect leaf like that of the Iris. It may be manifested by the position of the lateral members ; for instance, in many shoots (e.g. the Elm) the leaves are borne in two rows, right and left, one row on each flank. It may be manifested also in the internal structure. Thus, a transverse section of a walnut (Fig. 2 B) shows that internal, as well as external, isobilateral symmetry exists. But this does not § 4. SYMMETRY. 7 obtain in all cases ; the internal structure of isobilateral leaves is often not strictly isobilaterally symmetrical. b. Zygomorphic Symmetry. A zygomorphic or rnonosymmetri- cal body or member is divisible into two similar halves in one plane only (Fig. 2 (7). Of this there are two principal cases : — First, that in which the anterior and posterior halves are similar, whilst the right and left halves are dissimilar, in other words, when the plane of symmetry is lateral ; the body or member is then laterally zygomorphic (e.g. flower of Corydalis) : secondly, that in which the anterior and posterior halves are dissimilar, whilst the right and left halves are similar, in other words, when the plane of symmetry is antero-posterior ; the body or member is then said to be dorsiventrally zygomorphic, or, briefly, dorsi- ventral. Less frequently, as in some flowers, the plane of symmetry is neither lateral nor antero-posterior, but intermediate between the two, the zygomorphy being oblique. Of these possible forms of zygomorphic symmetry, the dorsi- ventral is the most common. The term is derived from the use of the terms dorsal and ventral to indicate, respectively, the dis- similar anterior and posterior halves of the body or member. The application of the terms dorsal and ventral to the two dissimilar halves of the body or a member requires some explanation. Generally speaking, the under surface of a dorsiventral body is the ventral, the upper the dorsal. In the case of leaves, however, the terms dorsal and ventral are used with reference to the parent stem : the upper or inner sur- face is here the ventral, the outer or lower, the dorsal. The difference between the dorsal and ventral halves may be exhibited in their external form. Thus, the dorsal and ventral halves of many fruits (peach, Fig. 2 C ] or a pea-pod) may be distinguished at once by their form. Or the difference may be in the nature of the members which they bear ; thus, creeping dorsi- ventral shoots commonly bear (adventitious) roots or root-hairs on their ventral, and branches or leaves on their dorsal surface ; or the one surface may bear lateral members, and the other none. Or, finally, the difference may exist in their internal structure ; thus, in dorsiventral foliage-leaves, the internal structure of the dorsal half is different from that of the ventral half. It must not be overlooked that the terms radial, isobilateral, and dorsiventral, may be all applicable to one and the same body or member, according to the particular feature which is taken into consideration. For example, a branch of the Silver Fir is, in its 8 PART I.— MORPHOLOGY. [§ & general appearance, dorsiventral ; a dorsal and a ventral half are readily distinguishable. But, since the leaves are arranged sym- metrically around it, it is in this respect radial. Again, since the lateral branches arise right and left upon its flanks, it is in this respect isobilateral. Hence it is important to distinguish clearly between the symmetry of any part of the body as a ivhole, and that of its constituent members. Thus in many isobilateral and dorsiventral shoots, the stem, regarded by itself, is radially sym- metrical ; the isobilaterality or dorsiventrality of the shoot being, in these cases, indicated only by the mode of arrangement of the leaves upon the stem. The causes which determine the symmetry of the body or of a member are mainly inherent ; but it has been ascertained in many cases that external conditions have a preponderating influence, such as the intensity and direction of the incident rays of light, or (as in certain dorsiventral flowers) the action of gravity. When a body or a member cannot be symmetrically divided into two similar halves in any plane whatever, it is said to be asymme- tric. The asymmetry in these cases is frequently associated with dorsiventrality ; as in some Mushrooms (e.g. Lcnzites abietina) ; in some foliage-leaves which are oblique, that is, the right and left halves of which are not symmetrical (e.g. Elm, some Begonias) ; and in some flowers (e.g. Aconitum, Delphinium). § 5. The Development of the Body and of the Mem- bers. The body, consisting of the primary members, is developed from the spore. It is not proposed to enter now into the some- what complicated details of the various modes of embryogeny occurring in the different groups of plants ; but rather to consider the development of the secondary members, that is, of those mem- bers which are produced directly or indirectly by the primary members. Whilst the plant is still an embryo, the whole of its protoplasm is capable of growth, and is said to be in the embryonic condition. As the development of the embryo into the plant proceeds, most of the protoplasm passes over into the adult state, and is no longer capable of growth. Certain portions of it, however, retain the embryonic condition and properties, remaining capable of growth : these portions of the protoplasm are termed growing-points, and may persist throughout the whole life of the plant. Such growing- points usually, but not exclusively, exist at the tip or apex of the prinmry stem and of each of its branches, as also at the apex of § 5. DEVELOPMENT. 9 the primary root and its branches. The leaf has, with rare excep- tions (e.g. some Ferns), no persistent growing-point, and this is also true of some stems. Members in which there is no persistent growing-point are said to have limited growth. The growing-point adds, in the first instance, to the size of the member to which it belongs, and is the means by which it grows in length. But, in the case of the stem, it does more than this ; it gives rise also to new members, either branches or leaves. It is, in fact, the only source of origin of such new members. When the growing-point is multicellular, the branches and leaves arise from the superficial layers of cells ; so that their mode of origin is exogenous. Moreover, these members are developed in a regular order, such that the youngest of them are to be found nearest to the growing-point, the older ones further away : this order of de- velopment is termed acropetal succession. As a general rule, the secondary members are developed laterally on the parent member, the latter continuing its growth in length. But in a few cases, more particularly when the body is a thallus or the stem is thalloid, the growing-point divides into two, each of which forms the growing-point of a new secondary member simi- lar to the parent. This mode of branching is known as dichotomy. In the root, the new members, except when dichotomy occurs, are not developed at or from the growing-point, but at some distance behind it. Unlike the branches and leaves produced by the stem, the branches of the root are not developed at the surface, but from a tissue lying deeply within the structure of the parent root. (See Part II.) This layer, although it is situated among cells which have become adult, retains its embryonic properties, and gives rise to the growing-points of the several lateral roots. Hence the origin of the secondary roots is endogenous, as their growing- points are developed within the tissue of the parent root, and have to force their way through it in order to reach the surface. The order of development of the lateral roots is acropetal. It sometimes happens that secondary members are developed out of their proper order or not in their proper place ; they are then said to be adventitious. This is rare in the case of stems and leaves, but is common in the case of roots. Roots are, for instance, frequently developed adventitiously on stems, instead of from the primary root which is their normal position. When they are de- veloped on stems their origin is almost always endogenous. The secondary members commonly persist ; but frequently they 10 PART I.— MORPHOLOGY. [§6 become separated from the member bearing them, and fall off after a time, when they are said to be deciduous. Leaves are nearly always deciduous. In most perennial plants the foliage-leaves all fall off at some season of the year, which, in temperate climates, is the autumn. But in " evergreen " trees and shrubs, the leaves, which may last for more than one year, do not all fall off at once. Those parts of the plant which are connected with reproduction are especially deciduous: for instance, the leaves forming the perianth of the flower, sometimes the whole inflorescence (e.g. catkin), sometimes the fruit (e.g. cherry), the seeds, etc. When a member thus falls off it leaves a more or less per- manent scar : the scar which marks the position of a fallen leaf is a leaf-scar. Hairs and reproductive organs are generally de- veloped as lateral out- growths upon the members, but they are occasionally developed directly from the growing-point. They are usually developed from one or more superficial cells, but \V \lfllllllffill / / in s°me cases the deePer ^PIllIP^ / / layers of cells also take part in their formation. All lateral members may be developed either singly or several together at the same level on the parent member. When in any cross-section of the parent member not one only, but two or more lateral members occur at the same level, they are said to form a ivhorl ; for instance, of secondary roots round a parent root, or of leaves round a stem as in Herb Paris (Paris quadrifolia). The members of a whorl may be developed either simultaneously, or one after the other ; hence a whorl may be either simultaneous or successional. Members not developed in whorls are said to be scattered. § 6. The Arrangement of the Lateral Members. The arrangement of the leaves on the stem is most intimately con- Fio. 3. — Diagrammatic longitudinal section through the growing-point of a stem: b the leaves; fcn their axillary buds; e epidermis; vascular bundles. § 6. ARRANGEMENT OF LATERAL MEMBERS. 11 nected with the acropetal order of their development ; and since the arrangement of the lateral shoots depends on that of the leaves, the same laws determine the arrangement of both these sets of members which apply generally to all acropetally developed members of plants. These laws are most conspicuously exhibited in the arrangement of the leaves, and they will be fully discussed with reference to these members only. The leaves are developed in very close apposition at the growing- point of the stem. The portions of the stem, termed internodes, which lie between the individual leaves may either remain quite short, as in the case of the rosette of leaves of the Plantain and of the Houseleek, of the fascicled leaves of the Larch, and in most flowers ; or they may undergo a considerable elongation so that the leaves become widely separated. The boundaries of the iuternodes — the places, that is, at which the leaves are inserted — termed nodes, are sometimes prominently developed, more particularly when the leaves are arranged in whorls, e.g. Labiatae, or when they ensheath the stem. The portion of the surface or the stem from which the leaf arises is the insertion of the leaf, and its organic centre is called the point of insertion. So long as the internodes have not begun to elongate, and the leaves are still folded together so as to cover the apex of the stem, the growing end of each shoot is known as a bud. The bud which lies at the apex of a shoot, the lower portion of which has already undergone elongation, is a terminal bud ; the lateral buds are the early stages of shoots developed laterally upon a growing main shoot, which often remain in this condition for a considerable time. The arrangement of the lateral buds, and consequently that of the branches which are developed from them, is closely related to that of the leaves ; thus whilst in the Pteridophyta the bud may be developed immediately below or by the side of a leaf, in the Phanerogams it is nearly always developed in the axil of a leaf, that is to say, in the angle made by a leaf with the internode above its insertion. In the latter case the buds make their ap- pearance at the first formation of the leaves (Fig. 3 kn). As a general rule, they are developed in the axil of every leaf, typical exceptions being the leaves that form the flower, and those of many of the Conifers. In some cases certain of the internodes do not elongate, and therefore the leaves, which have been really developed singly, or their axillary buds, appear to have been developed at the same level on the stem, thus forming a spurious 1-2 PART I. — MORPHOLOGY. [§6 whorl, as in the case of the upper leaves of the Tiger-lily and of the whorled branches of the Pines. The distribution of the lateral members over the surface of the parent axis is either (see § 4), multilateral, bilateral, or dorsi- ventral. 1. Radial Arrangement. The arrangement of the leaves on the stem (phyllotaxis') is very various ; this is particularly conspicu- ous in the cases where the leaves are arranged in whorls, for which reason these will be first discussed. If a whorl consists, for instance, of two leaves, it is obvious that they will be placed exactly opposite to each other on the surface of the stem, and that the distance between them, measured from the points of insertion, will amount to just half the cir- cumference of the stem. Similarly, if the whorl consist of three leaves, the distance between any two ad- jacent leaves will be one-third of the circumference, and so forth. The lateral distance between the points of insertion of two adjacent leaves, measured on the circum- ference of the stem, is called their divergence, and it is expressed in fractions of the circumference. Moreover, it is a rule, though not without exceptions, that the successive whorls alternate, so that the leaves of any whorl lie opposite to the intervals between the leaves of the whorls above and below it. Thus the leaves of alternate whorls are exactly above each other (Fig. 4). This arrangement, as in fact all relation of position, may be very plainly exhibited by means of diagrams (e.g. Figs. 5 and 6). Such a diagram consists of a ground-plan of the stem, regarded as being a cone, and looked at from above ; the insertion of each leaf will lie upon one of a series of concentric circles, and the higher the insertion of the leaf upon the stem, the nearer to the centre will be the circle of the diagram upon which its insertion is in- dicated. It may be perceived in the diagram Fig. 5, that when the leaves FIG. 4.— Stem of Lamium with whorls of two leaves : 1-1, 2-2, 3-3, the suces- eive whorls. § 6. ARRANGEMENT OF LATERAL MEMBERS. 18 FIG. 5. — Diagram of a stem with alternate two - leaved whorls : 0, 0, 0, 0, the four or- thostichies : 1, 1, 2, 2, 3, 3, the successive whorls. are arranged in alternate whorls, they form twice as many longi- tudinal series on the stem as there are leaves in each whorl, pro- vided, of course, that the number of leaves in each whorl is the same. The longitudinal series, which are indicated in the diagram by radii, are called or- thostichies. This particular arrangement of alter- nate whorls of two leaves occurs very frequently, and is termed the decussate arrangement. The two leaves of each whorl are said to be opposite. It is comparatively rare for equal successive whorls to be superposed ; that is, that the leaves of each whorl should lie ex- actly above or below those of the others, so that there are only as many orthostichies as there are leaves in each whorl ; but it sometimes occurs in flowers. Examples of decussate leaves : the Caryophyllaceae, the Labiatae, the Caprifoliaceae, to which belong Syringa (Lilac), Lonicera (Honeysuckle), and Sambucus (Elder); the Maple, the Horse-Chestnut, and the Ash. In Rhamnus cathartica the two leaves of each whorl are usually at a slightly different level. Alternate whorls of many leaves occur in Equisetum and Hippuris ; al- ternate whorls of 3 (irrespectively of flowers) occur in the common Ju- niper, in Catalpa, and occasionally in the Horse-Chestnut and the Maple. When successive whorls consist of unequal numbers of members, the relations of alternation become highly complicated, as in the shoot of Polygonatum verticilla- tum and in the flowers ° o of the Pomeae (Apple, etc.) When the leaves are arranged in a scattered manner it is easy to detect that, within a cer- tain region of the stem, their diverg- ence is constant ; that is, that the dis- tance between any leaf and its immediate predecessor and successor Fio. 6.— A, Diagram of a stem bearing leaves with a divergence of \ ; B, a utem bearing leaves with a diver- gence of J. 14 PART I. — MORPHOLOGY. [§6 is a certain fraction of the circumference. In the simplest case, when the divergence is i (Fig. 6 A\ starting with any leaf 0, the insertion of the next leaf, in succession on the stem, which may be numbered 1, will be on the opposite side to that of the leaf 0 ; and the next leaf, numbered 2, will be opposite to 1 and exactly above 0. Thus there are two orthostichies. But since each leaf is at a different level, in proceeding from leaf 0 to 1, 2, 3, and so on, always in the same direction, the circumference of the stem is traversed in a spiral which, in the course of each whole turn, touches the bases of two leaves and intersects the same orthostichy. This spiral will pass through the insertion of every leaf, and as it does so in the order of their development, it is known as the, (jcnetic spiral. The number of leaves through which the genetic spiral passes in its course between any two on the same orthostichy is termed a spire. When the di- vergence is ^, the leaf numbered 3 comes exactly above leaf 0, 4 over 1, 5 over 2, and so on : and there are three orthosti- chous lines, the spire being composed of three leaves. It might be said with equal accuracy that the di- FIG. 7.— Diagram of a stem with a constant di- Vergence is -|, since leaf 1 vergence of |: I. II, III, etc., the orthostichous is distant f of the cirCUm- lines. (After Sachs.) 3 ference from leaf 0, if the spiral be followed in the other direction. If it be continued in this direction, it will pass round the stem twice in each spire. For the sake of simplicity, the spiral is not traced in this longer way, but in the shorter way. When the numerator of the fraction of diver- gence is not 1, but some other rational number, the spiral passes round the stem more than once within the spire, in fact, just as many times as is expressed by the numerator of the fraction of divergence ; the denominator of the fraction expresses the number of the orthostichies. In Figs. 7 and 8, which represent a constant livergence of -»-, it is easy to see that eight orthostichies are pres- ent, leaf y being over 1, 10 over 2, and so on ; also that the spiral returns to a leaf on the same orthostichy after three turns, and thus goes thrice round the stem in one spire. ARRANGEMENT OF LATERAL MEMBERS. 15 If it is required to determine the arrangement of the leaves (phyllotaxis) on a stem, it is necessary to find the leaf which is exactly above the one, numbered 0, selected as a starting-point, and then to count the number of leaves which are met with in following the shorter spiral round the stem between thesfc two leaves. The number of the leaf which lies in the same orthostichy is the denominator of the fraction of divergence, and the numerator is the number of turns made by the spiral between the two leaves. When the number of orthostichies is greater than 8, it becomes very difficult to detect them, particularly when the leaves are closely arranged as in the rosette of the House-Leek, the florets in the capitulum of the Sunflower, or as the scales in a Mr-cone. Another set of lines lying obliquely then strike the eye, called parastichies, which also run round the stem in a spiral, but touch only some of the leaves ; for instance, in Fig. 8, the line which connects the leaves 3, 6, 9, and 12. It is evident that the number of parallel para- stichies must be as great as the difference between the numbers of the leaves in any one such line. Thus in Tig. 8, again, another parastichy connects the leaves 2, 5, 8, 11, and so on ; and a third, the leases 1, 4, 7, 10, etc. From this it is possible to deduce a simple method for ascertaining the phyl- lotaxis in complicated cases ; the parastichies which run parallel in one direction are counted, and the leaves in one of them are numbered according to the above-mentioned rule ; by repeating the process in another system of parastichies which intersects the first, the number of each leaf will be found. The commonest divergences are the follow- ing : i, i, *, I, TV, A, M- This series is easy to remember, for the numerator of each fraction is the sum of those of the two preceding, and it is the same with the denominator. There are, however, divergences FIG. 8.— Diagram of a stem the leaves of which have the constant diver- gence of f, the leaves of the anterior surface are indicated by their inser- tions, those of the pos- terior by circles ; they are connected by eight orthostichies, I, I ,11, IT, etc. 16 PART I.— MORPHOLOGY. [§ 6 which are not included in this series, e.g. £, f , f , etc. In some cases the construction of a spiral with a constant divergence is impossible, as in Salvinia. The causes of this regularity of arrangement of the leaves lie partly in the mode of origin of the leaves at the apex of the stem, and partly in the displacements which they undergo in the course of their subsequent growth. Instances of the divergence £ : all Grasses, and the smaller branches of the Elm,-the Lime, the Hornbeam, and the Beech ; in these, particularly in the last, the leaves undergo displacement, so that on the under side of the branch the divergence is less, and on the upper side it is greater than £. Divergence of £ is found in all the Sedges, and in the branches of the Alder and Aspen. Divergence of f may be regarded as the most frequent ; it occurs in many herbaceous plants and in most of the smaller branches of the Willow, the Poplar, the Oak, the Rose, the Cherry, and the Apple. The acicular leaves of the Firs and Spruces usually have a divergence of jj and ^3 : jfr occurs very commonly in the cones. Finally, it may be observed that the genetic spiral turns sometimes to the right and sometimes to the left on the stem ; in botanical terminology, a spiral is said to be right-handed when it runs in such a direction that if the observer ascended along it he would have the axis on his right ; and left-handed, when it runs in the contrary direction. It has been already pointed out that these laws of position stand in the closest relation to the progressive development of the lateral members. It can be demonstrated that the relation of position, when once established, is maintained, because each new lateral member arises just at the spot on the growing-point where there is the greatest amount of space between the members already formed, and it thus falls into the order which its predecessors have es- tablished. So long as the relation of size between the rudiments of the lateral members and the surface of the common axis remains constant, the divergence will likewise remain constant ; but if this relation be altered, if, for example, the newly developed members are smaller than their predecessors, it will be readily understood that the number of orthostichies and parastichies must be in- creased. Hence we find changes in the divergence taking place just in those regions in which the size of the lateral members alters, for instance, at the base and at the apex of Pine-cones, and at the base of the inflorescences of Compositse. Furthermore, sub- sequent alterations may be induced by the growth either of the axis or of the developing lateral members. § G. ARRANGEMENT OF LATERAL MEMBERS. 17 2. Isobilateral Arrangement obtains when similar lateral mem- bers arise on two diametrically opposite sides of the common axis, and thus form two rows or orthostichies. Usually the members of the two rows arise at different levels, so that they alternate (Fig. 6 A}. In this case, also, it is possible to. construct a genetic spiral ; such that at every half-turn it passes through the insertion of a lateral member, and connects all the existing members in the order of their age. Since the divergence is |, it is obviously quite immaterial in which direction this spiral may be traced. Examples of this alternate arrangement are given on the previous page. It is only rarely that the members of the two rows stand in pairs at the same level, thus forming superposed whorls of two members each ; this is the case with the leaves of many Naiadacese, probably in consequence of subsequent displacement. 3. Dorsiventral Arrangement. This arrangement of lateral members may be manifested in very different ways. In some cases the common axis bears lateral members on one side only ; in others, the common axis bears dissimilar lateral members on its different sides. As examples of the former, the flowering shoots of Vetches and their allies, which bear flowers on one side only, may be mentioned. The stem of Marsilea is an example of the latter ; it bears leaves on the dorsal surface, lateral branches on the flanks, right and left, and roots on the ventral surface : this relation holds good also in the case of Azolla and Pilularia. In Salvinia the dorsal surface of the stem bears the foliage-leaves, the flanks the branches, and the ventral surface the aquatic leaves : in Selaginella, likewise, the leaves are borne on the dorsal and ventral surfaces, and the branches on the flanks. In Utricularia and in the inflor- escence of the Boraginacese, the branches are borne on the dorsal surface, the leaves (when present) on the flanks. In the Lemnaceae the branches are produced on the dorsal, the roots on the ventral, side of the shoot. The members borne on the flanks, in these cases, are in rows, one on each flank; and a similar serial arrangement can usually be traced in the members borne on the dorsal and ventral surfaces. Thus, in the inflorescences of the Boraginaceae, the flowers are arranged in two longitudinal rows ; in those of the Vetches there may be two rows (Vicia Faba, commonly), or many rows (Vicia Graced). Similarly in some Ferns (Lygodium palmatum, Poly- podium Heracleum] there is a single dorsal row of leaves. In Azolla, Pilularia, and Marsilea, there are two dorsal rows of leaves ; M.B. C 18 PART I.— MORPHOLOGY. [§ 7 in Selaginella there are two ventral and two dorsal rows of leaves, and in Salvinia two ventral and four dorsal rows. The whorled arrangement is not excluded by dorsiventrality : for instance, in Salvinia, the leaves are arranged in alternating whorls of three, two of the leaves being borne dorsally, and the third ventrally, and thus the four dorsal and the two ventral rows of leaves are produced. The affinity between the dorsiventral arrangement and the isobilateral is indicated by the fact that whilst many axes develope their lateral appendages on their flanks, they eventually come to be dorsal. For instance, the creeping shoots of Butomus and other plants produce their leaves in two lateral rows, which, however, eventually undergo displacement on to the dorsal surface : again, in the twigs of the Beech, the two rows of leaves approach each other on the ventral surface, and the lateral branches approach each other on the dorsal (p. 16). Dorsiventral or isobilateral arrangement may not uncommonly be found in the same plant with radial arrangement, but in different parts : thus in the Hornbeam and the Elm the leaves of the primary shoot of the seedling are arranged radially, whilst on the twigs of the adult plant the leaves are arranged bilater- ally (see p. 16). § 7. Development of Branch-Systems. Just as it is possible to ascertain the laws governing the relative positions of all members growing in acropetal succession from a study of the leaves (which are alway developed in that order), so the study of the branching of stems will lead to the general laws which regulate branching. Any member with its branches composes a branch-system ; and every branching member is, with reference to its branches, the axis of a system. The following types of branch-systems may be distinguished, according to the arrange- ment of the members : — 1. The branching is termed a Dichotomy or Polytomy, when the direct apical growth of a member ceases, two or more grow- ing-points which are equally vigorous, at any rate at their first development, being formed at the apex. The member which bears the branches is called the base or podium, and each of these branches may become the base of a new dichotomy or polytorny. They may either continue to grow with equal vigour, and then, in the case of a dichotomy, the branching remains distinctly bifurcate (Fig. 9 A) ; as in the leaf of Schizcea dichotoma, where § 7. BRANCH-SYSTEMS. 1!) the branches all lie in one plane, and the roots of Selaginellar where the branches lie in various planes : or the system may be- come sympodial, if at each bifurcation one branch becomes more strongly developed than the other; in such a case the bases of the successive bifurcations appear to constitute an axis, which is called the pseud-axis or sympodium, on which the weaker branches appear as lateral branches (Fig. 9 B, Cf). The sym- podium may consist of bifurcations belonging to the same side of the successive dichotomies, either to the left or to the right (Fig. 9 -B), when it is said to be a hdicoid (bostrychoid) dichotomy, e.g. the leaf of Adiantum pedatum : or it may consist alter- nately of the right and left bifurcations of successive di- chotomies (Fig. 9 C), when it is said to be a scorpioid (cin- cinnal}, dichotomy, as in the stem of most Selaginellas. 2. The branching is said to be racemose when the member continues to grow in its ori- ginal direction, and produces lateral branches in acropetal succession behind its apex ; it is therefore the common base of all the lateral shoots, and hence the system is termed monopodial. Each branch may subsequently branch again in the same manner. The pri- mary axis continues to grow more vigorously than the la- teral axis, and each lateral axis stands in the same rela- tion to its lateral axes. 3. The branching is said to be cymose, when at an early stage the growth of each lateral axis begins to be more vigorous than that of the primary axis above the point of origin of the lateral axis, and when the lateral axis becomes more copiously branched than the primary axis. Hence two forms may arise : (a) There may be no pseud-axis ; this is the case when two or FIG. 9. — Diagram of the various modes of development of adichotomous branch-system. A One developed by repeated "bifurcation. B Helicoid dichotomy; here the left-hand branch is always more vigorous than the right (r). C Scorpioid dichotomy; the right and left branches are alternately more vigorous in their growth. 20 PART I.— MORPHOLOGY. more lateral axes are developed in different directions and grow with nearly equal vigour (Fig. 10) and more vigorously than the primary axis, ir* which soon ceases to grow ; such a system has a certain re- semblance to a dichotomy or polytomy, and is called a false di- chotomy (Di- chasium) or a false poly- tomy (Poly- chasium) : or (/3) a pseud- axis is formed / this takes place when only one lateral axis de- velopes vigorously in each case, as in Fig. 11 A where the lateral axis 2 has grown more vigorously than the mother-axis 1, and so on. (In the diagram the dark lines indicate the more vigorous FJG. 10.— Diagram of a False Dichotomy or Dichasium ; the Boman numerals indicate the order of development of the shoots of the *.\ wieni. (From Sachs.) In- ll.-Cyraose branchings represented diagrammatically. A B Scorpioid (cincinnal) cyme. C Dichaeial cyme. D Helicoid (bostrychoid) cyme. The numerals indicate the order of succession of the lateral shoots which spring from en.ch other. (Figs A B and D are ground-plans ; Fig. C is a projection hit •> the plane of the paper.) 8. COHESION AND ADHESION. •Jl growth.) The pseud -axis which is thus formed is at first crooked, but in most cases it subsequently becomes straight (Fig. 11 A becomes B\ If the stronger growth always occurs in the lateral shoots of the same side, the system is called a helicoid cyme (Fig. 11 D) ; if alternately in those of both sides,- it is called a scor- pioid cyme (Fig. 11 A,B). Such a branch-system is said to be sympodial. As examples of these various modes of branching, the inflorescences, which will be treated of subsequently (Part IV.), may be especially men- tioned; the following are selected from the vegetative organs: Racemose branching 'is very evident in Conifers ; the trunk is always more strongly developed than its lateral branches, and these than their lateral branches. False Dichotomy is exhibited in the stem of Viscum, the Mistletoe, the apex of which either terminates in a flower or else dies ; only the axillary buds of the two leaves develope into new annual shoots. As regards - the arrangement of the annual shoots, the same occurs in Syringa, the- Lilac, in which the axillary buds of the uppermost pair of leaves form the continuations of the stem, whilst the terminal bud dies; also in filiamnus cathartica, in which the- main axis is metamorphosed into a^ thorn. In this case the branching of each annual shoot is racemose, but the successive annual shoots form a cyme. The succession of the annual shoots of many trees, as the Birch, Elm, Beech, and Hazel, affords examples of the sympodial cyme; in these, each annual shoot either terminates in a flower, or it dies, and the uppermost lateral bud forms its continuation. Here, also, the branching of each an- nual shoot, apart from its apex, is racemose. § 8. Cohesion and Ad- hesion. It sometimes happens that the originally free edges of parts subsequently grow together ; for instance, the margins of the carpellary leaves to form ovaries. As a rule the rudiments of distinct members become united into one whole by the growth of their com- mon base. For example, a gamo- petalous corolla (see Part IV.) arises in this way, that the whorled leaf -rudiments are raised up by FIG. 12. — Flower of Petunia : A very young ( x 50) ; B mature (nat. size) ; k the calyx ; V the line along which the calyx has been removed; r the tube, I the lobes or teeth of the corolla. 22 PART I.— MORPHOLOGY. [§ 9, 10 the intercalary growth of their common base (Fig. 12 A, r), and come to be merely lappets on the rim of a tube (Fig. 12 £}. This explanation applies also to perfoliate and connate leaves (see Fig. 20). The union brought about in either of these ways may affect members developed at the same level, or members developed at different levels ; in the former case the term cohesion is used ; in the latter, the term adhesion. Examples of the former are afforded by gamopetalous corollas, syncarpous ovaries, «ta; and of the latter by epipetalous stamens, by leaves adhering to the shoots borne in their axils, as in the Lime, etc. § 9. The Thallus. The thallus offers considerable variety of form. It may be spherical ; or filamentous, branched or un- branched ; or a flattened expansion, branched or unbranched ; or a massive tuberous body. It commonly bears hairs. The sym- metry of the thallus is multilateral, isobilateral, or dorsiventral. Complete multilateral symmetry is exhibited when the thallus is spherical (e.g. Volvox, Fig. 1) ; isobilateral symmetry when the thallus is flattened (e.g. Desmids, Coleochsete) with similar sur- faces ; dorsiventral symmetry, when the thallus is flattened, with dissimilar dorsal and ventral surfaces (e.g. most Hepaticse, and Fern-pro thallia). The branching of the thallus takes place in accordance with the general laws laid down in § 7 ; the flattened thallus frequently branches dichotomously (e.g. some thalloid Hepaticse). The main axis and the branches maybe either limited or unlimited in growth. The branches of the thallus may be modified in form in connexion with some special function. Thus, the development of reproductive organs is in some cases confined to certain branches, and these then differ in form from the ordinary vegetative branches (e.g. some Hepaticse). § 10. The Shoot. The shoot may be either thalloid (see p. 3) or leafy. The morphology of the thalloid shoot hardly requires special consideration : it is much the same as that of the thallus. The general form of the leafy shoot varies widely. Even on one and the same plant there may be different forms of leafy shoots. the differences being due either to peculiarities in the conditions of development, or of function. Marked differences exist, for instance, between submerged, or subterranean, and aerial shoots ; also between vegetative shoots and those bearing the repro- ductive organs. § 10. THE SHOOT. 23 The form of the shoot depends largely upon the amount of elongation which the internodes of the stem undergo. Thus, there is in some plants (e.g. the Larch, Pine, and Taxodium, among the Coniferae ; and many Angiosperms) a well-marked distinction of two forms of vegetative shoots. These are the. ordinary elongated branched shoots ; and short shoots, termed dwarf -shoots, which elongate but little, branch scarcely at all, and are frequently of but short duration (see p. 9). Again, in some plants (e.g. most Ferns and Conifers) the primary shoot continues to grow throughout the life of the plant ; whilst in others, the growth of the primary shoot is limited, the further development of the shoot being effected by a lateral branch, itself of limited growth ; so that, by the repetition of this process a cymose branch-system is produced (see p. 21). This mode of development by innovation occurs in many so-called uniaxial plants whose primary shoot terminates in a flower ; and in the seedlings of the Lime, and of the Elm, which form no terminal bud at the close of the first year, the further development of the shoot being effected by the highest lateral bud. In plants which live for more than one year, the shoot may either persist from year to year, or it may die down to the surface of the soil in each year, the subterranean parts being alone per- sistent. Shoots which last only one year are termed annual. In those shoots of trees which are produced in one season's growth, the lowest internodes, especially those lying between the bud-scales, are very short ; so that it is easy, by noticing the closely-arranged scars of the bud- scales, to determine, in a shoot several years old, the amount of growth during each year. The terminal and the lateral buds of such an annual shoot usually remain in the bud-condition during the first year until the beginning of the next period of growth, so that the age of such a branch- system can be determined by the extent of the branching, the number of years corresponding to the number of times that branching has taken place. In some trees, however (e.g. the Oak), a second shoot, which had hitherto existed in the bud-condition, is regularly developed in the middle of summer. As a general rule, it is only the more anterior (near the apex) of the lateral buds on the shoot which develope in the subsequent year into branches, as is very clearly seen in the whorled branches of the Coniferse ; when, however, the more posterior lateral buds do develope, the branches produced are successively the shorter the further they are from the apex (e.g. Elm). Whilst in many trees (Coniferae, Oak) the ter- minal bud of a shoot always grows into a new shoot in the next year, in others (Lime, Elm, sometimes Beech) this is not the case, but the elongation of the shoot is effected in a sympodial manner by means of the highest lateral bud (see p. 21). In the Larch, the dwarf-shoots bear the fascicled leaves, and spring PART I. — MORPHOLOGY. 10 from the axils of the leaves of an ordinary shoot of the same year; they usually elongate but slightly each year, but they may, under certain cir- cumstances, develope into ordinary shoots. In the Scotch Pine, the dwarf- shoots bear only two green leaves, in addition to scaly leaves ; they arise in the axils of the scaly leaves of the ordinary shoots of the same year, and Fio. 13.— Various forms of shoots. A Tubers of Helianthm tuberoses (i nat. size); »lower part of the stem springing from last year's tuber fc'; in the axils of the upper leaves arise the buds Jen, and in those of the lower leaves the tubers fc with very small scaly leaves and buds. B Bulb of Hyacinthut orientals (reduced); fc the discoid stem, z the scaly leaves, » the stalk which subsequently elongates and bears the flowers above ground, with the bads b ; I foliage-leaves ; w roots ; Icn an axillary bud which becomes next year's bulb. C Elongated rhizome of Care* Arenaria (i) ; scaly leaves n ; a erect shoot with scaly and foliage-leaves I. D Runner « of the Strawberry, Fragaria (reduced), springing from the plant o, with scaly leaves ti, from the axil of which a new leafy shoot b arises. E Creeping shoot of the Ground Ivy, Gl«chomo hederacea (reduced) ; //decussate leaves ; the internodes are twigted ; a axillary shoot ; w root. they fall off when the leaves die. In dicotyledonous trees, these dwarf- shoots occur especially in advanced age, or when the growth of the tree is stunted. They are very conspicuous in the Apple, Pear, and other § 10. THE SHOOT. 25 similar trees, and are the only parts of the tree which produce flowers and fruit : they are the fruit-spurs. The Bulb and the Corm are examples of shoots with short stems ; they are, in fact, forms of the bud produced underground. The Bulb consist of a flattened discoid stem, bearing a number of scaly leaves closely arranged on its upper surface, and roots on the lower sur- face. The leaves may either invest each other, as in the Onion, when the bulb is said to be tunicate (Fig. 13 B) ; or they may only overlap at their edges, as in the Lily, when the bulb is said to be imbricate. Aerial buds develope in some plants into small bulbs, termed bulbils, as in Lilium bulbiferum, Denlaria bulbifera, and in some species of Onion. The Corm consists of a rounded or flattened stem which occupies a rela- tively larger proportion of space than that of the bulb, and is invested by only a few scaly leaves. It occurs in Crocus and other Iridese. The Tuber is a dwarf -shoot, consisting of a swollen stem bearing small, Fi&. 14.— A Rhizome, with unlimited growth, of Oxalis Acetosella (wood-sorrel) ; n scales ; I foliage-leaves ; V remains of older foliage-leaves ; bl flower ; 7i bracts. B Rhizome with limited growth of Potygonatum officinale (Solomon's Seal) ; I scar of last year's herbaceous aerial shoot; II aerial shoot of this year, which is the anterior portion of the shoot 2 ; III bud of next year's herbaceous aerial shoot, which is the continuation of the shoot 3 ; n, scales ; b and b' leaves from the axils of which the shoots 2 and 3 have arisen ; w roots. scaly leaves ; it is usually developed underground, as in the Potato and and the Jerusalem Artichoke (Helianthus tuberosus, Fig. 13 A). The morphological nature of the tuber is readily demonstrated by un- covering the underground shoots of a Potato-plant, when they develope into ordinary foliage-shoots. Again, if the development of tubers be pre- vented by cutting off the underground shoots, the buds in the axils of the leaves above the ground develope into tubers. The Flower is another form of dwarf-shoot, the leaves of which, when present, are arranged closely together. The morphology of the flower is discussed in subsequent paragraphs. 26 PART I.— MORPHOLOGY. Shoots may grow erect into the air; or they may grow horizontally either above or below the surface of the soil. A shoot which grows horizontally on the surface of the soil is terme creeping shoot (Fig. 13 E). The Runner or Stolon is allied to the creeping shoot. It is an elongated lateral shoot which takes root at some distance from the parent plant, and which by the dying away of the intermediate portion, becomes a new indi- vidual. The runner may grow either just above (Fig. 13 D), or just below the surface of the soil ; it bears sometimes scaly leaves, sometimes foliage- leaves (e.,j. Hieracium Pilosella). Banners usually spring from shoots with limited growth, but s o me times from those with unlimit- ed growth, e.g. Struthiopteris. When a shoot grows horizontally beneath the soil, it is termed a Rhi- zome. It is characteristic of those plants the subterran- ean parts of which alone are persistent. The growth in length of the rhizome is sometimes un- limited, some- times limited. When the former is the case, it con- tinues to elon- gate at its apex, and bears either only foliage-leaves (e.g. Pteris aquilina)] or foliage-leaves and scales in regular alternation (Fig. 14 A, I, w), in the axils of which annual shoots arise ; or only scales in the axils of which annual shoots bearing foliage-leaves and flowers arise, as in Herb Paris. More com- monly, the growth in length is limited, in which case the apex grows out into an aerial annual shoot, whilst from the axil of a leaf at its base one Fio. 16. -A Part of the shoot of the Vine ( nat. size) with two tendrils r r ; the upper one bears small leaves ?i and branches ; the lower one has become attached to a support x and has rolled up spirally ; b h petioles ; in this case the tendrils are branches which are peculiar in that they are opposite to the leaves. B Twining Bhoot of Ipomoea s, with leaves b and a bud fc; »x is the support. § 11. THE STEM. 27 or more subterranean shoots are produced which carry on by innovation the elongation of the rhizome. If the older portions of the rhizome persist for a long time, the basal portions of the annual shoots together form a sympodium (Fig. 14 B) ; if, however, they soon perish, then each annual shoot appears to constitute a distinct individual (e.g. Ranunculus acris, Neottia). It is by the simultaneous formation of a number of short innovation-shoots that the tufts of Grasses and Sedges are produced. The innovation-shoots commonly develope roots of their own, but they may remain connected with the main root of the plant as in Anemone PulsatUla. In rare cases (Psilotum among Vascular Cryptogams) the functions of roots are performed by subterranean shoots ; these shoots are more slender than the subaerial shoots, and bear the merest rudiments of leaves. Shoots which are unable to grow erect by themselves obtain, in some cases, the Advantages of that position by climbing. The structure of the shoot may be modified so as to subserve climbing. Branches are in some cases (Uncaria) developed in the form of hooks, and may or may not bear leaves ; these hooks serve to attach the plant to others. In other cases, branches bearing small scaly leaves are developed into tendrils, which twine round supports. In other cases the whole shoot twines round a support (Fig. 15 A, B). Branches are sometimes developed as thorns (Fig. 16). Thorns are hard, pointed structures ; they sometimes form the extremity of an ordinarj' shoot, as in Rhamnus cathartica ; or they are dwarf-shoots, as in Cratcegus coccinea ; they may bear branches which spring from the axils of scaly leaves, as in Gleditschia and the Sloe (Fig. 16). The morphology of the constituent members of the leafy shoot, namely the stem and the leaf, will now be considered. § 11. The Stem. The stem of an annual plant or of an annual shoot is succulent in texture, and is said to be herbaceous. A primary stem which persists for several years, though it is herbaceous at first, becomes hard and woody in tex- ture, and is termed a trunk. The stem is commonly branched ; but it may be unbranched, as in Tree-Ferns, Cycads, many Palms and Grasses. The form of the Stem varies Very, leaf -scar, from the axil of which widely. It may be short and much tbe thorny branch z 8prings ; on ,,. , , . J the thorn are // leaf -scars; in thickened, as in the bulb, corm and the axil of the upper one is the tuber, mentioned above (p. 24) and in branch z- in that of the lower- „. ... , the bud fc. some Uacti ; or a portion of it may be much thickened into a tuber, as in certain epiphytic Orchids, where one or more of the basal internodes form a pseudo-bulb 28 PART I. — MORPHOLOGY. [§ 12 and in Vitis gongylodes, where any internode may become tu- berous. The form of the elongated stem is commonly cylindrical or prismatic. The prismatic form is, in some cases, determined by the arrangement of the leaves ; thus, stems bearing decussate leaves (see Fig. 4, p. 12), that is, leaves arranged in four orthostichies, are quadrangular. When the stem has an angular form, the edges frequently grow out into a leafy expansion : such a stem is said to be winged. In some cases, as in Grasses, Bamboos, Pinks, etc., the stem presents a jointed (tumid) appearance at the nodes ; a stem with this peculiarity is termed a culm or haulm. When the development of the foliage-leaves of a shoot is de- generate, the stem performs the functions of the leaves : it is then of a green colour, and generally assumes such a form as to have a relatively large surface. Thus, the whole stem and its branches may become flattened, as in Opuntia (Cactacese) and in Genista sagittalis (Papilionacese) : or certain branches only, termed phyllodades, are flat- tened and leaf-like as in Ruscus (Liliacese), Phyllanthus (Euphorbi- acese), Miihlenbeckia (Polygonacese), Carmichaelia (Papilionacese), Phyl- locladus (Coniferse), and are either FIG. i7.-Phyiiociade of Ruscus isobiliterally or dorsiventrally sym- Hvpo^um (nearly nat. size) : . metrical. The phylbclades fre- stem; b leaf, m the axil of which the r J phyiiociade p is developed ; d leaf of quently bear flowers, but not always the phyiiociade bearinR flowers bl in in the Same position. Thus, in its axil. Ruscus androgynus the flowers are borne on the margin of the phyiiociade ; in Ruscus aculeatus and R. Hypoglossum, they are borne on the upper surface of the phyi- iociade ; and in R. Hypophyllum, on the under surface. Leaf-like branches are also formed in Asparagus ; they are not flattened, but are small and acicular ; something of the same kind also occurs in Equisetum. § 12. The Leaf. All leaves, except the primary leaves or cotyledons, are developed exogenously as lateral outgrowths upon the growing-point of a stem. In most plants the leaf undergoes differentiation or segmentation along its longitudinal axis or phyllopodium. In the most com- § 12. THE LEAF. 29 plete case, the phyllopodium is differentiated into three regions : a basal portion, the leaf-base or hypopodium ; an apical portion, the epipodium ; and an intermediate portion, the mesopodium, leaf- stalk, or petiole ; but the last-named portion is frequently absent. Most commonly the leaf assumes a flattened fcfrm in consequence of the development of a relatively thin membranous icing along one or other of these regions in the lateral plane : the epipodium is typically winged, and then constitutes what is known as the blade or lamina of the leaf ; the mesopodium is rarely winged, the hypopodium more frequently so. The growth in length of the leaf is at first apical in all cases ; it may continue to be apical (e.g. Ferns generally) ; or apical growth may be early arrested, further elongation being effected by basal growth (e.g. Iris, Onion, Myriophyllum, Poten- tilla anserina) ; or, more rarely, basal and apical (e.g. Achillea MillefoUum, and other Composite) growth may occur simultaneously. A characteristic feature of leaves is that their growth in length is limited ; but this is not without exception ; in fact, there are all intermediate forms between those which have limited and those which have unlimited growth. Thus, in most Phanerogams the leaves have limited growth ; the cells of the leaf are all actually formed at the time of its unfolding, and all that takes place subsequently is that the cells grow to their definitive size. In a few of these plants, however, (e.g. Gruarea and other Meliaceae) the pinnate leaves have an apical growing-point by which new cells are formed, and the growth in length of the leaf and the de- velopment of lateral branches is carried on after the leaf has unfolded. Long-continued apical growth appears to be the general rule in Ferns : in Pteris aquilina and in Aspidium Filix-Mas the leaf grows for three years ; and in Grleichenia, Lygodium, many Hymenophyllacese, and Nephrolepis, the leaf grows for many years after its appearance above the soil. The most striking example of long-continued basal growth is that of the two leaves of Wel- witschia which persist and grow basally as long as the plant lives, and consequently attain a great length. The leaves are inserted upon the nodes (p. 11) of the stem, the FIG. 18.-Leafof Ban- unculus Ficaria: v leaf- base (hypopodium) ; p petiole (mesapodium) ; t lamina (epipodium). 30 PART I.— MORPHOLOGY. 12 plane of insertion being usually transverse to the longitudinal axis of the parent stem. The Hypopodium or Leaf-Base. The leaf-base commonly de- velopes into a cushion of tissue, termed the pulvinus, which forms the articulation by which the leaf is attached to the stem ; in the Gooseberry the pulvinus developes into a spine. In many cases the leaf-base is sheathing, and embraces a part or the whole of the circumference of the node ; in the former case the leaf is said to be semi-amplexicaul ; in the latter, amplexicaul (e.g. Grasses, Onion, Fool's Parsley). The leaf-base sometimes produces a pair of opposite lateral Fto. 19.— 4 Part of a sessile leaf of Grass (Poa trivialis) with the ligule t ; a the haulm ; v the sheathing leaf-base ; I lamina of the leaf. B Leaf of a Willow (Soli* Caprea) ; a stem ; « 8 stipules ; p petiole ; / lamina ; fc axillary bud (nat. size). C Leaf of a Pea (Pisum arvente) ; a stem ; 8 s stipules ; r mesopodium or petiole ; // leaflets ; rf r/ the upper leaflets metamorphosed into tendrils ; r' end of the epipodium, likewise transformed into a tendril. branches which are termed stipules ; when they are present the leaf is said to be stipulate, and when they are absent, as is more commonly the case, the leaf is said to be exstipulate. The stipules are commonly winged appendages, similar in colour and texture to the lamina, and they are then said to be leafy (Fig. 19 B, O), as in the Willow, the Violet, and the Rubiacese where they are branched ; and they are especially large in plants, like the Pea, where the lamina is relatively small : in other plants, on the contrary, they are small brownish scales, which fall off soon after the leaf is un- § 12. THE LEAF. 31 folded, as in the Beech, the Elm, and the Lime. Sometimes the stipules appear as teeth on the upper margin of the sheathing leaf- base, as in the Rose. Occasionally the two stipules are connate, that is, they are more or less united ; when they cohere by their outer margins they form a single opposite stipule, opposite, that is, to the leaf to which they belong, as in Astragalus ; when they cohere by their inner margins they form an axillary stipule, that is, a stipule in the axil of the leaf to which they belong, as in Meli- anthus and Houttynia cordata ; in the Polygonacese they cohere by both their inner and outer margins, thus forming a tube, termed an ocrea, which surrounds the intemode above the insertion of the leaf ; when the stipules of opposite leaves cohere they form on each side an interpetiolar stipule, as frequently in the Rubiacese and in the Hop ; this may also take place when there are several leaves in a whorl, as in the epicalyx of cer- tain Rosaceae. In some cases (e.g. Smilax) the stipules are de- veloped in the form of tendrils, and in other cases (e.g. Ro- binia) as spines. Stipules are comparatively common in Dicotyledons ; they are absent in the Coniferse ; absent in Monocotyledons, excepting the Naiadacese and Smilax ; absent in most Pteridophyta, except the Marattiaceae among Ferns. In Tropoeolum majus only the two leaves which succeed the cotyledons have stipules. The leaflets of a compound leaf sometimes have stipules at their bases, as in Phaseolus, which are distinguished as stipels. In a leaf without a petiole it sometimes happens that the leaf- base is winged in continuity with the lamina ; the result is that the wings extend round the stem, either incompletely (Fig. 20 .4) when the leaf is said to be auriculate ; or completely (B) when it FIG. 20.— The insertion of sessile leaves. A Auricnlate leaf of Thlaspi per/oliotum. B Perfoliate leaf of Bupleurum rotundt- /olium. C Connate leaves of Lonicera Capri/oHum. 32 PART I.— MORPHOLOGY. [§12 is said to be perfoKate ; when this occurs in two opposite leaves, the leaves become connate (C ; see p. 22). There is, in some cases, a delicate membranous ventral outgrowth on the leaf at the junction of epipodium and hypopodium, termed the ligule ; it occurs in Grasses (Fig. 19 .4), in Selaginella and Isoetes, and in the perianth-leaves of some flowers (Narcissus, Lychnis). The Mesopodium or Petiole is commonly, but not always, present. When it is present the leaf is said to petiolate ; when it is absent, sessile. It is developed by intercalary growth in a portion of the primordial leaf lying between the hypopodium on the one side and the epipodium on the other. The most common form of the petiole is somewhat cylindrical ; though, where the dorsiventrality of the leaf is well-marked, it is convex on the lower (dorsal) surface, and flattened or grooved on the upper (ventral) surface. In the Aspen (Populus tremula) it is flattened laterally. Occasionally (e.g. Orange, Fig. 23 G ; Nepenthes, Fig. 28; Dionsea) the petiole is winged. In some cases (e.g. Australian Acacias) the petiole has somewhat the form of a lamina. Its flattened surfaces are directed laterally, the edges upwards and downwards, so that the symmetry is isobi- lateral. A petiole of this form is termed a phyllode. In such cases, the lamina, originally present, soon falls off. Tlie Epipodium may be either icinged or unicinged. The winged epipodium constitutes the lamina or blade of the leaf, and is typically flattened and expanded in form and dorsiventral in symmetry : but this is not always the case, for in some plants it assumes the form of a sac or pitcher (e.g. Utricularia, Nepenthes, etc.), and in others the symmetry is isobilateral (e.g. adult leaves of Eucalyptus Globulus). The form of the unwinged epipodium presents great variety ; thus, in Lathyrus Aphaca the epipodium branches into leaf- tendrils, and this is partially the case also in the Sweet Pea (Fig. 19 C) ; it may be cylindrical or prismatic, as in Onion, Sedum, Mesembryanthemum, Aloe; acicular as in Pinus ; narrow, and flattened anteroposteriorly (ensiform] so that the margins corres- pond to the dorsal and ventral surfaces of a dorsiventral leaf, with isobilateral symmetry, as in Iris and Gladiolus. The flattened dorsiventral lamina is normally so placed with regard to the parent stem that a plane, which includes the longi- § 12. THE LEAF. 33 tudinal axes of both the stem and the leaf, cuts the lamina into two lateral halves ; in other words, it is so placed that its upper (ventral) surface faces the apex of the stem, and its lower (dorsal) surface is directed away from it. As a rule, the two lateral halves of the lamina are symmetrical; but in some cases (e.g. Elm, Begonia) they are unsymmetrical, when the lamina is said to be oblique. The ultimate form of the lamina mainly depends upon the degree of elongation of the epipodium. When the epipodium elongates considerably, the lamina has a well-marked primary axis from which more or less numerous secondary axes of growth successively spring, and these in turn bear lateral axes of a higher Fie. 21.— A Pinnate leaf of the Beech, Fagus syloatica; m mid-rib, n lateral ribs. B Pal- mate leaf of Alchemilla vulgaris (nat. size.) C Pedate leaf of the Plane (} nat. size). 1, 2, 3, are the ribs or axes of the 1st, 2nd, and 3rd order. order : the resulting lamina is then of the pinnate type (Fig. 21 A). When, however, the epipodium remains short, it constitutes merely an intercalary growing-point from which a number of equal secondary axes spring, and the resulting lamina is of the palmate type (Fig. 21 B). The development of the peltate lamina, closely connected with that of the palmate type, is effected by a peculiar form of basipetal growth. In peltate foliage-leaves (e.g. Tropseolum, Nelumbium, Hydrocotyle, Cotyledon, Lupinus, etc.) the petiole is inserted in the middle of the under surface of the lamina, so that the long M.B. D PART I.— MORPHOLOGY. [§12 axis of the former is perpendicular to the plane of expansion of the latter. At first the development is that of a palmate leaf, the petiole being inserted at the base of the lamina, and at the point of insertion there is an intercalary growing-point from which spring several axes (Fig. 22 B, i 2,3) in basipetal succession. But in this case the last-formed axes(j-»,5-5, in the. figure) grow out in front of the petiole, with the result that the whole lamina gradually comes to lie perpendicularly to the petiole. The main axes of growth frequently grow thicker than the rest of the lamina, so that they project as ribs on the under surface. The thickened pri- mary axis (epipodium) of a pinnate lamina is termed a mid-rib. The Branching of the Leaf is commonly confined to the epipodium, and then it takes place in the lateral plane ; less commonly it occurs in the mesopodium (e.g. species of Ophioglossum, Botry- chium, Marsilea), and then (as in these plants) the branching frequently takes place in the antero-posterior (or dorsiventral) plane. The branching of the epi- podium is, like that of a stem or a root, either dichotomous or lateral, and essentially the same forms of branch-systems are produced. Dichotomous branching is comparatively rare ; it has been observed in the Hymenophyllacese, the branches either remaining distinct or forming sympodia. The two first leaflets of Marsilea are said to be formed by dichotomy. Lateral branching is the more common form, and the resulting branch- systems are typically monopodial. But in some cases (e.g. leaf of Plane, Fig. 21 C ; of Helleborus, and of some Aroids) there is apparently cymose branching with formation of a sympodium. The ribs of the lamina are branches of the epipodium. The Fio. 22.— Development of peltate leaf of Hy- drocotyle : A full-grown (nat. size) ; B very young; C somewhat older (B and 0x60); S petiole ; 1-5 primary axes of growth in young leaves, ribs in adult leaf; a secondary axes of growth. § 12. THE LEAF. 35 degree of segmentation of the lamina depends upon the relation between the growth of the various main axes and the marginal growth of their respective wings (see Figs. 21 and 22). When these keep pace with each other the lamina is_ altogether unseg- mented, that is, its margin is entire : when the growth of the axes is rather more vigorous than that of the corresponding wings, the margin becomes somewhat uneven (dentate, serrate} ; when the difference between them is considerable, the lamina is lobed ; and when still greater, it consists of a number of distinct seg- ments, leaflets, connected only by their common attachment to the mid-rib, in the case of pinnate leaves, or to the petiole in the case of palmate or peltate leaves. Whilst inequalities of the margin are indications of branching, the lamina is regarded as simple so long as the segmentation is incomplete ; it is only when the segmentation is complete, as in the last-mentioned case, that the leaf is said to be compound. The following examples will serve to illustrate the foregoing principles. The simple leaf of the Beech (Fig. 21) has an entire pinnate lamina; the leaf of the Shepherd's Purse (Capsella Bursa-Pastoris, Fig. 23 C) is simple, but the lamina is deeply pinnately lobed. Various forms of com- pound pinnate leaves are represented by Fig. 19 C and by Fig. 23 B, Z>, E, F, H, where the distinct segments or leaflets, termed pinna;, are inserted on the common primary axis. In H each pinna is itself compound, being segmented into pinnules which bear che same relation to the secondary axis of each pinna as that secondary axis does to the primary axis of the whole leaf ; such a leaf is said to be bipinnate, and when the segmentation is carried further the leaf becomes tripinnate, etc. In compound pinnate leaves, the leaflets are commonly opposite to each other. When only one pair of leaflets is present, the leaf is said to be unijugate; when two pairs, bijugate ; when many pairs, multijugate. When the axis (whether primary or secondary) is terminated by a leaflet, the leaf is said to be imparipinnate (Fig. 23 D) • when there is no terminal leaflet, the leaf is paripinnate (Fig. 23 E). When, as in the Potato and Potentilla anserina. pairs of small leaflets alternate with pairs of larger ones, the compound leaf is said to be interruptedly pinnate. The difference in size of the leaflets is simply due to the more active growth of the larger ones. The order of development of the leaflets of compound pinnate leaves depends upon the position of the growing-point in the longitudinal axis (see p. 29). When it is apical, the leaflets are developed in acropetal succession (e.g. Pea, Ailanthus, etc.) ; when it is basal, in basipetal succession (e.g. Myriophyllum, Poten- tilla anserina} ; when there is both an apical and a basal growing- 36 PART I.— MORPHOLOGY. [§12 point, in divergent succession, that is, both acropetally and basi- petally (e.g. Achittea MillefoUum, etc.). With regard to palmate leaves, Fig. 23 A is an example of a deeply lobed lamina; and B, of a compound palmate leaf of the Clover in which will be observed that there are three leaflets ; such a leaf is said to be FIG. 23.— Segmentation of leaves, p Petiole ; p' petiolule ; /' leaflet ; r phjllopodiutn. A Palmatifldor palmately lobed leaf of Geranium. B Temate (compound palmate) leaf of Clover. C Pinnaticected leaf of Shepherd's Purse (Capsella). Compound pinnate leaves : D Imparipinnate leaf of Hippocnpit como«o; t terminal leaflet. E Paripinnate leaf of Pistacia Lentitcus ; a wing of the phyllopodinm. F Imparipinnate unijugate lenf of Medi- cago. This differs from B, which is teraate, inasmuch as the secondary leaf-stalks p' do not all spring from one point, but the common leaf-stalk ji extends beyond the insertion of the single pair of pinnse ; » projecting rib, or mucro. G Leaf of the Oranae ; the articu- lation a between the blade and the winged petiole shows that it is really a compound leaf with a single terminal leaflet. H Bipinnate leaf of Acacia : r' secondary axis ; /" secondary pinnae or pinnules. § 12. THE LEAF. 37 ternate. This segmentation may be repeated in the leaflets, when the leaf is said to be Alternate, triternate, etc. On comparing Fig. 23 B and F, the close relation between pinnate and palmate leaves becomes apparent. A ternate leaf is usually considered to belong to the palmate type, but it might almost equally well be regarded as an imparipinnate unijugate leaf. Occasionally the margin of the lamina bears outgrowths which are not connected with branching, but are of the nature of emergences, as in the Cherry Laurel, Naias, various Conifers, etc.. A number of terms are used in Descriptive Botany for the purpose of precisely describing the various parts of plants. The more important of these terms, and those the meaning of which is not obvious, will now be denned. (1) The Outline of bilateral bodies, such as the lamina of the leaf, but of multilateral bodies, such as fruits, as well, is said to be linear when the two margins run nearly parallel to each other ; e.g. the leaves of most Grasses. If the margins are curved and intersect at each end at aoi angle, the outline is said to be lanceolate or elliptical, accordingly as the long axis is many times longer than, or only twice as long as, the transverse axis. If the two curved margins round off at each end, then the terms oblong and oval are to be substituted for the two preceding. If the longest transverse diameter lies relatively near to the base, then the outline is said to be ovate ; if relatively near to the apex, obovate. (2) The Apex may be either acute or obtuse ; when it is long drawn out it is said to be acuminate ; when there is a sharp projecting point, it is said to be mucronate (Fig. 23 F) 5 truncate, when it is, as it were, cut short across (Fig. 23 Z>) ; emarginate, when there is a depression in the obtuse apex ; obcordate, when the apical depression is deep. (3) The Base may be described by many of the preceding terms, but the following are especially applied to it: it is cordate when it is deeply indented in the median line ; sagittate, when the lobes on each side of the indentation are angular and diverge backwards ; hastate, when the lobes diverge outwards. (4) The Margin is said to be entire when it does not present any de- pressions or prominences ; when the prominences are slight and rounded, the margin is said to be crenate ; dentate or toothed, when the prominences are pointed and stand straight outwards ; serrate, when the pointed prominences slant forward. If the incisions in the margin are deep, the part, a leaf-blade for instance, or a gamosepalous calyx, is said to be lobed when the incisions do not extend to the middle ; if they extend to the middle, it is said to be partite; and dissected when they extend nearly to the base (Fig. 23 C). The segmentation of the lamina takes place in some Monocotyledons (Palms) in an altogether different manner from that described above. The lamina is at first entire, but it becomes divided by the dying away of strips of tissue. 38 PART I.— MORPHOLOGY. 12 The Venation of the Lamina. The mid-rib and other ribs of the lamina indicate the course of the larger vascular bundles ; and from these numerous branches are given off which permeate the tissue of the lamina, constituting its Venation. When the leaf decays, the ribs and the vascular tissue persist longer than than soft parts as a skeleton which retains the general form of the lamina. In Onvirandra fenestralis most of the softer tissue decays whilst the leaf is still living, so that the lamina consists of little more than the vascular skeleton. The main features of the venation are determined by the type of development of the lamina. In a pinnate lamina, the venation is pinnate ; in a palmate lamina, palmate ; in a pedate leaf, pedate ; in a dichotomously branching lamina, the venation is also dicho- tomous, or as it is specially termed, furcate. But there is considerable variety in the distribution of the smaller vascular bundle ; thus the venation of the individual segments of a palmate or a pedate leaf is fre- quently pinnate. According to the distribution of the veins and their branches, the following varieties of venation may be distin- guished ; they are, however, connected by intermediate forms. a. The venation is said to be free when the veins end free, without forming ana- stomoses, at the margin of the leaf ; this is the case in the leaves of many Ferns (Fig. 24) ; of Ginkgo (Salisburia), Arau- caria imbricata and others, among Coniferse ; of most Cycads ; of Water- Crowfoots, etc. b. The venation is said to be parallel, when numerous adjacent veins run parallel to each other towards either the apex (Fig. 25) or the margin of the blade, and then unite by curving inwards (Fig. 25 a). They are con- nected in their course by short veinlets (Fig. 25 «) which run usually at right angles to them. This form of venation is to be found in the leaves of most Monocotyledons, such as Grasses, Lilies, and Palms, with various modifications. For example in some cases (e.g. Orchis Mario) many veins enter the blade, but they branch scarcely at all ; in other cases lateral veins spring at an acute angle from a midrib which is prominent at the base at least, and then run towards the apex (e.g. Maize and other Grasses, Dracaenas, etc.) ; in others, again, the lateral veins spring almost perpen- dicularly from the well-developed mid-rib, and run out to the margin Fio. 21.— Leaf of a Young Fern, with free pinnate venation : m the midrib; » » the large lower lateral •veins; n the weak upper lateral veins, (x 3.) § 12. THE LEAF. 3!) parallel to each other, and then turn towards the apex of the leaf (e.g. Canna, Musa, etc.). c. The venation is said to be reticulate, when the veins branch repeatedly at various angles, and the branches for the most part anastomose (Fig. 26). Some of them, however, end blindly in the meshes of the network. This kind of venation is characteristic of Dicotyledons" ; but it also occurs in some Monocotyledons (e.g. Paris, Dioscorea, Smilax, many Aracese) and many Ferns. The Different Kinds of Leaves.— The leaves of different plants, as might be expected, are not alike, but differ more or less widely in size, shape, colour, and texture. But even the leaves borne on FIG. 25. — Apex of a Grass-leaf showing parallel venation : 711 middle vein ; a ana- stomoses ; D veinlets. (x4.) FIG. 26.— Portion of a leaf of Salix Ca- prea with reticulate venation : TO midrib ; n the larger lateral ribs ; c the anasto- mising veins (nat. size). one and the same plant are not all alike, the reason of their dissimi- larity being that, as there are different functions to be performed, the leaves are variously adapted in form and structure to the per- formance of these functions. 1. Foliage-leaves are usually known simply as leaves (Fig. 27 Li). They are conspicuous on account of their green colour, and in accordance with their nutritive function (see Part III.) they are expanded as much as possible to the sun-light. If they are small they are very numerous (Conifers), and the larger they are the fewer they are (Sunflower, Paulownia). They nearly always 40 PART I. — MORPHOLOGY. [§12 possess a well-developed lamina, which presents the various pecu- liarities of conformation previously described. The texture of the leaf is dependent upon the mode and duration of its existence. The texture of most leaves may be described as herbaceous. Leaves of this kind last usually for only a single season, and die or fall off in the autumn. Leaves of firmer .texture, which are said to be cori- aceous, survive the winter, and either fall off when the new leaves are de- veloped (the Privet), or continue to live for several years (Holly, Box, and most Conifers) ; the acicu- lar leaves of the latter may persist for as many as twelve years (Silver Fir). Fleshy or succulent leaves occur in plants in- habiting dry regions or positions, such as Aloe, Sedum, etc. It is worthy of note that foliage-leaves of dif- ferent form sometimes occur on the same shoot. For instance, it is com- monly the case that the first leaves of young plants are of a form dif- ferent from, and usually simpler than, that of those which are subsequently Produced> and exhibit a greater resemblance to fhn^P nf nlliVrl nlanfa jn°Se O1 alliea plants. Thus, Eucalyptus Globu- lus has at first oval dorsiventral leaves, and subsequently elon- gated isobilateral leaves. Again, the primary leaves or cotyledons, when they develope into foliage-leaves, are always different in form from the subsequently developed foliage-leaves, being much region; L region of the foliage-leaves; H hypso- phyllary region ; d the bracts i b the flowers in their axils ; » roots. § 12. THE LEAF. 41 simpler. In many water-plants, the submerged leaves are different from those which float at the surface; thus, in many species of Potamogeton, the submerged leaves are narrow and ribbon-like, whereas the floating leaves are broadly elliptical ; in many aquatic species of Ranunculus, the former are finely divided, whereas the latter have a circular lamina. Again, the submerged leaves of Salvinia are filamentous, whereas the floating leaves are flattened and oval. The simultaneous occurrence of two forms of foliage-leaf on a plant is termed Jietcrophylly . In certain plants the foliage-leaves assume remarkable forms in connection with their adaptation for catching small animals or for collecting water (e.g. Nepenthes, Cephalotus, Sarracenia, Utricularia, Dischidia, etc.). In these the lamina is metamorphosed into a pitcher or ascidium. The development of the pitcher begins in very much the same way as that of the lamina of a peltate leaf ; but instead of remaining flat, it becomes tubular by continued basal in- tercalary growth (see p. 34). The leaf may, as in Sarracenia and Darlingtonia, be sessile ; or it may be petiolate, as in Cephalotus and Nepenthes : in Ne- penthes (Fig. 28) the petiole is winged for some distance in its lower portion. The lid, when present, is a development of the apical, or sub-apical (Nepenthes), portion of the lamina ; as its first de- velopment it adheres firmly to the rim of the ascidium, from which it eventu- ally separates, except at the point of attachment ; the lid is bilobed. 2. Leaf -Tendrils (see p. 32) are leaves or parts of leaves which have a somewhat filamentous form, and which possess the property of twisting spirally round foreign bodies, thus fixing the plant (see Part III.). In species of Clematis, Tropseolum, etc., this function is performed by the petiole of the foliage-leaf ; but in the Vetches and Peas there is a division of labour of this kind, that the anterior leaflets of the pinnate leaf are modified into tendrils (Fig. 19 C, FIG. 28.— Pitchered leaf of Ne- penthes : a organic apex of leaf ; b leaf-base ; pet petiole, winged in its basal portion ; as ascidium ; I its lid; /r fringe of ascidium (reduced). PART I. — MORPHOLOGY. 12 rf) ; in Lathyrus Aphaca all the leaflets undergo this metamor- phosis, and the special functions of the foliage-leaves are discharged by the stipules. The tendrils of the Cucurbitacese are also metamor- phosed leaves. 3. Leaf-Spines are leaves or parts of leaves which are modified into pointed, hard, woody structures. Spiny teeth are often present on foliage-leaves (e.g. Holly, Thistles) ; in species of Caragana and Astragalus the phyllopodium of the pinnate leaf becomes a spine after the falling-ofF of the green leaflets ; finally, the entire leaf becomes spiny in Berberis (Fig. 29). 4. Scales or cataphyllary leaves (Fig. 27 N). These are usually of a yellow or brown colour, of simple structure, without projecting veins, and are attached to the stem by a broad base. They may be regarded in some case as leaf-bases, the laminse of which have not developed ; and in other cases, as entire leaves which have remained in a rudimentary condition. They al- ways occur on subterranean stems (e.g. the scales of the Onion, see also Figs. 13 and 14 n\ and sometimes on aerial stems. Many plants which are not green (Orobanche, Neottia) produce only cataphyllary leaves in addition to the floral leaves. The most common form in which they occur upon aerial stems is that of scales investing the buds of trees. In this case they are the lowest leaf-structures borne by the annual shoot, and usually fall off as the bud developes. Some few indigenous trees have naked buds without scales, as Viburnum Lantana, Cornus sanguinea, Rhamnus Frangula; their buds are protected by a dense growth of hairs. The following varities of bud-scales may be distinguished :— a. The bud-scales are the stipules of leaves which develop a lamina ; as m Alnus, Liriodendron, Marattiacese. 6. The bud-scales are the stipules of leaves which develop no lamina : Oak, Beech. c. The bud-scales are leaf-bases, the lamina not being developed ; Maple, Ash, Horse-Chestnut, Prunus Padus. FIG. 29.— Leaf-spines of Berberif tulgaru, at the base of a shoot of one year's growth : a leaf-spine with broad surface; b with a smaller surface ; fc fc axillary buds (nat. size). § 12. THE LEAF. 43 d. The bud-scales are laminae: Lilac, Privet, Abietineae. In the last case the bud-scales may be caused to develop into foliage- leaves by cutting off the top of the branch, or removing its leaves, at the time when the bud-scales are developing. Cataphyllary leaves are sometimes thickened "so as to serve as depositories for nutritive substances, as in the bulbs of the Onion, Lily, etc. 5. Bracts and Floral Leaves (Hypsophylls and Sporophylls ; Tig. 27 H). These leaves are generally peculiar in form, texture, and colour ; their morphology is discussed in connection with that of the reproductive organs in § 16, as also in Part IV. Vernation and Prefoliation. The forms of young leaves and their relative position in the bud, that is their vernation and pre foliation (aestivation and prefloration of floral leaves), require special consideration. The vernation (or (estivation) is said to be plane when the leaf is not folded at all; it is conduplicate when the two halves of the leaf are folded inwards face to face on the midrib as on a hinge (e.g. the Bean) : it is plicate when the leaf is folded in numerous longitudinal or oblique pleats (e.g. the Beech); it is crumpled, when the foldings are in all directions (e.g. the petals of the Poppy) ; it is involute, when the lateral halves are rolled inwards towards the mid-rib on the ventral surface (e.g. the Violet) ; it is revolute, when they are rolled inwards towards the mid-rib on the dorsal surface (e.g. the Dock); it is convolute when the whole leaf is rolled up from one lateral margin to the other, so as to form a single roll (e.g. Canna) ; or, finally, it is circinate, when the leaf is rolled longitudinally on itself from the apex downwards (e.g. Ferns). The prefoliation (or prefloration) is said to be valvate when adjacent leaves in the bud merely touch by their margins ; when some are overlapped by others it is imbricate ; an intermediate form is that in which one margin of each leaf is directed obliquely inwards, and the other obliquely outward overlapping the inner margin of the next leaf, and is termed contorted or twisted (e.g. petals of the Periwinkle.) Valvate prefoliation is only possible in the case of whorled leaves, whereas imbricate prefoliation is characteristic of spirally arranged leaves. A common form of imbricate prefoliation or prefloration is the quincuncial, which occurs in the many dicotyledonous flowers which have a $ calyx ; the five imbricate sepals are so arranged that two are wholly internal, two wholly external, and one partly internal and partly external, connect- ing the outer two with the inner two (see Part IV., Phyllotaxy of the flower). Where the phyllotaxy is distichous (^), the vernation of the leaves is generally conduplicate, and the margins of each older leaf over- lap those of the next younger leaf, giving rise to a form of imbricate prefoliation distinguished as equitant (e.g. Iris) ; in some cases the over- 44 PART I. — MORPHOLOGY. [§ 13 lapping is by one margin only, in which case the prefoliation is to be semi-equitant. § 13. The Root. It must be clearly apprehended that a sub- terranean member is not necessarily a root ; nor can a member be termed a root because it is found to absorb water and salts in solution, for in rootless plants this function may be discharged by shoots, or leaves, or hairs; nor can a member be termed a root because it serves as an organ or attachment to the substratum, for such organs may be emergences (see p. 48) ; only such members can be regarded as roots which bear neither leaves nor true reproductive organs. The root is sometimes wanting in plants where it might be expected to be present, in plants, that is, of which the body is not a thallus (e.g. Salvinia, Psilotum, Utricularia, Epipogum, Coral- lorhiza). There are certain peculiarities connected with the structure and development of the root which contribute to its morphological distinction. As a rule, the growing-point of the root is not ex- posed, like that of stems or leaves, but is covered by a structure termed the root-cap. As a rule also, the growing-point of the root, whether normal or adventitious, is developed, not at the surface, but in the interior of the tissue, that is, endogenously (p. 9). There are exceptions to both these rules. The primary root of some parasitic plants, such as Orobanche and Cuscuta, has no root-cap, as also the small lateral roots which spring from the larger roots of the Horse- Chestnut. In some cases (e.g. old roots of Azolla caroliniana, Hydrocharis, Pistia Stratiotes) a root-cap is present at first, but eventually disappears, the growth in length of the root being arrested. Exogenous development has been observed in the adventitious roots of Cardamine pratensis (roots of adventitious buds developed on leaves) ; of Neottia Nidus Avis ; of Nasturtium oflicinale and nilvestre ; of Ruppia rostdlata (embryo); Lycopodium, Phylloglossum. The root which is developed at the opposite pole of the embryo to the primary shoot, is termed the primary root. When the primary root persists and continues its growth (as in Oak, Rad- ish, Bean, etc.), it is termed a tap-root. In many cases (generally in Monocotyledons) the growth of the primary root is limited, so that it attains but feeble development. In other cases (e.g. Orchids, Selaginella) no primary root is developed, all the roots being adventitious. The symmetry of the root is most com- § 13. THE ROOT. 45 monly radial. In some cases, however, the root is isobilateral, as is shown as well by its internal structure as by the develop- ment of two opposite longitudinal rows of lateral roots. In other cases (e.g. attached aerial roots of epiphytic Orchids ; roots of Podostemacese) its symmetry is more or less distinctly dorsi- ventral. Roots branch either dichotomously (e.g. Isoetes), or laterally (see p. 9). In lateral branching the secondary roots are devel- oped in acropetal succession on the primary root, and so on. Since the lateral roots are not developed upon the growing-point of the parent root, the terminal apical portion of the parent root con- sequently bears no lateral roots. On anatomical grounds (see Part II.) the secondary roots are arranged in longitudinal rows on the primary roots ; an ar- rangement which also obtains in the branches of the secondary roots, and of higher orders. The form of the root is usually cylindrical ; when it is very delicate, as in Grasses, it said to be fibrous. The primary or the secondary roots may become much swollen, serving as- depositories for nutritive sub- stances ; the Turnip, the Carrot, the Beet, the Radish, have swollen primary roots ; the Dahlia has swollen secondary roots. FIG. 30.— The lateral roots n aris- ing endogenoualy from the peri- cycle of the tap-root of Vicia Faba (longitudinal sec. mag. 6 times) : / axial cylinder (stele) ; r cortex of the main root; h root-cap. of the lateral root. Various terms are employed to de- signate the different forms of swollen roots; that of the Turnip is termed napiform ; that of the Carrot, conical ; that of the .Radish, fusiform or spindle-shaped; those of the Dahlia and of some terrestrial Orchids, tuberous. Many plants have aerial roots which are peculiar both morpho- logically and physiologically. The roots of epiphytes, that is, plants (mostly Orchids and Bromeliaceae) which grow on trees without, however, being parasitic, never reach the ground, but serve as means of attachment : they frequently contain chlorophyll and serve as organs of assimilation, especially in Podostemacese. Some plants climb by means of aerial roots (e.g. Ivy, Tecoma radi- 46 PART I. — MORPHOLOGY. [§ 14 cans), which are developed on the ventral surface of the dorsiven- tral stem, and adhere closely to the tree- trunk or wall on which the plant is climbing. In some rare cases the aerial root is a tendril, as in Vanilla aromatica, Lycopodium rupestre and other species, and in some Melastomacefe (Mcdinilla radicans, Dissochseta). Roots are occasionally developed as thorns, as in the Palms Acanthoriza and Iriartea, and in Myrmecodia (Rubiaeese). In some species of Jussisea (e.g. J. repens] which live in swamps, some of the adventitious roots develope into floats, containing large intercellular spaces. § 14. Hairs and Emergences. Under these terms are included various appendages of a lower morphological value than the stem, the leaf, or the root, upon all of which they may be borne. (a) Hairs. Hairs are always developed from superficial cells ; a hair usually takes origin from a single superficial cell, but some- times from more than one. Their growth is generally apical, but sometimes basal. Hairs vary in form and structure ; they may be unicellular, when they are termed simple; or multicellular, when they are termed compound or articulate : they may be branched or unbranched ; they may be filamentous or scaly. They subserve various functions, being protective, secretory, or ab- sorbent. Various terms are used to describe hairs : filamentous hairs which are secretory have frequently a dilated apex, and are termed capitate ; flattened hairs which are star-shaped, are termed stellate ; discoid flattened hairs are termed radiate or peltate ; the erect flattened hairs of Ferns are termed palece or rametita. When hairs are stiff they are termed bristles or setce. Special terms are used to indicate the nature and the distribution of the hairs on a member. A surface which bears no hairs is said to be glabrous ; when the hairs are scattered the surface is pilose; when the hairs are close and short, villous ; when they are longer, tomentose. When the hairs are rather stiff, the surface is hirsute ; when bristly, hispid. When the hairs are borne on the margin only, the member is said to be ciliate. A surface with closely appressed hairs islepidote ; a member bearing ramenta is ramentaceous. The root-hairs demand special notice. Root-hairs are hairs which perform the functions of absorption and attachment ; they are com- monly developed on roots, though not always, for they are absent 14. HAIRS AND EMERGENCES. 47 from the roots of a number of aquatic plants (e.g. Butomus um- bellatus, Caltha palustris, Hippuris, Myribphyllum, Menyanthes, Nymphaea, Lemna) ; they may be developed on the thallus, or the thalloid shoot, in the gametophyte of Liverworts and homo- sporous Vascular Cryptogams ; on the stem, thpugh rarely (e.g. Fio. 31. — Different form of hairs. A Branched compound hair (Verbascum). E b Stinging-hair with basal cushion p ; ?i simple hair (Urtica). C Branched simple hair, seen from the surface ; e epidermis (Matthiola). D Scaly compound hair (Hippophae) ; a seen from the surf ace; b seen in section; c central cell; r radiating cells; s stalk-cell ; e epidermis. E Ramentum (Asplenium) ; b the point of attachment. Corallorhiza, Epipogum, Psilotum), or even on leaves. They are always unicellular, and it is only rarely that they are found PART I.— MORPHOLOGY. [§14 to branch. On roots, at any rate, they are developed in acropetal succession. (6) Emergences. These appendages differ from hairs in that they are developed not only from superficial cells, but from others lying beneath them. The commoner forms of emergences are prickles, (Fig. 32) and warts ; more specialised forms are the tentacles of the leaf of Drosera (Figs. 33 and 34) ; the ligule of the leaf of Grasses (Fig. 19 A), Selaginella, and Isoetes ; and the corona of Nar- cissus. The more highly developed emergences (e.g. many prickles, tentacles of Drosera) of Vascular Plants frequently contain vascular tissue. A remarkable kind of emergence is the organ of attachment, termed a hapteron, developed on the stalks of some Algae (e.g. Laminaria), on the stems and branches of Podostemacese and on the tendrils of some Ampelidese and Bigno- niaceae among Phanerogams : it contains no vascular tissue even in Vascular Plants. The suckers, or haustoria, of parasitic plants (e.g. Cuscuta, Orobanche, Thesium, Rhinanthus, etc.), are also emergences, being FIG. 32.— Prickles on the stem of the Rose (nat. size). Fio. 33.-Leaf of Drotera rotundifoha. A Expanded ; d the mdular tentacles of the edge of the leaf ; m the short ten- tacles in the middle. B The marginal tentacles have bent towards the middle at the touch of an insect, x. FIG. 34.— Tentacle of Drosera rotundifolia. (Af- ter Strasburger : x 60.) § 15. REPRODUCTION. 49 developed from the cortical tissue of the root or stem bearing them. Those of Rhinanthus, Thesium and Orobanche, are de- veloped exogenously ; those of Cuscuta, endogenously. They con- tain vascular tissue. § 15. Reproduction. Reproduction consists essentially in the development of one or more new organisms from the whole or from a part of the protoplasm of a parent organism. This may be effected either by the separation of a member or a portion of the body, which, by developing the missing members, constitutes a new individual ; or by the production of special re- productive cells termed spores. Two modes of reproduction are therefore distinguishable: vegetative multiplication, and spore- reproduction. 1. Vegetative Multiplication is essentially connected with the process of growth. The simplest modes of this occur in unicellular plants. In Pleurococcus, for instance, the cell divides into two, each of which constitutes a new organism. In this case the parent ceases to exist as an individual. In Yeast, the cell produces out-growths each of which becomes an independent unicellular organism. In this case the number of the progeny is not limited, and the parent organism persists. This is termed multiplication by gemmation. In more complex plants vegetative reproduction is commonly effected in this way, that the main axis of the shoot or of the thallus, dies away : the branches thus become isolated and consti- tute independent organisms. This occurs very commonly in the protonema of Mosses, in the rhizomes of many Phanerogams, etc. In those cases in which the leaves produce adventitious buds (e.g. JBryophyllum calycinum, many Ferns), the adventitious buds de- velope into independent plants after the leaf has fallen from the plant bearing it. In many plants special organs for vegetative multiplication are produced, which may be generally termed gemma?. In a few cases the gemmae are unicellular, as those of some Algae, Fungi, and Hepaticae. In other plants, multicellular gemmae are produced : as in other Hepaticae where they are developed in special receptacles (cupules) on the upper surface of the thallus (Lunularia, Marchantia), or on the margin of the leaves. In some Mosses flattened gemmae are produced in receptacles formed of leaves at the apex of the shoot, as in Tetrapliis pellucida, and Aulacomnion androgynum : and 50 PART I.— MORPHOLOGY. [§ 15 rounded tuberous gemmae are frequently formed on the protonema. The prothallia of some Ferns (Trichomanes) are reproduced by few- celled filamentous gemmae ; and that of Lycopodium Phlegmaria by ovoid tuberous gemmae. In the Vascular Cryptogams and Phanerogams, vegetative reproduction is generally effected by buds, the leaves or stem of which have become swollen, serving as depositories for nutrient substances. These buds may be subterranean, and then they are of considerable size, when they are termed bulbs or corms according to their structure (see p. 25) : or the buds may be borne on a swollen subterranean stem (e.g. potato-tuber) ; or be associated with tuberous roots (e.g. many terrestrial Orchids). Sometimes they are aerial, being borne on the stem ; on ac- count of their small size they are termed bulbils (e.g. Lilium bul- biferum, Dentaria, Nephrolepis tuberosa, etc.). In Psilotum, however, vegetative propagation is effected by small flattened gemmae, oval in shape, and consisting of a few large cells forming a single layer. 2. Spore- Reproduction. The term spore is applied to a specialised asexual reproductive cell which is capable, by itself, of giving rise to a new organism. There are two principal modes of origin of spores, and all plants produce spores in one or other of these modes. In the one, the spores are formed from the protoplasm of any part, or of some special part, of an organism ; in the other, they are formed by the fusion of two masses of protoplasm derived either from two dis- tinct organisms, or from distinct parts of the same organism. In the former case they are said to be formed asexually ; in the latter, they are formed sexually, the fusion of the two masses of proto- plasm being a sexual process (p. 2) ; the organs concerned are distinguished respectively as asexual and sexual, and are in all cases confined to the shoot. The spore is generally a single cell, consisting of a nucleated mass of protoplasm containing various nutritive substances (oil, starch, etc.). It generally has a cell-wall, which is commonly thick, and in some cases consists of two layers, an outer, the exine (or exospore], which is cuticularised, and an inner, the inline (or endospore), which is delicate and consists of cellulose. In some cases the spore has no cell-wall. It may then be cap- able of spontaneous movement. When motile, it usually swims by means of one, two, four, or many delicate protoplasmic filaments § 16. ASEXUAL REPRODUCTIVE ORGANS. 51 termed cilia. Motile spores are termed zoosporcs. Ciliated zoo- spores are common among the Algae, and they occur in some Fungi. The spores produced asexually by the sporophyte of any one plant are commonly of one kind only ; when this is the case the plant is said to be homosporous. But in some of the Pteridophyta, and in all Phanerogams, which are therefore said to be heterospor- ous, there are two kinds of asexually produced spores, which differ in size and in the nature of the organisms to which they respec- tively give rise, and are distinguished as microspores and macro- spores. In the Phanerogams, the microspores are commonly termed pollen-grains ; and the macrospores, embryo-sacs. § 16. General Morphology of the Asexual Reproduc- tive Organs. In the great majority of plants the asexual pro- duction of spores takes place in the interior of an organ termed a sporangium. Whilst in some plants the asexual production of spores is not limited to any particular portion of the body, in others it is so limited. When this is the case, the portion of the body which performs this function differs more or less widely in form from the vegetative portions, and it is distinguished as the sporophore. When the body is differentiated into root and shoot, the sporophore is always part of the shoot. In those plants in which the shoot is differentiated into stem and leaf, the development of spores is commonly confined to the leaves. A leaf bearing one or more sporangia is termed a sporo- phyll. (a) The Sporangium. In unicellular plants (e.g. Yeast, Haema- tococcus) the cell, that is the whole body of the organism, becomes one sporangium. In this case the development of spores closes the life of the organism, for the protoplasm is used in the formation of the spores, and the cell-wall is ruptured to allow of their escape. In simple multicellar plants ( e.g. Ulva, Ulothrix) each cell eventually acts as a sporangium, giving rise to spores. WTith the formation of spores the life of each cell is closed ; so that when all the cells have formed spores the life of the organism is ended. In plants of higher organization the formation of spores is limited to certain cells, so that the formation of spores no longer necessarily puts a term to the life of the organism. It is in these plants that distinct sporangia are to be found. In the Algae and Fungi, the sporangium, when present, usually consists of a single cell. In all plants higher than the Algae and 52 PART I.— MORPHOLOGY. [§ 16 the Fungi, the sporangium is multicellular. It is, however, unilo- CM/ar, that is, it contains but one cavity in which spores are developed, though this is sometimes chambered by incomplete walls (trabeculce) as in Isoetes. In the Bryophyta, where the sporophyte apparently produces only a single sporangium, termed the capsule or theca, this organ constitutes the whole (Riccia) or a considerable portion of the sporophyte. Its structure is simple in Riccia and other Hepaticse, but it becomes highly elaborate in the true Mosses (e.g. Polytri- chum). It must, however, be borne in mind that the theca of the Bryophyta does not correspond to a single sporangium of a Fern or a Phanerogam, but to at least a cluster (sorus} of such sporangia : hence the exceptional complexity of its structure. In the Pteridophyta and the Phanerogams the sporophyte pro- d uces a number of sporangia. In the heterosporous forms there are two kinds of sporangia which respectively produce the two kinds of spores ; those which produce macrospores are termed macrospor- angia ; those which produce microspores, microsporangia. In the Phanerogams the macrosporangium is commonly termed ovule, and the microsporangium pollen-sac. When the shoot of the sporophyte is differentiated into stem and leaf, the sporangia are generally borne on the leaves (sporophylls) : but in some plants they are borne on the stem. This is the case in most Selaginellas, among the Pteridophyta : the macrosporangia (ovules) are borne on the stem in various Phanerogams ; among Gymnosperms, in the Taxese ; among Angiosperms, in the Poly- gonace?e, Chenopodiacese, Amaranthacese, Primulacese, Composite, Graminese, Naiadaceae, Piperacese, and others, the macrosporangia being either terminal or lateral : the microsporangia are less com- monly borne on the stem, but this is the case- in some Angiosperms, such as Naias and Casuarina. The sporangia may be borne singly, or, as is more commonly the case, in groups, each such group being termed a sorus. The spor- angia of a sorus are generally quite distinct from each other ; but in some cases (e.g. most Marattiaceae, Psilotum, Tmesipteris) the sporangia are coherent, forming what appears to be a multilocular sporangium but is really a synangium. In those heterosporous plants in which the sporangia are in sori, the two kinds of sporangia are borne in distinct sori, the only ex- ception to this rule is afforded by the Marsileacese, where each sorus includes both microsporangia and macrosporangia. § 16. ASEXUAL REPRODUCTIVE ORGANS. 53 The mature sporangium of these higher plants is either borne upon a stalk (sometimes termed funicle) ; or it is sessile, and then it is commonly more or less imbedded in the tissue of the parent member, as in the case of the sporangia of the Ophioglossacese, and of the pollen-sacs of most Phanerogams. The development of the sporangium begins, in all cases, at the surface of the parent member. The area which bears the spor- angium, especially when a number of sporangia are developed close together, generally projects more or less as a cushion of tissue to which the term placenta is applied. In the Ferns (except Marat- tiaceae, Ophioglossacese, and Isoetacese) and in the Hydropteridese (Rhizocarps), the sporangium is developed from a single superficial cell : in the rest of the Pteridophyta and in the Phanerogams it is developed from a group of superficial cells, and in some cases from cells of deeper layers as well. These Ferns and the Hydropteridese are hence distinguished as leptosporangiate from the rest of the Vascular Plants which are said to be eusporangiate. The most important morphological feature in the development of the multicellular sporangium is the differentiation of the sporogen- ous tissue, that is, of the mother-cells of the spores. These are derived from a hypodermal cell or group of cells, termed the arclie- sporium, which may be distinguished at an early stage in the de- velopment of the sporangium, by the highly granular protoplasm and the large nucleus. The mother-cells of the spores are usually formed by the division of the archesporial cell or cells, but oc- casionally the archesporial cells themselves become spore-mother- cells. The sporogenous cells, as they develope, become more or less completely invested by a layer of highly granular cells, form- ing a membrane termed the tapetum, which temporarily separates them from the wall of the sporangium ; the tapetum may be derived wholly or in part from the archesporium or from the wall of the sporangium. In most cases the asexually-produced spores are set free from the organism producing them. An exception to this is offered by the macrospore (embryo-sac) of Phanerogams, in which plants the macrospore remains permanently enclosed in the macrosporangium (ovule), and the macrosporangium remains attached for a consider- able time to the plant bearing it. It is on account of this peculi- arity that seeds are produced in Phanerogams. The production of seeds is the characteristic difference between Phanerogams and Cryptogams. The spores are usually set free by the rupture or 54 PART I.— MORPHOLOGY. 1G dehiscence of the sporangium. In some cases the wall of the sporangium simply degenerates ; in other cases there is a special mechanism, sometimes very elaborate, for its dehiscence. In a few cases the whole sporangium falls off from the parent plant, e.g. the microsporangia and macrosporangia of Salviniacese ; here the spores never become free from the wall of the sporangium, but germinate inside it. (b) The Sporophore. Beginning with the lower plants, a well- marked asexual spore-producing organ is to be found in many Fungi, where it represents, in fact, the shoot of the body, and is a specialized, erect-growing branch of the mycelium. It may be simple (e.g. Mucor, Peronospora, Eurotium) or compound (Agaricus). The sporophyte of the Bryophyta affords a good example of a highly specialised sporophore in an organism the shoot of which is not differentiated into stem and leaf. The entire shoot of the sporophyte constitutes the sporophore, which consists (except in Riccia) of a longer or shorter stalk (seta\ bearing a terminal cap- sule (theca) of more or less complex structure. In the majority of the higher plants, in which the shoot of the sporophyte is differentiated into stem and leaf, there are well- marked sporophores (see Fig. 27). The sporophore may be the terminal portion of the primary shoot or of a branch ; or it may be an entire branch. It is commonly known, among Phanerogams, as the inflorescence, but there is no reason for confining the use of this term to this group of plants. The sporophore or inflorescence is characterised by its limited growth in length, and can usually be distinguished from a vegetative shoot by peculiarity of form, and, when they are present, by the nature of its leaves. The distinction of an inflorescence from a vegetative shoot is easy when the former is an entire branch borne latterly on the latter ; but when a monopodial vegetative shoot terminates in an inflorescence, the tran- sition from the one region to the other is so gradual, that it is difficult to determine where the one begins and the other ends. The sporophore may be simple, or it may be branched, and it then affords some of the most striking examples of the various branch-systems (see p. 18). When the branch-system is such that there is a well-defined main axis, this is termed the rhachis, of the inflorescence. The rhachis and the branches of the inflorescence are commonly elongated and cylindrical, or flattened, or prismatic in form ; but they are in many cases dilated at the apex into a flattened, depressed or conical receptacle, as in the Composite, etc. § 16. ASEXUAL REPRODUCTIVE ORGANS 55 The sporophore may be destitute of leaves (e.g. Salisburia adian- tifolia). When it bears leaves they usually differ more or less widely in form, colour, and structure from the foliage-leaves of the plant. Of these leaves there are two kinds : those which bear sporangia, hence termed sporophylls ; those which do not bear spo- rangia, termed hypsophylls (see p. 43). When no sporophylls are present, the sporangia are borne directly by the rhachis or the branches of the sporophore, at or near the apex, in a cluster if there are several. When sporo- phylls are present, they are also usually collected together at the apex of the rhachis or of a branch, in consequence of the short- ness of the terminal internodes. Any axis of the sporophore, bear- ing one or more sporangia or sporophylls, is termed & flower (p. 25). When hyposophylls are present, some of them are commonly aggregated round the sporangia or the sporophylls, as in most Angiosperms, constituting what is termed the perianth of the flower. When the rhachis is unbranched, it bears a single terminal flower (e.g. Equisetum, Violet) ; when it branches, each axis, of whatever order, terminates in a flower. It is on this account that the growth of the axes of inflorescences is limited. It occa- sionally happens, as a monstrosity, that the axis grows through the flower and produces foliage-leaves ; this is termed prolifera- tion. When the rhachis bears a single terminal flower it is commonly termed the peduncle of the flower ; when the rhachis is branched, the branches may be so short that their flowers appear to spring directly from the rhachis and the flowers are said to be sessile; when the branches are longer and bear terminal flowers, they are termed pedicels, and the flowers are said to be pedicillate. For further details as to inflorescences, see Part IV. When no sporophylls are present, the form of the flower is ex- tremely simple. When sporophylls are present, the form of the flower varies with the degree of elongation attained by the termi- nal internodes of the axis. When they elongate to some extent, the flower forms a cone, as in Equisetum, Lycopodium, Selaginella, Pinus. When they remain short, as generally in Angiosperms, the apex is more or less broadened, forming a flattened, depressed, or shortly conical torus on which the sporophylls and the perianth- leaves are borne. The various forms of flowers are described in detail in Part IV. 56 PART I. — MORPHOLOGY. In heterosporous plants it is commonly the case that the two kinds of sporangia are borne together on the same axis of the sporophore ; that is, they are included in the same flower (e.g. Sela- ginella, most Angiosperms), but they are frequently confined to distinct axes, as in the Gymnosperms, and in some Angiosperms (e.g. Beech, Birch, Oak, Walnut, etc.) ; these distinct flowers are distinguished, according to the kind of sporangia which they respectively bear, as micro sporangiate or macrosporangiate ; in some cases one individual bears exclusively microsporangiate, and another exclusively macrosporangiate flowers, as in the Hemp, the Yew, etc. (c) The Sporophylls. In many cases, most Ferns and Lycopo- dinae, for example, the sporophylls are similar to the foliage- leaves, differing only in that they bear sporangia ; but more com- monly the sporophylls are distinguished by some peculiarity in form or structure. Thus in the Flowering Fern (Osmunda rcgalis) the sporophylls differ from the foliage-leaves in that no green leaf-tissue is developed in them; and in the Phanerogams they differ widely from the foliage-leaves. The distribution of the sporangia among the sporophylls in heterosporous plants is an important point. In the Hydropteridese (Rhizocarpse), both the microsporangia and the macrosporangia are borne by the same sporophyll ; but in all other heterosporous plants they are borne by distinct sporophylls, which may be dis- tinguished respectively as microsporophylls and macro sporophylls. In the Phanerogams the microsporophyll is termed a stamen ; the macrosporophyll, a carpel ; but there is no reason for applying special terms to this group of plants. In heterosporous plants, both kinds of sporophylls are gener- ally present in one and the same flower : when, however, the flower includes only microsporophylls, it is termed microsporophyl- lary or staminate ; and when it includes only macrosporophylls, it is termed macrosporophyllary or carpcllanj. In some cases the sporangia are borne, not upon, but in close relation with, a leaf, which is nevertheless regarded as a sporophyll. Thus, in Selaginella, the sporangium is in the axil of the sporophyll. Again, the leaves which invest the macrosporangia (ovules) of Polygonacea?, Primulaceae, etc., are termed carpels, though they do not actually bear the sporangia. The distribution of the sporangia on the sporophyll is various. They may be borne exclusively on the under (dorsal) surface, § 16. ASEXUAL REPRODUCTIVE ORGANS. 57 as in most Ferns, Equisetum, and Gymnosperms (pollen-sacs) ; or exclusively on the upper (ventral) surface as in the Lyco- podirise, Marsileacese, macrosporangia of Coniferse and of some Angiosperms (e.g. Butomus) ; or on both surfaces, as in Osmunda ; or on the lateral margins, as in Ophioglossum ' and the Hymeno- phyllacese, and in many Angiosperms (e.g. Leguminosse, Violacese, Liliacese) ; or on the apices of segments of the sporophyll, as in the Salviniacese. The number of the sporangia borne by a sporophyll also varies widely. In some cases there is only one, as in Selaginella, Lyco- podium, Isoetes ; or two, as in most Coniferse ; or four, as in most Angiosperms (microsporangia) ; or many, as in the Filicinse. In most cases the sporangia are free on the surface of the sporophyll ; but in some cases they are enclosed in a cavity formed either by the infolding and junction of the margins of the sporo- phyll, or by the junction of the margins of adjacent sporophylls. The sporangia of the Marsileaceae are thus enclosed by the sporo- phyll, as are also the macrosporangia of all Angiosperms. In the latter group the resulting structure is termed the ovary. (b) The Hypsophylls (Fig. 27, p. 43). Under this common term are included bracts and perianth-leaves. Bract. This term is generally applicable to the leaves, other than the sporophylls and perianth-leaves, which are borne by the rhachis or branches of the inflorescence : those which are borne on the pedicels of individual flowers are, however, distinguished as bractcoles or prophylla. The bract is frequently not distinguishable from a foliage-leaf ; but it may be reduced to a scaly leaf ; or it may be very large and even highly coloured, when it is said to be petaloid. An example of the occurrence of bracts in the Pteridophyta is afforded by Equisetum, where there is a whorl of small bracts, forming what is known as the ring, just below the cone or flower. In some Monocotyledons (e.g. Palms, Arums, etc.) there is a large bract, termed a spathe, which invests the whole inflorescence ; it is usually not green in colour, as in the Trumpet Lily (Zantedeschia cethiopica) where it is white. In some cases the bracts are arranged in whorls round the inflorescence (e.g. Composite) forming an involucre. The bracteoles sometimes form an investment, termed an epicalyx, to the flower (e.g. Malva, Camellia, etc.). The Perianth-leaves are leaves developed in immediate relation 58 PART I. — MORPHOLOGY. [§ 17 with the sporophylls, or with the sporangiferous axis, of a flower, to which they form an investment termed the perianth. A perianth is present only in Phanerogams, and is confined almost exclusively to the Angiosperms: the Grnetacese are the only Gymnosperms in which it is represented. The leaves may be arranged in a single whorl, or in two or more : they may be all alike, either green and inconspicuous, or of other bright colours and conspicuous ; but most frequently the leaves of the outer whorl (sepals constituting the calyx] are small and green in colour, being especially protec- tive in function, whilst those of the inner whorl (petals consti- tuting the corolla) are large and brightly coloured, being especially attractive in function. (For further details, see The Flower, Part IV.) § 17. General Morphology of the Sexual Reproductive Organs. The general morphology of the sexual reproductive organs agree in many respects with that of the asexual reproduc- tive organs. In the great majority of plants the sexual reproductive organs give rise to sexual reproductive cells, termed gametes (p. 2) ; hence the organs may be generally termed gamctangia. In some cases the formation of gametangia is limited to a certain portion of the body of the gametophyte which differs more or less from the vegetative portions of the shoot and may be distinguished as a gameiophore. When a part of the body is differentiated as a shoot, the gametophore is part (or the whole) of the shoot. (a) The Gametes. A gamete is a sexual reproductive cell — a reproductive cell, that is, which is incapable by itself of giving rise to a new organism ; in this respect it differs from a spore. A spore is, however, formed from the fusion of two gametes of different sexes : that is, by a sexual process (see p. 50). In those of the lower Algae and Fungi in which sexual spore- formation takes place, the gametes produced by the organism are all externally similar ; hence these plants are termed isogamous ; the sexual process, which consists here in the fusion of two similar gametes, is termed conjugation ; and the spore formed by conjuga- tion is termed a zygospore. In all the higher plants, hence termed heterogamous, the gametes are not all alike ; but there are two kinds, the male and the female. The male and female gametes may be generally distinguished by their difference in size, the male being the smaller, and by the greater activity of the male gamete in connection with the sexual § 17. SEXUAL REPRODUCTIVE ORGANS. 59 process which is here termed fertilisation, the male gamete being considered to fertilise the female ; product, an oospore. The gametes of isogamous plants, in those cases in which they are set free from the gametangium and are free-swimming, are well defined, ciliated, somewhat pear-shaped masses of protoplasm destitute of a cell-wall (e.g. Botrydium, Ulothrix, Ectocarpus, etc.), and are distinguished as planogametes. When, however, they are not free-swimming (as in the Conjugate Algae) they have no defined form nor are they ciliated. The gametes of heterogamoiis plants. The male gamete, when the conditions are such that it must of necessity be free-swimming, is generally a well-defined ciliated mass of protoplasm, termed a spermatozoid. Spermatozoids occur in the heterogamous Green and Brown Algse (e.g. Vaucheria. Volvox. Sphseroplea, (Edogonium, Chara, Fucus), in the Bryophyta, in the Pteridophyta, and in a few Gymnosperms. In the lower forms the spermatozoid is more or less rounded or pear-shaped, somewhat resembling a planogamete of the isogamous forms : but in the higher it is club-shaped or fila- mentous, thicker at the posterior end, pointed at the anterior end where the two or more cilia are borne, and more or less spirally coiled. It has no cell-wall. When, owing to the proximity of the male and female organs at the time of fertilisation, the male gamete has no considerable dis- tance to traverse (e.g. most Phanerogams), it is not differentiated as a spermatozoid, but is simply an amorphous cell without a cell-wall. The female gamete, or oosphere, is not ciliated, nor is it, as a rule, set free, but remains in the female organ until after fertilisa- tion : but in some Algse (e.g. Fucus), the oosphere is extruded from the female organ before fertilisation. It is, generally speaking, spherical in form, as its name denotes. It has no cell-wall. The gametes are developed from one or more mother-cells in the gametangium. In isogamous plants, as a rule, each mother-cell gives rise to more than one gamete, and commonly to a considerable number (e.g. Botrydium, Ulothrix) ; but in Ectocarpus and some other Phseosporous Algse, each mother-cell produces but a single gamete. Whilst in the higher heterogamous plants the male gametes are each developed singly from a mother-cell, in the lower it is the rule that the male gametes are produced several together from one mother-cell. The female gametes are developed singly in the mother-cell, except in the Saprolegniacese among Fungi, and in some genera of Fucacese (Pelvetia, Ozothallia or Ascophyllurn, 60 PART I.— MORPHOLOGY. [§ 17 Fucus), in which from two to eight (Fucaceae) or up to twenty (Saprolegniacese) oospheres are produced from one mother-cell. (b) The Gametangia. The general morphology of the gainetan- gia is very much the same as that of the sporangia. With regard to the terminology employed in designating these organs, they are said to be male when they contain protoplasm which is capable of effecting fertilisation ; and female, when they contain protoplasm capable of being fertilised. When there is no external indication of the physiological nature of the organ, it is simply termed a gametangium. But when the male and female organs respectively are clearly differentiated, special names are given to them in order to indicate peculiarities in their structure or function, or the group of plants to which they belong. In the first place a distinction must be drawn, in the case of these differ- entiated gametangia, between those which give rise to clearly differentiated gametes, and those the protoplasm of which does not undergo such differentiation. To the former category belongs the male organ, termed antheridium, in which spermatozoids are developed, and the female organs, termed oogonium or archegon- ium, in which one or more oospheres are differentiated. To the latter category belong the male organ termed pollinodium (e.g. in Peronosporaceae and some Ascomycetes), and the female organs termed pi-ocarp (Florideae) or archicarp (Ascomycetous Fungi). In the lowest plants in which the sexual formation of spores takes place, the whole cell, when the organism is unicellular, or any cell, when the organism is multicellular, becomes a game- tangium, without being specially modified for the purpose. This is the case, not only in isogamous plants (e.g. Pandorina, Ulothrix, Conjugate?), but in some heterogamous plants (e.g. Sphseroplea) in which the gametes are perfectly differentiated into spermato- zoids and oospheres. In plants of higher organisation there are specialised game- tangia. In the simpler forms of these the male and female gametangia are externally similar, as in the Volvocaceae, Ecto- carpus, and Cutleria, among the Algae, and in the Zygonrycetes and some Ascomycetes (e.g. Eremascus) among the Fungi. In the more complex forms, the male and female gametangia are dis- similar. The undifferentiated gametangia are generally unicellular and unilocular; but they are multicellular and rnultilocular in some Phaeosporous Algae (e.g. Ectocarpus, Cutleria). § 18. THE FRUIT. 61 The differentiated gametangia are of various structure. The antheridium is unicellular in most of the lower plants (Green Algte, except Characese ; Fucacese). In all the other cases it is multicellular, and of simple structure, except in the Characese, where the structure is extremely complex. In some cases (Rhizo- carps) the antheridium consists almost entirely of the mother- cells of the spermatozoids ; in most cases the mother-cells are sur- rounded by a parietal layer of cells. The pollinodium is generally unicellular. The oogonium is unicellular. The archegonium is generally multicellular, consisting of a cellular wall investing the oosphere, usually prolonged into a tubular neck ; but in most of the higher plants, the archegonium is reduced to a single cell, the oosphere. The archicarps and procarps are unicellular in some cases, multi- cellular in others ; in most cases the organ is prolonged into a filament, the trichogyne, by means of which fertilisation is effected. The oogonia (except those of Peronosporacese, Saprolegniacese, and Characeae) and the archegonia, open, so that their contents are in direct relation with the surrounding medium ; in the procarps and archicarps this is not the case. Further details are given in Part IV. in connexion with the plants to which the various organs belong. (c) TJie distribution of the Sexual Organs. The male and female organs are either borne by the same gametophyte, or they are borne by distinct male or female gametophytes ; in the former case the organism is said to be monoecious, in the latter dioecious. When in monoecious plants the male and female organs are both present in the same sorus, as in some Algae (e.g. Fucus platy carpus, Halidrys and other monoecious Fucacese) and in some Mosses, the sorus is said to be bisexual or hermaphrodite, and the plant is said to be monoclinous ; when they are borne in different sori on the same plant (e.g. in Hepaticse generally, some Mosses), the sorus is said to be unisexual, male or female as the case may be, and the plant diclinous. These terms are also applied to the flowers of Phanerogams. §18. The Fruit. Although the forms of fruit occurring among plants are so various in their form and in their structure, it is possible to include them all in a single definition. A fruit is the product of a process of growth initiated as a consequence of a sexual act in structures which are not themselves immediately concerned in the sexual act. 62 PART I.— MORPHOLOGY. [§ 19 To begin with instances among the lower plants ; in most of the Red Algse and Ascomycetous Fungi, the effect of the fertilisation of the female organ is not merely that the female organ gives rise to sporangia ; but the adjacent vegetative tissues are stimulated to growth, forming an investment to the structures developed directly from the fertilised female organ, the whole constituting a fruit, known in the one case as a cystocarp, in the other as an ascocarp. Similarly, in the Bryophyta, and to a less extent in the Pterido- phyta, the effect of the fertilisation of the oosphere is not merely to cause the formation of an oospore and the development of an embryo, but the wall of the archegonium is stimulated to fresh growth and forms an investment, the calyptra, which encloses the embryo ; sporophyte for a longer or shorter period, the whole constituting at this stage a fruit. The most remarkable instances of fruit-formation are, however, to be found in the Phanerogams. Here, as a result of the fertilisation of the oosphere, various parts of the flower are stimulated to growth ; most commonly it is only the macrosporophylls (carpels) which are so affected, but the stimulating influence may extend to the perianth-leaves or to the axis of the flower, the resulting tissues being either hard and Woody, or soft and succulent (see Part IV., under Phanerogams). The peculiar feature of the fruit of these plants, as contrasted with those of the lower plants, is that here the tissues affected all belong to the sporophyte, whereas in the lower plants they belong to the gametophyte ; this is the necessary result of the peculiar relation of the female gametophyte to the sporophyte which obtains in the Phanerogams (see p. 3). § 19. The Seed. As this is a structure which is peculiar to Phanerogams (p. 53), its morphology is discussed in connection with that group (see Part IV.). PART II. ANATOMY AND HISTOLOGY. § 20. Introductory. The body of a plant, like that of an animal, consists essentially of living matter termed protoplasm. The body may consist simply of a mass of protoplasm, as the plas- modium of the Myxomycetes ; or it may consist of a mass of proto- plasm invested at the surface by a definite membrane which is not protoplasmic (e.g. Phycomycetous Fungi and Siphonaceous Algse) ; or it may consist of a mass of protoplasm segmented into portions by non-protoplasmic partition-walls. A body of this last type of structure may be conveniently distinguished as septate, from those of the two former types which are unseptate. On examining the protoplasm of any plant, it will be found to contain certain well-defined protoplasmic bodies termed nuclei ; it is, in fact, the case that all protoplasm is nucleated. In an un- septate body, such as those mentioned above, the nuclei, which are very numerous, are scattered irregularly throughout the proto- plasm. In the septate body of certain plants (e.g. higher Fungi ; some Algse, such as Cladophora and Hydrodictyon) the septation of the body and the distribution of the nuclei stand in no direct rela- tion to each other, the protoplasm being segmented into portions each of which includes a number of nuclei ; such a plant-body may be designated as incompletely septate. In the rest of the septate plants, the septation of the protoplasm and the distribution of the nuclei stand in a direct relation to each other, such that each of the portions into which the protoplasm is segmented contains but a single nucleus ; a plant-body of this structure may be described as completely septate. The portions of protoplasm which are delimitated by the septa in the body of a completely septate plant, are, both morphologically and physiologically, units of protoplasm. They are frequently spoken of as ceZ/s, but it is more accurate to reserve this term to the protoplasmic unit together with the wall (cell-icall) by which it is invested. The structure of the body or any part of it can 64 PART II. — ANATOMY AND HISTOLOGY. [§ 20 only be accurately described as cellular when it consists of one or more such cells, that is, when it is either unicellular (e.g. Yeast, Hsematococcus, etc.) or multicellular. The body of an unseptate plant (such as the Phycomycetous Fungi and the Siphonaceous Algae), as also a segment of the body of an incompletely septate plant (such as Cladophora, Hydrodictyon, etc.), is not a single cell, but is an aggregate of protoplasmic units enclosed within a common wall. Such a body, or part of a body, may be conveniently distin- guished as a coenocyte, and the plants in which it occurs may be said to have coenocytic structure. Even in typically cellular plants structures occur which are coenocytic. Thus, in the early stages of its development in the embryo-sac of a Phanerogam, the endosperm is generally unseptate, consisting of a layer of protoplasm with many nuclei scattered through it ; it eventually becomes a cellular tissue by the delimit- ation of the constituent units by means of cell-walls. Again, a " laticiferous cell " of a Euphorbia (and other Phanerogams) is essentially a coenocyte like the body of a Vaucheria or a Botry- dium. On the other hand, there is such a thing as a multinucleate cell. It has been observed, for instance, in old internodal cells of Chara, and in old parenchymatous cells of Lycopodium and of various Phanerogams (e.g. Tradescantia, Taraxacum, Cereus, Solanum, etc.) that, from being uninucleate, they become multinucleate by the direct division or fragmentation of the nucleus. The distinction between a coenocyte and a multinucleate cell would appear to be this : that the former is either multinucleate from the first or becomes so at a very early stage in its develop- ment, whilst the latter becomes multinucleate at a quite late period. There is another kind of structure occurring in cellular plants which has to be distinguished from both the cell and the coenocyte : that is the syncyte. This structure is formed by the fusion of already-formed cells, the cell-walls, when present, being more or less completely absorbed, so that the cavities of the fused cells becomec ontinuous. The commonest case of this occurs in the de- velopment of vessels, where the transverse septa of a longitudinal row of cells are absorbed so that a continuous tube is formed. But even in the fully-developed cellular plant-body it appears to be very frequently the case that the protoplasm in one cell is not absolutely cut off from that of the adjacent cells, but that there is 20. INTRODUCTORY. 65 continuity of the protoplasm] that is, that the protoplasm of one cell is connected with that of the contiguous cells by means of very delicate protoplasmic fibrils which traverse the pits or pores of the intervening cell-walls (Fig. 35). This connection appears, how- ever, to exist from the first development of the cells, and thus differs from the case of the syncyte where the absorption of the intervening cell-walls is a secondary process. The term tissue is generally applied to any continuous aggregate of cells ; but it is essential to define the term more accurately. A true tissue is an aggregate of cells which (1) have a common origin, whether formed simultaneously (e.g. development of endosperm of Phanerogams), or successively, as in the case of a tissue developed from a growing- point ; which (2) are coherent from the first and are governed by a com- mon law of growth ; and which (3) are physiologically in- terdependent and cannot, in fact, exist otherwise than as part of the tissue. The tissue of which the body of a plant consists may be either homo- geneous or hetero- geneous ; that is, the cells may be all alike, constituting therefore but one kind of tissue ; or they may not be all alike, the different kinds of cells being more or less grouped together so as to form different kinds of tissue. A body which consists of different kinds of tissues is said to be histologically differentiated. The structural differences between the various forms of tissue in a histologically differentiated body are essentially connected with the special adaptation of each form of tissue to the performance of some particular function in the economy. It is a remarkable fact that, whilst the cells of the various M.B. F FIG. 35 (highly magnified, after Gardiner).— Continuity of the protoplasm of contiguous cells of the endosperm of a Palm-seed (Bentinckia) : a contracted protoplasm of a cell; b a group of delicate protoplasmic fibrils passing through a pit in the cell- wall. 66 PART II.— ANATOMY AND HISTOLOGY. [§ 21 tissues of a histologically differentiated body present characteristic peculiarities of form, size, and relative arrangement, the most striking distinctive peculiarities are exhibited, not, as in animals, by the protoplasm of the cells, but by the cell-walls in respect of their thickness, their chemical composition and physical properties, and their markings. Inasmuch as the cellular plants are the more numerous, and present greater variety of structure, the following account deals almost exclusively with them. And since the cell is the structural unit of these plants, it will be advantageous to study the cell as such first, and then to proceed to the study of the tissues. CHAPTER I THE CELL § 21. The Structure and Form of the Cell. In a fully developed living cell the following three principal constituents may be distinguished (Fig. 36 B, C and Z>) :— (1) A closed membrane, ihe^ell-wall (7i), consisting generally of a substance termed cellulose. (2) A layer of semi-fluid substance, the jtrotojolasm (p\ lyingjn close contact at all points with the internal surface of the cell-wall ; the protoplasm gives the chemical reactions of proteid. In it lies a nucleus ,(fc), in which one or more smaller bodies, nucleoU (kk) may generally be distinguished. (3) Cavities, one or more, in the protoplasm, termed vacuolcj (s), which are filled with a watery liquid, the cell-sap. The structure of a ccenocyte is essentially the same as that just described, except that several (sometimes very many) nuclei are present. The young cell presents a somewhat different appearance (Fig. 36 A). At this stage the protoplasm occupies the whole cell- cavity. But, in the subsequent development of the cell, the in- crease in bulk of the protoplasm does not keep pace with the superficial growth of the cell-wall. Hence, since thejjrptopjasm must remain in contact with the cell-wall at all pjjinJts, the result is that cavities, the vacuoles, are formed which become filled with cell-sap (Fig. 36 5). The vacuoles, small at first, increase with the growth of the cell, and may fuse together to a greater or less §21] CHAPTER I. — THE CELL. 87 extent owing to the gradual withdrawal of more and more of the protoplasm into the now extensive parietal layer. Cells such as these are examples of the kind of cells which com- pose the succulent parts of plants, such as the cortex of stems and roots, the tissue of leaves, succulent fruits, etc.," in fact the bulk of the actually living tissues of the plant. In the higher plants it is generally the case that a considerable number of the cells of the body eventually lose the whole of their protoplasmic contents, con- taining, in fact, nothing but air or water ; such are cork-cells and vascular wood-cells. Such structures are no- longer living cells, but are merely their skeletons, and are of use only in virtue of the mechanical properties of their cell-walls. On the other hand, there are frequently found in connection with the pro- cesses of repro- duction, what have been termed FIG. 36. — Cells and their structure. A Young cells from tbe ovary of Sj/mphoricarpu* rticemosus (x 300); B cells from an older ovary of the same plant ( x 300) ; C and D from the fruit of the same plant (x 100); 7i cell-wall; p protoplasm; fc nucleus; fefc nncleolus; » vacnole. In C there is a f>injrle large vacuole, the whole of the protoplasm forming the parietal layer. In D there are several vacuoles, and the nucleus lies in a central mass of protoplasm connected with the parietal layer by numerous strands. such as zoospores, gametes, sperma- tozoids, and oospheres (see p. 59), each of which is simply an unit of protoplasm without any cell- wall, though the zoospores event- ually secrete a cell-wall when they come to rest, as do also the oospheres after fertilisation. The size and form of the cell vary widely. While some cells are so small that little more than their outline can be discerned with the help of the strongest magnifying power (about O001 of a 68 PART II.— ANATOMY AND HISTOLOGY. [§ 22 millimeter in diameter), others obtain a considerable size (from 0-1 to 0-5 millim.), so as to be distinguishable even by the naked eye (e.g. in pith of Dahlia, Impatiens, Sambucus). Many grow to a length of several centimetres, as the hairs upon the seed of Gossypium (cotton) ; and if coenocytes be included, such as the laticiferous tubes of the Euphorbiacese, the Siphonaceous Algse, and the Phycomycetous Fungi, very much larger dimensions in length are attained. The Form of such cells as constitute an entire individual, or exist independently, not forming part of a tissue (e.g. spores), is generally spherical, or ovoid, or cylindrical. The different organs of highly organised plants consist of many varieties of cells, and even in the same organ cells lie side by side which are of very different form. The two main types of cells are, first, such as are spheroidal or polyhedral, with nearly equal or slightly differing diameters (Fig. 36), as in pith, juicy fruits, fleshy tubers; and secondly, such as are narrow and greatly elongated (Fig. 72), as in the case of fibres. § 22. The Protoplasm. The protoplasmic contents of a cell present certain clearly differentiated portions. In the first place there is a nucleus ; and there are more or less numerous plastids. These all lie in the general protoplasm of the cell which may be distinguished as the cytoplasm. a. The Cytoplasm is of viscid tenacious consistence, but it is not a fluid. Chemical examination shows that it consists (at least, when dead) of proteid substance ; intimately associated with this are varying quantities of other organic substances, such as fats, and carbohydrates, together with water, and a small proportion of inorganic ash- constituents. As it is the seat of all the nutritive processes of the cell, it must ob- viously contain at different times all the various chemical substances which enter FIG. 37.— Resting nucleus from the young endosperm lnto> or are formed within the Cell. of F,;iai«ria imperial*. b. The Nucleus is always situated in the •bowing the flbrillar net- i =rr * work with iu chromatin- cytop^sm. It consists of various proteid granules, and several i.u- substances. Its structure, when at rest, xTl)!^ ™y ^ generally described as follows. It is bounded at the surface by a membrane which belongs, however, to the cytoplasm. It consists mainly of 22] CHAPTER I. — THE CELL. 69 a semi-fluid clear ground-substance, the nucleohyaloplasm. In the nucleo-hyaloplasm lies a fibrillar network in which are dis- tributed a number of granules of a substance termed chromatin. One or more small granules, termed nucleoli, are to be seen lying in the ground-substance. On treating the nucleus with staining reagents, the fibrillar network becomes stained on account of the absorption of the colouring-matter by the chromatin-granules, as also do the nucleoli. Its^form is most commonly spherical, but it may be lenticular, or elongated, and straight or curved. A formation of a nucleus de novo does not take place under any circumstances ; hence all the nuclei in a plant have been derived by re- peated division from the nucleus of the spore from which the plant was de- veloped. The nucleus divides into not more than two parts, which are similar to each other in all respects. c. The Plastids are differentiated portions of the protoplasm which, like the nucleus, are not formed de novo, but multiply by division. Their form varies widely. Structurally, they seem to consist of a ground- substance, denser at the surface, with imbedded fibrils. The .plastids may either be_colourless, when they are termed leiLcoplastids ; or coloured, when they are termed chromatopliores. The chromatophores are distinguishable as chloroplastids, when they contain the green colouring-matter chlorophyll ; or as chromoplastids when they contain no chlorophyll, but some other colouring-matter. Plastids are not found in theJFungi, nor, ap- parently, in the Cyanophycese among the Algse. The Leucoplastids may be spheroidal, fusiform, or cylindrical in PIG. 38.— Chloroplastids in the cytoplasm of the cells of the prothallium of a Fern. A Optical section of the cells ; B part of a cell seen from the surface- Some of the plastids have begun to divide. ( x 400.) 70 PART II.— ANATOMY AND HISTOLOGY. [§22 shape ; they are especially numerous in the neighbourhood of the nucleus. In parts of plants which, in the ordinary course, eventually become exposed to light, the leucoplastids__deyelope - Into chloroplastids. Conversely, when a part which is normally exposed to light is kept in darkness, the chloroplastids become_ replaced by leucoplastids. The essential function of the leuco- plastids is to form starch-grains. The Chloroplastids or Chlorophyll-bodies, are of various form. The characteristic feature of them is their function, which is two- fold. In the first place, they can, like the leucoplastids, generally produce starch-grains ; in the second place, they are capable, in virtue of the colouring-matter present in them, of constructing organic substance from carbon dioxide and water under the in- FIG. 39.— Gronp of rod-h'ke leuco- plastids, each bearing a pyramidal starch-grain, collected round the nu- cleus in a cell of the pseudo-bulb of an Orchid (Phajus grandifolius). (x860: after Schimper.) FIG. 40. — Isolated chloroplastids with starchy contents from the leaf of Funaria 'kygrometrica (550). a A young corpuscle ; b an older one, V and V have begun to divide ; c d e old corpuscles in which the starchy contents fill almost the whole space ; /and g after maceration in water by which the substance of the corpuscle has been destroyed and only the starchy contents remain. (After Sachs.) fluence of light (see Part III.). Their function is thus not only starch-forming or amyloplastic, but also assimilatory. These two functions may be, and usually are, carried on simultaneously ; hence when, under the influence of light, organic substance is being produced in the chloroplastid, it usually becomes filled with starch-grains, and sometimes to such an extent that the substance of the chloroplastid constitutes but the wall of a vesicle i^Fig. 40). But starch-grains may be formed in a chloroplastid, as in a leucoplastid, in the absence of light ; the organic substance required for the building-up of the starch-grain being not produced 22] CHAPTER I.— THE CELL. 71 in the chloroplastid itselfj but supplied from other parts of the plant. These plastids are termed chloroplastids, because the colouring- matter upon which their assimilatory function depends is most commonly the familiar green colouring-matter, chlorophyll. But th£y_arejioXalways green. In some of the Algae they are red or brown, because in addition to chlorophyll there is present in the one case (Rhodophyceae), a red colouring- matter, phycoerythrin, and in the other (Phseophyceae) a brown colouring-matter, phycoxanthin or phycophcein. These substances are, however, related to chlorophyll. When the colouring-matter is dissolved out by alcohol or some other solvent, the protoplasmic plastid is left colourless, but un- changed in form or size. Thej^hlorpphyll appears to exist in an ojlv__splutipnj and to be con- fined to the fibrillar portions of the plastid, in the form of draplets (#ro2ia). The most common form of chloroplastid — the only one oc- curring in the higher plants — is the chlorophyll -corpuscle (Fig! 40), which is flattened and discoid. Usually, many corpuscles are present in a cell, but occasionally (e.g. Antho- ceros) there is only one. In the Algse the chromatophores, though sometimes small and discoid (e.g. Vaucheria, Fucus, etc.), are more commonly large, occurring singly, and of very various form. The chromatophores of the Algse present a great variety of form. Generally speaking, those of the higher forms are small corpuscles of a more or less discoid form ; while in the lower forms the chromatophores are few in number, often single, in each cell, and are relatively large, assuming commonly the shape of a flattened plate, sometimes elongated and straight or spirally coiled (Fig. 41). In the latter case the large flattened chromatophores present one or more spherical thickenings, each of which is termed a gjtrenoid (Fig. 41), and consists of a homogenous colourless mass of proteid substance. The pyrenoid is generally sur- rounded by a layer of starch-grains : this is, in fact, the only part of the chromatophore in which starch can be detected. FIG. 41.— Spirogyra majuscuZa (after Stras- burger: x2VO). A cell of a filament, showing the nucleus suspended in the centre ; also the spirally-wound chromatophore with py- renoids. 72 PART H. — ANATOMY AND HISTOLOGY. •23 Chromatophores multiply by division into two, effected by median constriction (Figs. 38 B ; 41) : pyrenoids, when present, are multiplied in the same way. The chloroplastids ultimately undergo degeneration, when, as in the case of falling leaves, for instance, all that remains of them is a few yellow granules. In many cases the green colour of parts of plants containing chloroplastids is masked by the presence of other colouring-matters held in solution in the .cell-sap (e.g. the leaves of Amaranthus, Coleus, Copper Beech, Copper Hazel, etc.). The Chromoplastids are generally derivatives of chroi which have undergone a change both in form and colour. They occur most commonly in the cells of yellow floral leaves, such as those of Tropseolum (Fig. 42) : in the super- ficial cells of many fruits of a red or orange colour (e.g. berries of Solanum, fruit of Tomato). The yellow colour of the root of the Carrot is due to the presence of leuco- plastids, in each of which there is a large orange-coloured crystal of carotin. The chloroplastids of many Coniferse (e.g. Biota orientalis) assume a reddish colour at the beginning of winter. Many of the primordial reproductive cells are motile (zoospores, planogametes, spermatozoids), and move by means of cilia. A cilium is a delicate filament of protoplasm which is con- tractile, and by its oscillations serves to propel through the water the body to which it be- longs. The number of cilia borne by these cells varies considerably: there may be a single cilium (e.g. zoospores of Botrydium, and occa- sionally those of Hydrodictyon) : or a pair (generally in planogametes ; frequently in zoospores ; less commonly in spermatozoids, as those of most heterogamous Algae, of the Bryophyta, and of Lycopodium and Selagin- ella): or four (e.a. zoospores of certain green Algae, Ulothrix, Cladophora, Ulva); or many (e.g. all motile cells of (Edogonium ; zoospores of Vaucheria; spermatozoids of Filicinse and Equisetinae). Cilia also occur in free-swimming Algae, such as Volvox, etc. § 23. The Cell- Wai I is a non-protoplasmic membrane_con- sisting, at least at its first formation, of an organic substance termed cellulose, of water, and of a small proportion of Inorganic FIG. 42. — From the upper side of the calyx of Trojxeo- lum majut. The inner wall of an epidermal cell with the chromoplastids. (After Strasburger : x 540.) § 23] CHAPTER I. — THE CELL. 73 mineral constituents. Its growth, as well as its first formation, is the result of the vital activity of the protoplasm ; it is, in fact, formed from and by the protoplasm. 1. 'The Growth of the Cell- Wall. The cell-wall grows in surface and in thickness. a. The growth in surface of the cell-wall may take place in either of two ways, both of which are, however, dependent upon pressure exerted from within upon the wall. In the one case the stretched wall grows continuously by means of material supplied to it by the cytoplasm, the wall remaining unbroken. In the other, the stretched wall is ruptured at certain parts, new portions of cell-wall being at once intercalcated to close the gap. The former is of more common occurrence : the latter has been observed in some Algae, for instance in the growth of the cells of (Edo- gonium, and in connexion with the apical growth and with the development of lateral members in Caulerpa, Cladophora, and Polysiphonia. Growth in surface takes place to such an extent that the volume of the cell not infrequently becomes a hundred-fold greater than it was originally. Thus, for instance, in a leaf still enclosed in a leaf-bud, the cells of which it will consist when fully developed are all actually present, and it is simply by their increase in volume that the leaf attains its full size. In the comparatively rare cases in which the superficial growth of the cell-wall is equal at all points, the cell preserves its original form : but more commonly the cell-wall grows more vigorously at certain points than at others ; thus, for instance, a primarily spheroidal or cuboidal cell may become tubular, cylin- drical, fusiform, stellate, etc. b. The growth in thickness of the cell- wall is effected by the deposition of sue- cessive layers on the internal surface of the first-formed layer. The cell-wall does FIG. 43.— Ripe pollen-gram not usually begin to thicken until after of cichorium, intybus ; the ai- Its growth in surface has ceased, the cell m°8t 8Pherica; 8urf^e of th« , ? cell-wall is furnished with having then attained its definite size ; but ridge-like projections pro- cases of simultaneous growth in surface longed int° spines, and form- — j . , i • i ing a network. (After Sachs.) and iu thickness have been observed. The growth in thickness of the cell-wall is also rarely uniform ; the cell-wall commonly becomes more thickened at some points 74 PART II.— ANATOMY AND HISTOLOGY. [§23 than at others, and thus acquires inequalities of surface. In the case of isolated cells or of free cell-walls, the prominences existing in this way on the external surface appear as warts, tubercles, spines, etc. (Fig. 43). Cells that are united to form tissues have their inequalities on the internal surface of the cell-wall, the prominences sometimes having definite form, and projecting into the interior of the cell ; such are the annular (Fig. 44 r) and spiral thickening (Fig. 44 «) of the walls of certain vessels ; in the so-called reticulated cell-walls, the thickening is in bands which are united into a network, so that circular or oval thin spaces are left. In other cases, isolated and relatively small thin spaces are left in the wall in the course of the growth in thickness, which appear, when seen on the external surface, as bright spots, commonly called pits, and are seen in section to be canals of greater or less length, according to the relative thickness of the walls (Figs. 45, 46). Very frequently the pit, when_seen from the surface, presents the appearance of two concentric circles, or ellipsesj_j.or this reason, that the opening of the .canal into the interior of the cell is .narrow, whereas the external opening is broad (Fig. 48 A}. Such bordered pits occur in the wood-cells of Conifers (Fig. 49), in the walls of many vessels (Fig. 48), and elsewhere. The scalar if orm thickening of the walls of many vessels arises from the regular and close arrangement of bordered pits which are much elongated transversely. The Structure of the Cell-icall. When the cell- wall is at all thickened it presents indications of structure. It presents, in the first place, a layered appearance when ex- amined in longitudinal or transverse action (Fig. 46). This layer- ing or stratification of the cell-wall is readily intelligible when it is remembered that the thickening of the wail is due to the depo- sition of successive layers from within. It presents, secondly, a delicate striation, when examined in surface-view, the lines running at a larger or smaller angle to the long axis of the cell, sometimes even transversely. The planes of striation are commonly different in the different layers constitu- Fia. U.—r Annular, » spiral thickening of the walls of ves- sels ; r seen from outside, s in longitudinal section highly magnified (diagrammatic). §23] CHAPTER I. — THE CELL. FIG. 45.— A cell with pitted walls, from the wood of the Elder (Satn- bucus). A longitudinal section showing the pits in tbe lateral walls as channels, a; and in the farther wall as roundish spots, b. ( x 240.) FIG. 48. — Transverse sec- tion of a bast-cell from the root of DaJiU'a «arinbi!is (x 800); I the cell-cavity ; A' pit- canals which penetrate the stratification ; sp a crack by which an inner system of layers has become separa- ted. (After Sachs.) Fio. 47.— Cells from the endo- sperm of OrnitJwgalum tunbella- tum showing simple pits : m pits seen in surface view ; p closing membrane seen in lon- gitudinal section ; » nucleus. (x2X): after Strasburger.) ting the thickness of the wall, and these seem in the surface-view to cross each other (Fig. 50). The cause of striation appears to be this, that when a considerable area of cell-wall has to be formed, it is deposited by the protoplasm not as one continuous sheet, but FIG. 49.— Oval bordered pits in the wall of a vessel of Helianthus. A In longitudinal section. B As seen from the surface ; t the pit ; h the pit-chamber. (x 600). FIG. 49. — Circular bordered pits on the wood-cells of the Pine. A Seen from the surface. B In section ; s the closing membrane ; m the middle lamella. C An earlier stage, in section. ( x SOO.diagram.) 76 PART II.— ANATOMY AND HISTOLOGY. in the form of delicate spirally-wound bands with their edges in contact. The lines of the striation are the planes of contact of the edges of these spiral bands. A well-marked illustration of the spiral mode of deposition of cell-wall by protoplasm is afforded by the spiral vessels already mentioned (Fig. 44 s). 3. The Chemical Composition of the Cell-ivall. As a rule, the organic constituent of the newly formed cell- wall is cellulose (C6 H]0 05), a carbohydrate, the characteristic reaction of which is that it turns blue when treated with sulphuric acid and iodine, or with a mixture of iodine, iodide of potassium, and chloride of zinc (chlor-zinc-iod). It is, however, commonly the case that when a cell-wall has undergone thickening, some at least of its constituent layers do not consist of cellulose. The chemical changes which are pre- sented by cell- walls may be distinguished as follows : — a. The cell- wall may un- dergo cuticularisation: e.g. walls of epidermal cells or cork-cells, of spores. The cuticularised or corky cell- wall contains a substance termed cutin. It is but slightly permeable to water ; FIG. so.-surface view of the wall of a ceil it is extensible and highly showing striation, from the pith of Dahlia varia- elastic ; it turns yellow btlw. ( x 249: after Strasburger.) / when treated with sul- phuric acid and iodine, or with iodised chloride of zinc. The cuti- cularisation of the cell- wall is most marked in the external layers ; in fact the external layer consists entirely of cutin, whilst the internal layers (of which there may be several, as the cuticularised wall is often much thickened) consist more and more largely of cellulose, the innermost layer consisting frequently of pure cellu- lose, though it is sometimes more or less lignified (cork). This can be shown by treating the cuticularised tissue with strong chromic acid for some time, or by warming it in a mixture of nitric acid and chlorate of potash, when the cutin is removed, and the re- maining tissue gives the characteristic cellulose-reactions. § 23] CHAPTER I. — THE CELL. 77 ft. The cell- wall may undergo lignification; that is, the cell-wall becomes impregnated with a substance termed lignin, which makes it hard and elastic, and though readily permeable to water it cannot absorb or retain much in its substance. The character- istic tests for lignin are, that a cell-wall containing it (a) turns yellow when treated with aniline chloride and hydrochloric acid, and (&) turns pink when treated with phloroglucin and hydro- chloric acid. When a lignified cell-wall is macerated in a mixture of nitric acid and chlorate of potash, or in a strong solution of chromic acid, the lignin is dissolved out and the wall ceases to give the lignin-reactions, and now gives the cellulose-reactions. Lignincation takes place in the sclerenchymatous and tracheal tissues, less commonly in the parenchymatous tissue, of the sporophyte of the Pteridophyta (Vascular Cryptogams) and Phanerogams ; it does not occur in any of the lower plants, nor in any gametophyte. y. The cell-wall may become more or less mucilaginous] in its dry state it is then hard and horny; when moistened, it absorbs a large quantity of water, becoming greatly increased in bulk and gelatinous in consistence ; it usually turns blue when treated with sulphuric acid and iodine, or with iodised chloride of zinc, but in some cases it does not give this reaction ; and in yet others (e.g. asci of Lichens, bast of Lycopodium, endosperm of Peony, and cotyledons of various leguminous seeds) it turns blue with iodine alone. Mucilaginous cell-walls are common in the coats of seeds (e.g. Flax or Linseed, Quince) ; they are very remarkable in the case of the macrospores of Pilularia and Marsilea ; in tissues^ Ihey are^ well seen in the Malvaceae: they occur in all sub-divisions of the vegetable kingdom. In some cases the change goes so far as to result in the con- version of the cell-wall into gum, soluble in water, as in some species of Astragalus and in certain Rosaceous trees (Cherry, Plum, Almond, Peach, etc.). These modifications may occur either singly or together in the different layers of one cell-wall, as in corky, or suberised cell-walls, where cuticularisation and lignification occur simultaneously. S. Mineral matters are also frequently deposited during growth in considerable quantity in the cell-wall, particularly salts of lime and silica ; they are usually intercalated between the solid organic particles of the cell-wall, so that they cannot be directly detected, but remain, after burning, as a skeleton which retains the form of PART II. — ANATOMY AND HISTOLOGY. [§24 the cell. Silica is present in the stems of Grasses and of Equi- setacese. Calcium oxalate sometimes occurs in a crystalline form (Fig. 51.) Calcium carbonate is also frequently deposited in cell- walls, as in certain Algse (e.g. Ace tabu laria, Coralline^ Jama, etc.) ; also in hairs of some of the higher plants (e.g. many Boraginacese) ; but most peculiarly in the cystoliths present in the epidermal cells of the leaves of Ficus clastica, and of the Urticacese and Acan- thacese : it may occur either as granules or as crystals. A cystolith (Fig. 52 A) consists of a basis of cellulose incrusted with calcium carbonate. On treating a section, containing a cystolith, with acid, the calcium carbonate is dissolved with evolution of bubbles of CO2, leaving the cellulose basis (B) which presents both striation and strati- fication. The cellulose basis is, in fact, a local thickening of the cell-wall. Fio.] 61.— Crystals o calcium oxalate in the wall of the bast-cells of Cephalotaxut Fortune*. (x600:afterSolms.) FIG. 62.— A cystolith from the leaf of Celtis Tala ( x 200) . A Normal condition ; c cysto- lith ; e epidermal layer; p palisade-tissue. B The cystolith after treatment with hydro- chloric acid which has dissolved the calcium carbonate, leaving the stratified cellulose basis. § 24. Cell-Contents. The following are the principal cell- contents which are not protoplasmic and are, in fact, not living : they are moreover not universally present in cells, but are con- fined to special cells, and frequently to special plants: starcL- grams ; fatsj proteid grains and crystalloids ; mineral crystals ; the cell-sap, and the substances dissolved in it. a. Mm-cli-grains are small solid granules of various shape- rounded, oval, lenticular, etc.— consisting of starch_with a certain amount of water and a small proportion of incombustible ash. They are specially abundant in those parts of plants which serve § 24] CHAPTER I.— THE CELL. 79 as depositories of reserve-materials, e.g. rhizomes, jmd roots_of perennial plants during the winter, tubers of the potato, seeds . such as those of the cereal and leguminous plants. They canjbe extracted by maceration from the organs in which they occur, and then appear as a white powder which is known as' starch. Starch is a carbohydrate ; its percentage composition is the same as that of cellulose, and may be represented as C6 H10 05, but its molecule is smaller and less complex. It is readily detected by the cha- racteristic blue colour which it assumes on treatment with an aqueous solution of iodine. When boiled with water, or when treated with potash, the grains swell enormously and form a paste. The substance of the starch-grain is always stratified, being disposed in layers round an organic centre, the hilum + this stra- tification, as also in the case of cell-walls, is the result of the deposition of successive layers one on the other. The hilum is the most watery portion of the grain, whilst the external layer is the most dense. It is, as already mentioned (p. 70), the rule that starch-- grains are produced by means of plastids : in parts of plants ex- posed to light, by chloroplastids ; in parts of plants not__ex|)osed to light, by leucoplastids. In the former case the_ grains are usually formed in the interior of the plastid (see Fig. 40) ; in the latter case, on its surface (Fig. 39). It not uncommonly happens th&t^compound starch-grains are to be found. Sjmn'ouxly compound grains are simply grains which have become adherent in consequence of mutual pressure : they occur frequently in the interior of the plastids (see Fig. 40). The truly compound grains (Fig. 53 B — E) are formed in this way, that one plastid produces simultaneously two or more rudimentary starch-grains ; as these increase in size, they eventually come into contact ; the further deposition of starchy layers must necessarily be of such a kind that they surround, not each individual grain, but the aggregate of adjacent grains ; the young grains thus be- come bound together by investing layers, and a grain is produced which has apparently a number of hila. The form of the starch-grains is characteristic in the different plants in which they occur ; thus those of the Potato (Fig. 53) are excentrically oval ; those of leguminous plants (Fig. 55), concen- trically oval ; those of Rye, Wheat, and Barley, lenticular (Fig. 56). The distribution of starch throughout the different classes of 80 PART II. — ANATOMY AND HISTOLOGY. [§24 plants is a matter of considerable interest. Generally speaking, it is confined to plants which possess chloroplastids, though a sub- stance turning blue with iodine has been found to occur, diffused throughout the protoplasm, in certain Schizomycetes (Clostridium butyricum, Sarcina ventriculi, Bacterium pastorianum). But, on the other hand, it is not always present in plants which possess typical chloroplastids ; thus, it is absent, for instance, from__the Onion, species of Vaucheria, etc. In the case of plants which have other colouring-matters besides chlorophyll, starch may be altogether absent (Cyano- phycese, Diatomacese) ; or it may be replaced by some other substance (most Phseo- phycese and Rhodophycese). /?. Fats occur very com- monly in the cells of plants as oily drops scattered throughout the cytoplasm. They are more particularly abundant in seeds, in many of which oil is the form in which the non-nitrogenous reserve material is deposited (e.g. Palm, Castor-Oil plant, Rape, Flax, etc.) ; it is also present in some fniits (e.g. Olive). y. Proteid Grains, or Ale- juron, are granules of various sizes, oval or spherical in form, which occur in seeds, and are of physiological im- portance in that they are the source from which the em- bryo is supplied with nitro- genous food when the seed germinates. They consIsT'oTlTmTxture of proteid substances belonging to the globulins and the albumoses. They present no indications of structure, and are much larger in oily_than in starchy seeds. The proteid grain generally contains a mass of mineral matter. Most commonly this is a rounded body, the globoid (Fig. 54), con- FIG. 63.— Excentric starch-brains from the tnber of a Potato ( x 800). A A fully developed simple grain, B-E Compound grains ; a f> young simple grains ; c young compound grain. (After Sachs.) 24] CHAPTER I.— THE CELL. sisting of double phosphate of lime and magnesia ; lessjrequently there is a crystal, or a cluster of crystals, of calcium oxalate. In the large grains of oily seeds it is frequently the case that a portion of the proteid (globulin) of the grain crystallises out, con- stituting the crystalloid ; there are occasionally two or more crystalloids in the grain (Fig. 54). 8. Mineral Crystals are frequently found in the cells of plants. They sometimes consist, but in comparatively few cases, of calcium carbonate ; for example, the crystals in the protoplasm of Myxo- mycetes, and the crystalline masses occurring in the cells of the FIG. 64. — Cells from the endosperm of Bicinii* communia (x 800) : A fresh, in thick glycerine ; B in dilute gly- cerine ; C warmed in glycerine ; D after treatment with alcohol and iodine ; the grains having been destroyed by sul- phuric acid, the cytoplasm remaining behind as a net-work. In the grains the globoid may be recognised, and in B C the crystalloid. (After Sachs.) Fie. 55.— Cells of a very thin section through a cotyledon of the embryo in a ripe seed of Pisum sativum ; the large concentrically strati- fied grains St are starch-grains (cut through) ; the small granules a are aleuron, consisting of proteids ; i the intercellular spaces. (After Sachs.) pericarp and testa of some plants (e.g. Celtis australis, Litlio- spcrmwn officinale, Cerinthe glabra). In all other cases the crystals consist of calcium oxalate, which crystallises in two systems according to the proportion of water which it contains ; to the one system, the quadratic, belong the octahedra (Fig. 57 fc) ; to the other, the clinorhombic, belong the acicular crystals, distinguished as raphides, which occur in bundles in the cells of Monocotyledons more especially (Fig. 58), and are generally associated with mucilage in the cell. M.B. G 82 FART II.— ANATOMY AND HISTOLOGY. [§ 24 It sometimes happens that the crystal, or group of crystals, be- comes surrounded by a layer of cellulose attached to the wall at one or more points (e.g. „ _^^ leaf of Citrus vulgaris, ^<^=?^c^~I^:-) pith of Kerria japonica). J^rf^^TrTTTY^ )P c. The Cell-Sap satur- -t ates ~tEe celPwall, the protoplasm, in fact the whole organised struc- ture of the cell ; it also fills the vacuole, when present, in the cyto- plasm. It is a watery solution ^f~the~most va- rious substances. In all cases it holds salts in solution, consisting mainly of alkalineTblises in combination either with inorganic acids, such as the nitric, phos- phoric, or .._snlpliuric1_ or with organic acids, such as malic (e.g. apple and other fruits), citric (lemon, etc.), and others. It frequently contains tannin, and nitrogenous substances j, such as asparagin. It very commonly ) so as to constrict the cytoplasm somewhat, and the newly-formed walls become attached to these projections. In all cases the protoplasm in each of the four special mother-cells PART II. — ANATOMY AND HISTOLOGY. [§25 surrounds itself with a proper wall which becomes the coat of the pollen-grain or of the spore. b. The number of the nuclei derived by repeated division from the nucleus of the mother-cell before any cell-wall is formed is indefinite. Each of them becomes surrounded by a portion of the cytoplasm. FIG. 63.— Zoosporangia of 'an Achlja ( x 560) : A still closed ; B allowing the zoospores to escape, beneath it a lateral shoot c ; a the zoospores just escaped ; b the abandoned membranes of the zoospores which have already swarmed ; e swarming zoospores. (After Sachs.) FIG. 64.— Cell-formation in the asei of P«irn convorula ; af successive steps in the develop- ment of the asci and spores: sh mycelium. (After Sachs : x 550.) It is in this way that the zocspores of many Algse and Fungi are formed (Fig. 63), and it is usually not until some time after their escape from the mother-cell that they become clothed with a cell- wall. The spores formed in the asci and sporangia of Fungi (Fig. §25] CHAPTER I. — THE CELL. 87 64) are also developed in this way, but in this case the cells are always invested by a cell- wall before they are set free from the mother-cell. A further example of this is to be found in the development of the endosperm-cells in the embryo-sacs of phanero- gamous plants. This mode of cell-formation is" known &sfree cell- formaiion. II. Rejuvenescence. The whole protoplasm of the mother-cell may undergo rejuvenescence, when it contracts and reconstitutes itself as the new protoplasmic body of a daughter-cell, which usually does not surround itself with a new membrane for some time. It is in this manner that the single zoospores of many Algse are formed, as in Vaucheria, Stigeoclonium, (Edogonium (Fig. 62), as well as isolated sexual cells such as oospheres. Fio. 05.— Conjugation of the cells of Spirogyra (x 400). A The cells of two filaments which are prepared for conjugation. At a the filaments have begun to swell towards each other. The spiral bands of chlorophyll are recognisable at cl, and the nucleus at K. At B the protoplasm of the cell p is fusing with that of the other p'. At C is a perfectly-formed zygospore Z. III. Conjugation. In conjugation the protoplasmic contents of two or more cells coalesce to form a new cell, which acquires a membrane. This process occurs in a typical manner in various groups of Algae, e.g., Spirogyra (Fig. 65), and of Fungi. The formation of new cells does not therefore necessarily imply an increase in number ; this is the case only when division into two, four, or many cells occurs ; in the process of rejuvenescence the number is unaltered, and in conjugation it is actually diminished. PART II. — ANATOMY AND HISTOLOGY. [§26 CHAPTER II. THE TISSUES. § 26. The Connexion of the Cells. According to their arrangement in space, the following combinations of cells may be distinguished : A. Filaments, where the cells are connected only by their con- tiguous ends, and so form a filament, e.g., many Algae, as Spirogyra (Fig. 65), (Edogonium (Fig. 62), and many hairs. B. Surfaces, when the cells form a single layer and are in con- tact in two directions of space (length and breadth), e.g. many Algae and the leaves of many Mosses. C. Masses, when the cells are in contact on all sides. The tissues commonly consist of cells which have originated from common mother-cells by their repeated division into two, and which have been connected from the first in consequence of the mode of formation of the septa (Fig. 60). In a few special cases tissues are formed otherwise (spurious tissues) ; either cells which have been hitherto isolated become adherent and then continue their growth in common ; or filaments consisting of rows of cells become inter- woven and exhibit a common growth, without however having be- come adherent in every case (Fig. 64 sK). The Common Wall of cells combined into a tissue is, in the first instance, usually extremely thin and delicate, and appears under the strongest magnifying power as a simple plate (Fig. 60). As it increases in thickness a middle lamella usually becomes visible (Fig. 66), which divides the wall into two parts, one of which apparently belongs to each of the contiguous cells. This middle lamella is nothing more than a specially differentiated part of the wall which belongs to both of the cells in common. Its chemi- cal composition, which is different to that of the remainder of the wall, permits of its solution (in nitric acid and chlorate of potash), FIG. 66.- Transverse section of the cortical cells of Trichomaneg speciosum (x600). Middle lamella (m) ; ti the cell-wall adjoining the lamella; I cell cavity; bordered-pits which meet in adjoining cells ; the pits on each side are divided by the middle lamella. § 27] CHAPTER II.— THE TISSUES. 89 so that the individual cells may be separated. When the common wall of similar cells is pitted, the pits on each side accurately meet (Fig. 66 £) ; if, however, certain cells of a tissue undergo a special modification, as in the vessels, the unequal thickening of the membrane may be confined to one side only of the common wall ; in the case of spiral thickening of the cell-wall this is self-evident. In certain cases the septa between the cavities of adjacent cells become wholly or partly absorbed, as, for instance, occasionally the thin partition between bordered-pits ; the transverse walls of such cells as combine to form the vessels are wholly absorbed, if they lie at right angles to the long axis of the vessel (Fig. 73 C a V) ; if they lie obliquely, they are broken through in various ways. In a similar manner the transverse septa (and more rarely isolated areas on the longitudinal wall also) of the sieve-tubes (§ 28, Fig. 74 B) are perforated by closely-set and very fine open pits, and are then known as sieve-plates. Hence a vessel is a syncyte (see p. 64). § 27. Intercellular Spaces are lacunae between the cells of a tissue. They may be formed in two ways : either by a splitting of the common wall of adjacent cells, that is schizogenously ; or by the disorganization of certain cells, that is lysigenously. They contain either air or certain peculiar substances. The intercellular spaces which contain air are usually formed schizogenously. They occur almost exclusively in parenchymatous tissue, at the angles of junction of a number of cells (Fig. 67 z)> Sometimes these spaces — then called air-chambers — at- tain a considerable size, so that whole masses of tissue are separated from each other, as in the petioles of the Water Lily and of other aquatic plants. The large cavities in the Stems and FlG- 67.-Intercellular spaces (*) between cells from the leaves of Rushes and stem of Zea Mais ( x ;6£0) ; gto the common wall. (After of other allied plants, are produced lysigenously by the drying-up and [rupture of con- siderable masses of cells : this is true also with reference to the 90 PART II.— ANATOMY AND HISTOLOGY. [§ 28 cavities extending through whole internodes of many herbaceous stems (Grasses, Umbelliferae, Equisetacese), and those occiirring in leaves (Leek). The intercellular spaces which contain certain peculiar sub- stances are treated of under the head of Glandular Tissue in § 28. §28. Forms of Tissue. According to the form and arrange- ment of the constituent cells, the thickness and chemical composi- tion of their walls, the nature of their contents, etc., it is possible to classify the forms of tissue in various ways. Taking, first, the capacity for growth and cell-formation, embryonic tissue or meristem is distinguishable from adult or permanent tissue. The former consists of cells (e.g. in the growing- point of a cellular plant) which grow and divide: whereas the latter consists of cells which have ceased to grow and divide, having attained their definitive form and size ; and whilst meristem consists entirely of true cells, permanent tissue may consist wholly or in part of cells which have lost their protoplasm. Taking, next, the form of the indi- vidual cells and the mode of combination into a tissue which their form determines, two forms of tissue termed parenchyma and proscnchyma are distinguished. In parenchymatous tissue there_Js^ no great difference in the three axesjrf_jjie_sojne- what cubical cells, and the_ £ells_are in contact by broad flat surfaces (Figs. 60, 68). In prosenchymatous tissue, on the other hand (Figs. 68, 70/), the cells are much longer than they are broad, having pointed ends which overlap and dovetail in between those of other cells of tli and in this way a continuous system of laticiferous vessels is formed. This occurs in the greater Celandine (Chelidonium majus}, and in the Banana (Musa) where, however, the latex is not milky. 100 PART II. — ANATOMY AND HISTOLOGY. [§28 More commonly, as in the Cichoriese (e.g. Dandelion, Scorzonera), the Campanulacese, and in most Papaveracese (e.g. Poppy), the cell-fusions take place in all directions, producing a dense network (Fig. 80). Structures apparently of the nature of laticiferous vessels occur in certain Basidiomycetous Fungi (e.g. Lactarius). (e) Laticiferous ccenocytes, commonly known as ''laticiferous cells," occur in some Euphorbiacese (the Spurges), in the Urti- cacese (Nettles), Apocynaceae, and Asclepiadacese. As already ex- plained (p. 64), these " cells " are really coenocytes ; they are visible in the early stages of the development of the embryo, and they grow and branch in the tissue as if they were independent organisms (Fig. 81). As they extend from one end of the plant to the other, they attain a very considerable length in many cases. Their walls are frequently thickened (e.g. Euphorbia), but, like those of the laticiferous vessels, they are not lignified. They contain protoplasm with many nuclei which multiply by division, and in the older parts latex is abundantly present. The latex of the Euphorbiacese contains curious rod-like or dumb-bell- shaped starch-grains. (/) Epidermal Glands. Whilst all the preceding forms of glandular tissue are developed in the internal tissues of plants, somewhat similar glandular structures are developed from the super- ficial layer (epidermis), most commonly in the form of hairs (p. 46), either unicellular or multicellular. When the multicellular hair consists of a single row of cells, the secretion is generally confined either to a large terminal cell, or to several of the distal cells ; in any case the secretion begins with the terminal cell, and extends backwards to other cells towards the base. The gland, though epidermal in origin, does not, however, always pro- ject from the surface, but may be more or less sunk in the internal tissue (e.g. glands in the leaf of the Psoralea hirta}. FIG. 81.— A portion of a lati- ciferous coenocyte dissected out of the leaf of a Euphorbia. ( x 120: after Haberlandt.) § 29] CHAPTER II.— THE TISSUES. 101 The secretum (which, may consist of mucilage, or gum-resin, or ethereal oil, balsam, etc.) is accumulated either in the cavity of the secreting cells (e.g, mucilaginous hairs at the growing-point of Liverwort gametophytes and of Fern-sporophytes), or between the external cuticle and the deeper layers of the cuticularised cell- wall (e.g. mucilaginous hairs [colleters] on the buds of many Phanerogams, resinous hairs gener- ally ; Fig. 82). § 29. General Morphology of the Tissue- Systems. When a form of tissue constitutes a complex which extends continuously throughout FIG. 82. — Gian- the body of a plant, or over considerable areas, dular hair from the J baseofaramentum it is spoken of as a tissue-system : for instance, Of A*pidium mix a laticiferous system; a resin-duct system; a mas (X2):«, «"* , . , ,. secretum, lying be- sieve-tube system; a mechanical tissue-system tween the outer and (stereom) including collenchyma and scleren- inner layers of the chyma ; a glandular tissue-system ; a tegumen- tary tissue-system : or a more elaborate system may be produced by the combination of two or more systems: for instance, the sieve-tube system and the tracheal system taken together con- stitute the vascular tissue-system. These tissue-systems are, however, characterised rather by their function, that is physiologically, than by their development, that is morphologically. From the latter point of view the following primary tissue-systems are distinguished: (1) The Tegumentary Tissue; (2) The Ground-Tissue; (3) The Stele. The study of these tissues will be begun by the consideration of the structure of the growing-point in stems and roots. As already pointed out (p. 8), the growing-point consists, in the higher plants, of embryonic tissue, the cells of which may be of approximately uniform size, constituting a small-celled primary meristeyt ; or there may be at the organic apex a cell conspicuously larger than the rest, the apical cell ; or a group of several larger initial cells. a. Grouping-points consisting of small-celled meristem are, with rare exceptions, to be found in the roots and stems of Phanerogams, as also in the root of Lycopodium and Isoetes among the Pteridophyta. Although the cells are all embryonic, they nevertheless present such a degree of differentiation as to make it possible to distinguish the three primary tissue-systems. 102 PART II.— ANATOMY AND HISTOLOGY. [§ 29 In the stem, a growing-point of this kind usually presents the appearance shown in Fig. 83. It consists, in the first place, of a well-defined superficial layer which, on being traced backwards, is seen to be continuous with the primary tegumentary tissue (epidermis) of the older parts ; this layer is, in fact, the embryonic epidermis or dermatogen (d} ; it is quite distinct, morphologically, from the subjacent cells, and is characterized by the fact that its cells only undergo division in planes perpendicular to the surface (anticlinal), and not in any plane parallel to the surface (periclinal). Consequently, whilst the dermatogen increases in area, so as to keep pace with the growing tissues within, it does not become many-layered, but remains a single layer of cells. In the middle of the growing-point is a solid mass of somewhat elongated cells (pp) con- stituting the plerome, and terminating in one or more initial cells ; on trac- ing this backwards into the older part of the stem it is found to give rise to a single axial cylinder of tissue, the stele, in which the vascular tissue is de- veloped. Such a stem is /*• P said to be monostelic : Fxo. 83.-Medianlongitudinal section of the grow- ing-point of the stem .of Hippuris vvlgaris. The point of this structure growing-point census of a small-celled meristem & ^^ without excep. differentiated into dermatogen a, plerome p p, and ^ " periblem consisting of the five layers of cells between tion, monostelic. the plerome and the dermatogen ; I rudiment of a Between the dermato- leaf. (After de Bary : x225.) gen externally and the plerome internally, is a layer — less frequently several layers — of cells constituting the periblem ; below the apex the cells undergo divisions both anticlinally and periclinally, so that both the area and the number of the layers are increased. On tracing the peri- blem backwards into the older parts, it is found to be continuous with the ground-tissue which, in monostelic members, is known as the primary cortex. The growing-point of the root (Fig. 84) of one of these plants essentially resembles that of the stem in its structure ; the small- celled meristem is differentiated, at least primarily, into dermatogen, §29] CHAPTER II.— THE TISSUES. 103 plerome, and periblein. But there is this distinctive peculiarity about the dermatogen of the root, that its cells undergo division, not anticlinally only, as in the stem, but periclinally also, so that the epidermis of the root is many-layered (except in Hydrocharis FIG. 81. — Median longitudinal section through the growing-point of the root of Hbrdettm vulgare (Barley) : r root- cap ; fc initial cells of the dermatogen of the many-layered epider- mis; d-eculiarform_of stoma is found in some plants, known as a icater- stoin*t_(~FiK.7&). In consist^ of two large, almost sphericalTguard-cells which cannot alter their form so as to close the aperture. Water-stomata -K'cur on the 1, .fives of some of those plants (e.g. Alchemilla, Crasnla, Flcus, Saxifraira, Colocasia, Papaver, Tropreolum) which excrete water in the form of drops; they are situated over the termination" of the vascular 30] CHAPTER II. — THE TISSUES. 109 bundles on the margins or at the apex of the leaf ; when chalk-glands are pivsunt i"p. !)7), water-stomata are developed in connexion with them. ~Tn some plants (e.g. Grasses) which excrete drops of water, the water escapes through fissures in the epidermis of the leaf. The epidermis of the submerged shoots of water-plants differs from that of land-plants in that it is not cuticularised, in the absence of stomata, and in that its cells frequently contain chloro- plastids. The epiblemof_the subterranean root is commonly known as the piliferous layer be- TTause it is the layer from which the root-hairs (see p. 46), when present, .are developed. Its cell-walls are not cuticularised, but are frequently (especially in the root-hairs) more or less mucilaginous. It is generally of but short du- ration, and to be found only on the younger parts of roots which are the regions of active absorp- tion : on its disappearance the exodermis becomes the superficial layer. In aerial roots (Orchids, etc.) where the epiblem persists as a velamen (see Fig. 87, p. 107) of one or several layers of cells, the walls are thickened, cuti- cularised (especially the superficial layer), and somewhat lignified. The many-layered root-cap (see p. 103), in^its younger, more internal part, consists of parenchymatous cells, with cell-walls cf cellulose, forming a compact tissue without intercellular spaces. As the cells grow older, and come to be situated more externally, they lose their protoplasmic contents. TEe" disintegration of the root-cap is due, iu some cases, to the mucilaginous degeneration of the middle lamella of the cell-walls ; whilst in other cases, where the cell-walls become cuticularised, the superficial layers of the cap are successively split off and exfoliated by the pressure of the internal growing tissues. Hairs (see p. -46), are frequently developed on the primary — Water-stoma from the margin of the leaf of Tropccolum majus, with surrounding epider- mal cells. (After Strasbureer : x 240.) 110 PART II. — ANATOMY AND HISTOLOGY. [§80 tvmunentary tissue, and are generally formed each as a,u outgrowth of a single superficial cell (Fig. 90; see also Fig. 31, p. 47; and F'ig. 82, p. 101). The hairs of the subaerial parts of plants are, like the epider- mal cells, cuticularised. In many cases the protoplasmic contents disappear at an early stage (as in Cotton, the hairs on the outer coat, or testa, of the seed of Grossypium) and are replaced by air. Sometimes the cell-wall contains deposits of lime or of silica. The hairs are frequently glandular (see p. 100). FIG. 90.— Hairs on a young ovary of Cucurbits (x 100): b glandular hair ; eef early stages of development. wl FIG. 91.— Root- hairs (h) on the primary root (ic) of a seedling of the Buckwheat : he hypocotyl ; c c cotyledons. The root-hairs (Fig. 91 ; also see p. 46) are developed each from a single cell of the piliferous layer ; they are not developed in the immediate neighbourhood of the growing-point, but at some little distance behind it. Moreover, as they grow older, the root-hairs die off ; hence they are only to be found on a very limited region of a primary or a secondary root. § 31. The _ Primary Ground-Tissue is constituted, by the tissue which belongs neither to the epidermis, on the one hand, nor to the stele on the other. § 31] CHAPTER II. — THE TISSUES. Ill Morphology. The limits of this tissue vary with the structure of the part concerned. The external limit of the ground-tissue is the layer of cells lying immediately beneath the primary tegumentary tissue. Again, when the member is monostelic, the internal limit of the ground-tissue is the layer termed the endodermis, which abuts upon the central stele ; in this case the ground-tissue consists of several layers of cells bounded externally by the true epidermis (if present), or reaching to the surface, and bounded internally by the stele, when it is spoken of as the cortex of the member of which it forms part. In a polystelic member, the internal limit of the ground-tissue is still the eudodermis, but each individual stele is invested by a distinct endodermis ; here the primary ground- tissue includes not merely the superficial layers (cortex), but also the tissue between and among the steles. The following are the regions or layers of the primary ground- tissue which can be distinguished morphologically. 1. The hypoderjnff, is the external region of the ground-tissue: the external layer of the hypoderma is distinguished as the exodermis. 2. The general ground-tissue. 3. The cndodermis, the layer of the ground-tissue which abuts on a stele ; in monostelic members the endodermis is the inner- most layer of the cortex. Structure. Speaking generally, the ground-tissue consists mainly of parenchymatous cells which have cellulose walls and retain their protoplasmic cell-contents ; however, supporting- tissue (stereom) is largely differentiated in the ground-tissue, whether as eolUnicliyma or as sclerenchyma. In cylindrical members isteins. roots, etc.) the cells are generally somewhat elongated in the direction of the long axis of the member. 1. The hypoderma of stems and leaves commonly consists of either collenchymatous or sclerenchymatous stereom (see p. 92) : collenchymatous hypoderma is especially characteristic of the stems and leaf-stalks of herbaceous Dicotyledons (see Fig. 69, p. 91), but it occurs also among Pteridophyta in the petioles of the Marattiacese : sclerenchymatous hypoderma may form a continuous layer of more~^oFn[ess prosencnymatous cells (e.g. stem of some Ferns, Equisetum hiemalc, most Selaginellas, leaf of many Cycads, Conifers, some Orchids, etc.) ; or it may form numerous isolated strands (e.g. stems of Cyperacesee, species of Juncus [Fig. 112 PART II.— ANATOMY AND HISTOLOGY. [§ 31 98 C], some Umbelliferae and Papilionacese, many Equisetums ; leaf-blade of Cyperaceae, Typha, Sparganium, many Palms). The spines of leaves (e.g. Holly), also entire spiny leaves or stipules, various emergences, such as the warts of Aloe verrucosa and the prickles of the Rose, and the thorny branches of many plants (e.g. Hawthorn, etc.) owe their hardness mainly to the development of sclerenchymatous hypoderma, the cells of which are generally elongated and fibrous, though they may be short as in Aloe verrucosa and the Rose. The hypoderma of the root commonly consists of a single layer of cells, which is then the exodermis ; but in some plants the hypoderma consists of several layers (e.g. the Date, Pandanus, Asparagus, etc.). The walls of the exodermal cells generally undergo cuticularisa- tion and frequently become very much thickened, especially on the lateral and external walls, in view of the position which it eventually occupies as the external layer of the root (see p. 109). In some cases it presents a peculiar localised thickening in the form of a band extending round the upper, lower, and lateral walls of the cells, a thickening which is therefore confined just to the surfaces which are in contact with other cells belonging to the same layer, and which appears in a transverse section as a dark dot on the radial walls of the cells. In some cases the cells of the exodermis are prosenchymatous and sclerenchymatous (e.g. species of Carex). When the exodermis is invested by tegumentary tissue, as in aerial roots of Orchids (Fig. 87), some of its cells retain then-Thin unaltered walls, and are the passage-cells, by means of which water can penetrate into the interior of the root. 3. The general ground-tissue, of stems, leaves, and roots, lying within the hypoderma, consists mainly of parenchymatous tissue, with, frequently, a considerable differentiation of masses of fibrous sclerenchymatous stereom. In aerial stems and foliage-leaves, the more external, at least, of these cells frequently take part in the assimilatory processes of the plant ; the cells contain chloroplastids and constitute assimi- latory tissue. Towards the most highly illuminated surface of the member, the cells are frequently so arranged that their longer axes are perpendicular to the surface, that is, are parallel to the incident rays of light; assimilatory tissue of this structure is termed palisade-tissue : the whole of the internal ground-tissue of a leaf-blade is termed generally mesophyll. M.B. 114 PART II. — ANATOMY AND HISTOLOGY. [§31 In view of its great physiological importance a somewhat detailed ac- count of the structure of the mesophyll of the leaf-blade seems necessary. The mesophyll consists of parenchymatous thin-walled cells of various form. When the blade is thin, the whole mesophyll consists of assimila- tory tissue ; but when it is more or less fleshy and succulent, the more central part consists of cells without chloroplastids, the assimilatory tissue being confined to the surface. When the mesophyll is altogether assimilatory, the arrangement of the cells is correlated with the symmetry of the leaf-blade. In a dorsiventral lamina (Fig. 92) the structure of the mesophyll is different an relation with the upper (ventral) and the lower (dorsal) surfaces. Towards the upper surface, Avhich is more directly exposed to light, the somewhat elongated cylindrical cells form a compact palisade-tissue one or more layers in thickness ; whereas, towards the lower shaded surface, the cells are less regular, frequently somewhat stellate in form, leaving lai-ge intercellular spaces between them, constituting what is known as the spongy parenchyma. The loose structure of the mesophyll towards the lower surface of the blade is correlated with the presence of nu- merous stomata in the epidermis of that surface (see p. 108). When the palisade-parenchyma consists of several layers, the transition from the one form of tissue to the other is gradual. The vascular bundles run along the junction of the two forms of tissue. When it so happens that all sides of the leaf are equally exposed to light, the palisade-paren- chyma is developed in relation with both the dorsal and the ventral surfaces ; this is true, not only of isobilateral and of radial leaves, but also of dorsiventral leaves (e.g. leaf-blade of Anchusa italica, Linosyris vulgaris, Silene inflata, Dianthus Caryophyllus, etc.) ; in which case the spongy parenchyma is either absent, or consists of a few layers in the middle of the blade, but the intercellular spaces between the palisade- cells are, however, relatively large. In some cases, the mesophyll is not differentiated into palisade and sj>ongy parenchyma, but consists of rounded cells (e.g. succulent leaves, such as those species of Crassula, etc.). The cells of the assimilatory tissue sometimes present other forms and arrangements. Thus the assimilatory tissue of the leaf of Pimis and Cedrus consists of polyhedral cells, the walls of which present infoldings. the elfect of which is to increase the surface of the cell-wall. In other cam* it consists entirely or in part of elongated cells. reseml>Hng_palisade- ci-lls, which are arranged with their long axes parallel to the surface. either parallel to the long axis of the leaf (e.rj. Galanthus nil-alls, the Snow- Fio. 93. — Diagrammatic transverse section of the acicular leaf of a Fir : e epidermis ; es sclerenchy- matous hypoderma ; »p sto- mata ; /i resin-ducts ; s en- dodermis enclosing the single meristele ; gwood; b bast. 31] CHAPTER II. — THE TISSUES. 115 drop; Leucojum vernum, the Snow-flake;) or transversely (e.g. Iris germa- nica. Erythronium Dens-Canis, species of Gladiolus and Tritonia). Tin' colourless mesophyll of succulent and coriaceous leaves consists of large cells containing much watery sap constituting in fact an aqueous tissue (e.g. leaves of Alo'^, Mesemhiyanthemum, some Myrtaceae etc.). In some Orchids (e.g. Oncidium maximum), the cells of the aqueous tissue are scatterecFamong the assimilatory cells ; in many Orchids the cells of the aqueous tissue are tracheidal, having spirally-thickened walls, as they are also in the stem and leaf of Nepenthes. In jirainvjcases, especially in aquatic plants, the ground-tissue has large air-cavities, either lysigenous or schizogenous (see p. N!i : generally , they schizo- genous origin in aquatic plants, of lysigenous origin in land- plants. These cavities fre- quently extend throughout the whole length of the root or the leaf and through an entire inter- node of the stem ; but they may be inter- rupted at inter- vals by__dia- p h r a g m s (e.g. leaf of some Monocotyledons ; root of Hydrocharis ; stem of Alisma, Pontederia, Marsilea). When these cavities^are largely developed the member becomes a float (e.g. root of Jussisea). 4. The^Endodermis is, in the great majority of cases, a single layer of cells ; it is but rarely altogether wanting ; it sometimes consists of t\vo layers, formed by the tangential division of the cells of the primitively single layer (e.g. root of Equisetum ; stem of some Pteridophyta, such as Aspidium, Pteris, Salvinia and Azolla). td Fio. 94. -Trans verse section of central portion of the root of Bonunculus repent (x 300): ed the endodermis, enclosing the single central stele ; its radial walls show the sections of the cuticularised thickening-bands ; x the four protoxylem-bundles ; t the solid xylem; s the four phloem-bundles ; pc the pcricycle ; r the cortical tissue. 116 PART II.— ANATOMY AND HISTOLOGY. Most commonly the cells of the endodermis are thin-walled, with a suberised thickening-zone extending round the lateral and upper / and lower surfaces of the wall, and showing in transverse section (Fig. 94) as a black dot on the radial wall. This peculiar mark- ing is by no means always present : it is frequently wanting in the endodermis of the stem, in which case the endodermisjsan, in many cases, be distinguished by the presence of starch-grains in its cells. When the endodermis is double, this marking is confined to the outer of the two layers. This marking is not confined to the endodermis ; it sometimes occurs also in the exodermis of roots (see p. 112), and in one or more layers of the internal cortex in some roots (one layer, next the endodermis, in Cupressus, Taxus, Prunus, Rosa, Lonicera, etc. ; several layers, Juniperus, Seqxioia, many Cruciferse such as Mustard and Wallflower, Fig. 95). The walls of the endodermal cells frequently become sclerotic either over their whole surface, or more frequently on the internal lateral sur- faces. WThen this is the case, some of the cells remain thin- walled, as passage-cells, opposite to the wood-bundles within. §32. The Stele. The plerome, constitut- ing the young stele, always gives rise to vas- cular tissue and usually to a certain amount of other tissue which is termed conjunctive tissue. The first indication of the development of vascular tissue in the stele is afforded by the differentiation of a varying amount of pro- cambium, consisting of somewhat elongated narrow cells formed by repeated longitudinal division, which is the embryonic tissue from which the vascular tissue is eventually formed. The procam- bium frequently constitutes one solid central strand, surrounded by more or less conjunctive tissue constituting the pericyclc; this is sometimes the case in slender roots (see Fig. 94), in slender monostelic stems (e.g. many aquatic Phanerogams, such as Callitriche, Elodea, etc.; among Pteridophyta, Isoetes, Sal- viniacese, Lycopodiaceae, Hymenophyllum, Schiztea), and gene- rally in the steles of polystelic stems. More commonly, however, the procambium of stout roots and monostelic stems is developed as a number of strands variously arranged in the stele, generally Pio. 96.-A cell cortex of tbe root of the Mustard, seen obliquely from tho internal surface, showing the su- berised thickening zone. (After van Ticghcm: x 350.) 32] CHAPTER II.— THE TISSUES. 117 in a circle or in several circles ; the strands thus forming an in- complete hollow cylinder enclosing a central mass of conjunctive tissue, the medulla or pith, whilst the conjunctive tissue between the strands constitutes the medullary rays. In all cases the stele (whether one or more) Is at anjearly jtage marked off from the ground tissue, the layer of the grQund:tis8ue which abuts on the stele being specially differentiated as a sheath, the endodermis (p. 115), which forms a continuous covering to the stele or any isolated portion of it. All primary stems are typically monostelic to begin with, but as they increase in bulk this type of structure is departed from in various ways ; typical monostely is, however, the rule in the stem of Phanerogams, and is frequent in that of Pteridophyta (in Hymenophyllum, Osmunda, Lycopodium, Isoetes, some species of Selaginella). Some stejns are, however, polystclic. In these the original single stele passes over, as the stem, grows and enlarges, into a varying number of steles which can be traced to the growing-point as distinct plerome-strands. Poly- stely is rare in stems which have a growing-point without an~apical cell, whilst it is com- mon in steins where the grow- ing-point has an apical cell, or a group of common initial cells : hence it is rare in Phanerogams (occurs in Auricula and Gun- nera), and is common in Pteridophyta (especially Leptosporangiate Ferns, and some Selaginellas : see p. 102). A common modification of the polystelic structure is that which is termed gamostelic ; in this case the several steles are not dis- tinct for any considerable distance in their longitudinal course ; but some or all of them fuse with each other at more or less frequent intervals ; this is common in Ferns. The general morphology of the tissues of the leaf is essentially the same as that of the stem which bears it. When the stem is FIG. 96.— Part of a transverse section of the stele of a root of Iris florentina : e scle- rotic endodermis, with / a thin-walled pass- age-cell; v bast; « wood-vessel; c cortical ground-tissue; p pericycle. (After Stras- bnrger:x240.) 118 PART II.— ANATOMY AXD HISTOLOGY. polystelic, one or more complete steles enter the petiole of the leaf which is, consequently, either monostelic or polystelic. ~lVEen the stem is monostelic, each leaf receives a portiqn^Jermed a meristefc, of the stele of the stem; thisjneristelejnaj be either entire, or be split up into a number of parts, each of which may consist of but a single vascular bundle. The Conjunctive Tissue. The morphology of the conjunctive tissue of the stele varies somewhat in accordance with the develop- ment of the vascular tissue. When a solid vascular cylinder is produced, there may be no conjunctive tissue at allj the whole of t he plerome having developed into vascular tissue ; or the conjunc- tive tissue may be limited to one or more peripheral layers, th.e pericycle, investing the vascular cylinder ; or, again, in addition_J,o the pericycle, the Conjunct i ve tissue may extend inwards to some extent between the bundles (interfascicu- lar) of the stele. On the other hand, when the vas- cular cylinder is hollow (see Fig. 97). the central space is occupied by me- dullary conjunctive tis- sue, constituting the }>ith, and connected with the pericvcle by interfascicti- lar conjunctive tissue con- stituting the medullary rays. Pith and medullary rays are generally absent from the steles of a poly- stelic member. FIG. 97. — A transverse section of a young stem of Arixtolochia Sipho, illustrating the arrangement of the primary tissues in a monostel'c stem, in which the vascular cylinder is hollow, enclosing a pith (after Strasburger: x 9) : c cortical tissue, with collenchyma cl ; e endodennis ; pc pericycle, continuous by means of interfascicular conjunctive tissue (medullary rays) with the medullary con- junctive tissue m (pith) ; sfc ring of sclerenchyma belonging to the pericycle ; fv vascular bundles in an interrupted circle ; they are open and collateral ; cb bast ; p protophloem ; fc fascicular cambium ; »/c interfascicular cambium ; t>l wood ; the central pointed end of each wood-bundle consists of protox.ylem, and the central ends of the whole ring of wool-bundles constitute the medullary sheath. A remarkable form of tis- sue is that which invests the two vascular bundles in the acicular leaves of Pinus, and, to a less degree, of other Conifers (Fig. 93). The tissue consists of parenchyma with some fibrous sclerenchyma: in the parenchyma two special kinds of cells can be distinguished, which constitute what is some- § 32] CHAPTER II.— THE TISSUES. 119 times termed the trannfusion-tisstte] namely, cells with unlignified and unpitted walls, distinguished by their abundant protoplasmic and proteid contents ; tracheidal cells with slightly lignified walls and bordered pits, without protoplasmic contents ; the former may be regarded as an ex- tension of the sieve-tissue of the bundle, the latter-as an extension of the tracheal tissue. The Pericycle is altogether wanting in a few cases only ; it is absent when the endodermis consists of two layers (see p. 115) ; it is also absent from the slender roots and stems of some water- plants. It is usually a continuous membrane; but in some cases it is interrupted by projections of the vascular tissue (e.g. by the- xylem-bundles in the root of some Graminese and Cj^peraceae). It may consjst throughout of a single layer of cells (e.g. roots of most Angiosperms [Fig. 96] and of some Vascular Cryptogams) ; or of more than one layer throughout (roots of some Dicotyledons, e.g. Vine, and of Gymnosperms generally ; commonly in the stem and leaf-stalk) ; or in part of one layer and part of more than one (e.g. root of some Perns and Leguminosse). The pej-icycle-jnay be hamogeneous or heterogeneous ; that is, it may consist of the same kind of tissue throughout, or of several kinds of tissue. The typical homogeneous pericycle consists of thin-walled parenchymatous cells, with protoplasmic contents, which are capable of becoming merismatic. In some cases the primarily thin-walled cells eventually become sclerotic, either throughout the whole pericycle, or in certain parts only ; this commonly occurs in the roots of Monocot}rledons. Generally speaking, the pericycle of the root is homogeneous ; when it is heterogeneous, it is so in consequence of the presence of glandular tissue (secretory ducts) (e.g. Umbelliferae, Hyperi- cacese) ; ^t never contains fibres. The pericycle of the stem and of the leaf-stalk, on the contrary, is generall}'- heterogeneous, owing principally to the differentiation of a portion of it into collenchyma (e.g. some Composite, Bark- hausia foetida, Sonchus oleraceus}, or into fibres (Fig. 97) which are generally sclerotic, but not in all cases (e.g. Apocynacese, Con- volvulacese, Flax) ; or it may be heterogeneous in consequence of the presence of secretory ducts (e.g. Hypericum, some Umbelli- ferse) ; or, in consequence of the presence of both secretory ducts and of fibres (e.g. Ligulifloral and Tubulifloral Composites). The Pith (or medulla) consists, typically, of parenchymatous 120 PART II.— ANATOMY AND HISTOLOGY. [§32 cells with thin walls and protoplasmic contents; but in many cases sclerenchyma is differentiated in it. The most important fact with regard to the parenchyma of the pith is that, in many cases, the cells forming the central portion of the pith soon die, or even the whole of them (e.g. Elder). When this is the case, the dead cell-walls freqviently undergo disorganisa- tion, so that the stem becomes hollow. The sclerenchyma of the pith may consist of scattered strands (e.g. stems of some Palms) ; or it may form a ring connecting the inner ends of the bundles of the hollow vascular cylinder (e.g. Bongainvillea spectabilis, woody Piperacese. The bulk of the pith varies very much. It is relatively very large in tuberous shoots, such as Potato, Apios, etc. The Interfasci- cular Conjunc- tive Tissue con- sists typically of p a r e n chy matous cells with thin walls and proto- plasmic contents ; but it is fre- quently scleren- chymatous where it abuts on the vascular bundles, thus contributing to the formation of a more or less complete sheath of sclerenchyma round them (e.g. many Mono- cotyledons) : in woody plants the cells of the medullary rays become lignified. The various systems of sclerenchymatous supporting-tissue (stereom) described above, the hypodermal, the cortical, the pericyclic, the inter- fascicular, and the medullary, may be connected with each other in various combinations. Thus, the hypodermal and the cortical systems may be continuous ; or the hypodermal, cortical, and pericyclic ; the pericyclic and the interfascicular, etc. ; as the mechanical conditions of the member may render necessary (see Fig. 98). FIG. 98.— Diagram (after Schwendener) illustrating the dis- tribution of the supporting-tissue or srereom, as seen in trans- verse section of stems: A of Arum moculatum having isolated cortical stereom-strands ; B of Allium vineale, with continuous pericyclic stereom-ring ; C of Juncws glaucus (hollow), with hypodermal stereom-strands and conjunctive stereom-strands ; / vascular bundles ; 8 stereom-strands ; 1 air-cavities. § 33] CHAPTER II. — THE TISSUES. 121 § 33. The Primary Vascular Tissue. The primary vas- cular tissue is differentiated from the procambium of the stele in the form of strands or bundles, vascular bundles. The vascular tissues of the bundles are either tracheal tissue (p. 93), which is always lignified, and is termed icood or xylem ; or sieve-tissue (p. 94), which is termed bast or phloem. A vascular bundle may consist, either exclusively of wood or of bast ; or of both wood and bast, when it is said to be a conjoint bundle. It is generally the case that a varying proportion of sclerenchyma (stereom) is associ- ated with the vascular tissue; hence the bundles are frequently spoken of as fibro-vascular bundles. As a rule, an equal number of wood-bundles and of bast-bundles are differentiated in a stele, whether they be isolated or conjoined ; there may be only one of each (e.g. finer branches of the dichotomous roots of most Lycopo- diums) or there may be a very considerable number (e.g. stems of Monocotyledons). With regard to the occurrence of vascular tissue in the gametophyte- generation, and in the sporophyte of the lower plants, it may be stated that lignified vascular tissue (i.e. wood) does not occur in any gameto- phyte, nor in the sporophyte of any plant below the Pteridophyta. How- ever, in the stem of the gametophytic shoots of some Mosses there is a solid central stele consisting of tissue which is functionally vascular tissue; the same is true of the stem (seta) of the Moss-sporophyte in certain cases. Sieve-tissue has been found in some of the larger Brown Seaweeds (p. 96). The primary vascular tissue-system extends continuously throughout the body of the sporophyte of the higher plants ; the vascular bundles of root, stem, and leaf are all in direct com- munication. The arrangement and course of the vascular bundles are in- timately connected with the morphology of the plant and with the differentiation of its members. In elongated members (stems, petioles, roots) the bundles run longitudinally, so that a transverse section of such a member shows transverse sections of its vascular bundles. In the primary root the longitudinal course of the bundles is simple ; there is an axial vascular cylinder, either solid or hollow, consisting of straight. more or less distinct, bundles of wood and bast, and extending from the growing-point backwards to where the root merges into the stem ; from this cylinder there arise 122 PART II.— ANATOMY AND HISTOLOGY. [§33 lateral offsets, which constitute the steles of the lateral branches of the root. In the stem the course of the bundles is more complicated, on account of the fact that the stem bears lateral members, leaves, which differ from itself or from its branches. In some cases, the bundles of the stem, when traced upward toward the growing- point, are found to terminate in the young leaves ; whilst in other cases the bundles end (like those of the root) in the plerome of the growing-po i n_t ; bundles of the former kind are distinguished as common (i.e. com- mon to stem and leaf), and, in their course in the stem, are termed leaf- traces ; bundles of the latter kind are dis- tinguished as cauline (i-e. confined to the stem). Stems with common bundles are generally monostelic ; the leaf- traces do not, how- ever, follow a uniform course in all cases. Thus, they may pro- ceed to the centre of the stem and form a solid vascular cylinder (e.g. Isoetes among Pteridophyta ; and certain aquatic Mono- cotyledons, such as species of Potamoge- ton, etc.). Or they may form a hollow cylinder. In the simplest case FIG. 99.— Diagram of the course of the vascular bundles in stems. A Longitudinal section through the axis of a Palm-stem, showing a transverse section of half of it. The leaves (cut off above the insertion) are hypothe- tically conceived of as distichous and amplexicaul, and so are seen on both sides of the stem, 1 m 2 m 3 m being the median line of each. B Outside view and transverse section of Cerastium (hypothetically transparent, to show the internal bundles). The decussate leaves (1, 2, 3) are cut off. The bundle proceeding from each leaf divides into two above the leaf immediately below it, and the branches of all the bundles unite to form the four thin bundles which alternate in the Hection with the thicker ones. In the section, m is the pith, r the cortex, v the medullary ray. The xylem in the nbro-vascular bundles is indicated by shading. § 33] CHAPTER II. — THE TISSUES. 123 of this (as in Osmundacese, most Gymnosperms and Dicotyledons) the bundles (leaf-traces) entering the stem from a leaf are few in number, or even only one ; they penetrate to an egu^l depth in the stem, and run vertically downwards through one or two internodes, joining at a node with the bundles entering the -stem from a lower leaf ; sometimes their lower ends are bifurcate so that they join with the bundles of the lower leaves (Fig. 99 JB}. When the leaf-traces entering the stem from a leaf are more numerous, they penetrate to various depths in the stele, and tljeir course is usually not vertical, but more or less curved : they may then form two circles (e.g. Cucurbitacese, Phytolacca, Piperacese) ; or many circles, more or less irregular, trenching on the pith (e.g. many Ranunculacese, such as Cimicifuga, Thalictrum ; Nymphseacese ; Monocotyledons generally). A good example of this is afforded by a Palm stem (Fig. 99^4). The median leaf-traces first tend to- ward the centre of the stem ; they then bend outward, thinning out gradually as they descend, and coalesce with the lateral bun- dles, which do not penetrate so deeply, in the pericycle at a point much lower down. Furthermore, each bundle is somewhat twisted in its course, so that the lower end lies toward a different side of the stem from that on which it entered it. In these cases, when there is a well-defined external ring, the more internal bundles are termed medullary bundles. The relative position of the phloem and of the xylcm in a con- joint bundle is subject to some variation ; they may either be side by side, when the bundle is said to be collateral ; or the one may more or less completely invest and surround the other, when the bundle is said to be concentric. In the collateral bundle, the wood and the bast are so situated that they both lie on a straight radial line drawn through the bundle from the centre of the member to the surface, the wood being nearer the centre, and the bast nearer the surface (see Fig. 97). This type of bundle is common in the stems and leaf- stalks of Phanerogams and of some Pteridophyta (Osmundacese, Ophioglossacese, Equisetum). In some stems (e.g. Solanacese, most Convolvulacese, Cucurbitacese, etc.) there is a second bast-bundle on the inner (medullary) side of the wood of the conjoint bundle ; such a bundle is distinguished as bicollatcral. In a concentric bundle, either the bast is surrounded by the wood, or the wood by the bast, more or less completely : the 124 PART II. — ANATOMY AND HISTOLOGY. [§33 bicollateral bundle is, in fact, a structure intermediate between the collateral and the concentric bundle. The former type of con- centric bundle occurs in the rhizomes of various Monocotyledons (Acorus, Iris, Cyperus, Carex, etc.), and in the medullary bundles of the stem of some Dicotyledons (Rheum, Statice, Ricinus, Piper, etc.). The latter type is rare in Phanerogams (e.g. the cortical FIG. 100.— Transverse section of an open, collateral, conjoint, vascular bundle of the stem of Ranunculus repent : s spiral vessel of the protoxylem at the inner (central) end of the wood j m pitted vessel of the wood ; c cambium ; v a sieve-tube of the bast with adjacent granular companion-cells; vg sheath of sclerenchymatous conjunctive tissue. (After Strasburger: x 180.) and medullary bundles of the Melastomacese) ; but it prevails in the Filicinse and in Selaginella, when the bundles (two_or jnore) of each stele of the polystelic stem, form a central mass of wood completely, or nearly completely, surrounded by a ring of bast. §33] CHAPTER II. — THE TISSUES. 125 The relative position of the plilo em-bundles and xylem-bundles tchen they are dis- tinct from each other is such that they alternate with each other so that a radius drawn from the centre to the surface of the member cuts through either a phloem or a xylem- bundle, but not through both (Fig. 102). This ar- rangement obtains only in mono- stelic mem- bers ; it is common to all roots, and oc- curs in the stem of Lyco- podium and Psilotum though in a less regular manner than in roots. It is commonly termed the radial a r- rangement. The Differ- entiation of the Primary Vascular Bun- dle. The first Fia. 101. — Transverse section of a concentric bundle, with external wood, from the rhizome of Iris (x 350) : t trachea; V protoxylem ; s sieve-tubes ; g companion-cells, of the in- ternal bast.] •vi FIG. 102. — Part of a transverse section of the stele of the Sar- saparilla-root (Smilax) : r cortex; ed endodermis with passage- cells d ; the pericycle and the interfascicular conjunctive tissue o are sclerenchymatous ; v' the pith ; * the protoxylem, and t a pitted vessel of a wood-bundle: s a bast -bundle. The alternation, or radial arrangement, of the wood and bast-bundle is shown. ( x 300.) 126 PART II.— ANATOMY AND HISTOLOGY. [§ 33 indication of the development of vascular tissue in the plerome is the differentiation of one or more strands of narrow elongated merismatic cells, the procambium (p. 116) ; each procambium- strand_pf the plerome becomes a vascular bundle of thejitele. The development of the vascular tissue does not take place simultaneously throughout the whole transverse section of the procambium-strand, but b«gins at one definite point, and extends in one or more directions from that point. The development of the xylem-bundle (or part of a conjoint bundle) begins with the differentiation of one or a few tracheids or tracheae, constituting the protoxylem ; the walls of the corre- sponding procambium-cells become spirally thickened and liguified, and the protoplasmic contents of the cells disappear. It is an important generalisation that spiral or annular vessels (or tra- cheides) are characteristic of, ' and absolutely confined, to, the protoxylem of the bundle. The remainder of the primary wood (i.e. the wood which is developed from the procambium) is then gradually differentiated, the walls of the tracheides or trachese presenting one or other of the various kinds of pitted marking (p. 74). Similarly, the development of the phloem-bundle (or the phloem of a conjoint bundle) begins with the differentiation of a small group of sieve-tissue, constituting the protophloem, which does not, however, differ in any marked manner from the rest of the primary phloem, but their cavities soon become obliterated, so that they then look like strands of swollen cell-wall (Fig. 103). The details of the differentiation of the primary vascular tissue are essentially the same as in the case of the secondary vascular tissue described on p. 145. The longitudinal differentiation of the primary vascular tissue does not take place in the same order in all cases. In roots, and in stems with cauline vascular tissue, the longitudinal differenti- ation proceeds acropetally. In stems with common bundles the differentiation usually begins in the procambium-strand at a node, proceeding both downwards in the internode of the_stem, and out- ward into the young leaf. In the majority of instances, the whoU of the procambium-strand becomes differentiated into permanent tissue, either wood or bast : this is true for all roots, and for the stems of nearly all Pterido- phyta and Monocotyledons (Fig. 103). Bundles of this kind are § 33] CHAPTER II.— THE TISSUES. 127 said to be closed. In the stems of most G-ymnosperms and Dicgty- " ' ledons, on the other hand, the whole of the procambium is not converted into the primary wood and bast of the collateral conjoint bundle, but a portion of it persists as an embryonic merismatic tissue, the cambium, forming a transverse zone- Jbetween the wood Fio. 103. — Transverse section of a conjoint, collateral, closed, vascular bundle of the stem of a Monocotyledon (Zea Mais) : a outer or peripheral end of the bundle ; i inner or central end ; p conjunctive tissue, the portion immediately investing the bundle being sclerenchy- matous ; I lysigemus intercellular space ; g r spiral and annular vessels constituting the protoxylem ; g g large pitted vessels, between which lie the smaller pitted vessels of the wood ; v v v sieve-tubes of the bast with intervening companion-cells ; just outside the bast, and within the sclerenchymatous sheath, the remains of the protophloem are visible. (After Sachs : x 553.; on, the inner (central) side and the bast on the outer side (see Figs. 97, 105). Such a bundle is said to be open. 128 PART II. — ANATOMY AND HISTOLOGY. [§ 33 Some few Dicotyledons have closed bundles (i.e. no cambium) in the stem, e.g. Adoxa, Ranunculus Ficaria, Nymphseacese, Myriopliyllum, Utricularia. etc. The position of the protoxylcm and of the protophloem in the transverse section of the bundle is not the same in the different members. The protophloem is in all cases superficial : and though the protoxylem is also generally superficial, it is sometimes in- ternal (as in the bundles in the petiole of Cycads, in the stem of Isoetes, and in the concentric steles of stems and petioles of many Ferns), being more or less surrounded by the rest of the primary xylem. In members, whether monostelio or polystelic, in which the primary bundles or the steles are arranged in one or more circles (or other figure corresponding to the sectional outline of the mem- ber), the orientation of the bundles in the stele, as indicated by the position of the protoxylem, bears a definite relation to the symmetry o^ the transverse section of the member. For instance^ in medullate monostelic stems (Fig. 97) the protoxylem forms the innermost or central portion of the bundle ; the broken circle of protoxylem- groups is sometimes specially designated the medullary sheath. In the root, whether the vascular cylinder be medullate or not, the protoxylem is always outermost or peripheral, abutting on the pericycle (Fig. 102). This is also the case in monostelic stems which are not medullate (e.g. stem of Lycopodium). The proto- phloem is always external, abutting on the pericycle._ The transition from the root to the stem. Inasmuch as, generally speaking, the type of primary structure of the root differs so con- siderably from that of the corresponding stem, the transition from the one to the other is a matter of some importance. Taking as an illustration the case of a plant with a monostelic stem, the passage from the radially arranged separate bundles of the primary rooj to the collateral conjoint bundles of the stem is effected on this wise : —generally speaking, on tracing the wood- and bast-bundles of the root upwards into the stem, the wood-bundles are found to twist on themselves so that the protoxylem of each bundle, from being peripheral in the root, comes to be central in the stem ; at the same time they change their position somewhat so that they come to lie on the same radii as the bast-bundles, or the bast-bundles may also deviate somewhat from their straight course, and thus the conjoint collateral bundles come to be constituted. As a rule, these changes of position are accompanied by an increase in number of the bun- §33] CHAPTER II. — THE TISSUES. 129 dleSj each of the bundles of the root bifurcating above, so that there are commonly twice as many bundles in the stem as in the corresponding root. The structure of the primary bundle. The primary wood (whether in an isolated or a conjoint bundle) consists essentially of lignified tracheal tissue (tracheae, or tracheids, p. 93), together with a varying proportion of wood-parenchyma, more or less lignified, the cells being occasionally somewhat fibrous. The protoxylem (see p. cp n' FIG. 104. — F?adial longitudinal section of a conjoint, closed, collateral bnndle from the stem of a Monocotyledon (Zea Mais ; after Strasburger, x 180) ; to the right is the central (medullary) limit of the bundle ; to the left the peripheral (cortical) limit : c p protophloem ; v sieve-tubes of the bast, with companion-cells * ; «p a a', the protoxylem ; a a' remains of ruptured annular vessel lying in the lytigenous lacuna I ; vg sheaths of sclerenchymatous conjunctive tissue. (Compare this with Fig. 103.) 126) is usually a conspicuous feature; in transverse section, on account of the relative smallness of its tracheae (or tracheids) ; j.n longitudinal section, on account of the loose spiral or annular thickenings of their walls. The looseness of the spiral or annular markings is due to the fact that these vascular cells are the first formed constituents of the bundles, and that consequently they are considerably stretched by the continuance, for a time, of the M.B. K 130 PART II. — ANATOMY AND HISTOLOGY. [§ 33 growth in length of the adjacent undifferentiated tissues; hence the successive thickenings become more or less widely separated, and the wall of the vessels may be torn and destroyed (Fig. 104). The primary bast or phloem consists essentially of jsieye-tissue (p. 94) and of parenchyma. The sieve-tissue consists in_alLeases mainly of sieve-tubes of simple structure (Fig. 74, p. 95), con- stituting the vascular tissue of the bast, with which companion - cells are associated in Angiosperms but not in Gymnosperms and Pteridophyta. In some Angiosperms, particularly in the closed bundles of Monocotyledons (Fig. 103), there is no bast-parenchyma, the whole bast consisting of sieve-tubes and companion:cells : but this tissue is generally present, and is readily distinguishable from the companion-cells by the larger size of its cells. In some cases (e.g. some Palms) the bast-parenchyma is to some extent replaced by sclerenchymatous fibres ; otherwise the occurrence of fibres in the primary bast is rare. The cambium is present in the collateral primaryjbundles of the stem of most Gymnosperms and Dicotyledons; it is never present in primary bundles of any other type of structure; nor, on the other hand, is it always present in a collateral bundle (absent in Equisetum, Monocotyledons, some herbaceous Dicotyledons, see p. 128). It lies between the bast externally and the wood in- ternally, and consists essentially of a single layer of merismatic embryonic cells rich in protoplasmic contents, and with walls of cellulose. In transverse section (see Fig. 100) the cells are oblong, with their longer axes placed tangentially ; in longitudinal section the cells are seen to be elongated and somewhat prosenchymatous, like the procambiurn-cells, where they abut on the wood or on the bast; but where they abut on primary medullary rays they are short and parenchymatous. The Termination of the Vascular Bundle. The gradual thinning out and termination of the vascular bundle can nowhere be more satisfactorily studied than in leaves. The bundles, when traced towards their ultimate ramifications, are seen to diminish in bulk in consequence, partly, of a reduction in number of the constituent elements, and partly also to the smaller size of the elements which still remain. The mode of termination of the vascular bundles in foliage-leaves is briefly as follows. In many cases the bundles have only free ends, as in most Pteridophyta (e.g. Adiantum, Selaginella), and generally in small reduced leaves. In othejs, there are no free ends, but the finer branches anastomose with each 33] CHAPTER II.— THE TISSUES. 131 other to form a closed system ; this is characteristically the case where the venation is parallel (e.g. Monocotyledons, see p. 38). In others, again, the finer branches anastomose, forming a network from the meshes of which the ultimate branches project among the mesophyll-cells as free ends : this obtains generally among Dicoty- FIG. 105. — A Tranverse section of an open conjoint, collateral, vascular bundle in the stem of the Sunflower, if Pith. X Xylem. C Cambium. P Phloem and pericycle. R Cortex ; s small, and s' large spiral vessels (protoxylem) ; t pitted vessels ; t' pitted vessels in course of formation from the cambium ; h wood-fibres ; sb sieve tubes ; b fibres of the heterogeneous pericycle; e endodermis or bundle-sheath; tc inter-fascicular con- junctive tissue. B Radial vertical section through a similar bundle (somewhat simplified) lettered like the former. ( x 150.) ledons. The free ends of the bundles consist of one or two rows of short tracheids with close spiral markings ; no sieve-tubes can 132 PART II.— ANATOMY AND HISTOLOGY. [§ 34 be traced quite to the extremity ; they disappear further back, and their place is taken by parenchymatous cells. Bundles often terminate in connection with glandular Jissue ; for~instance, in chalk-glands and nectaries. § 34. Histology of the Development of Secondary Members. It has been already pointed out (p. 9) that the growing-point is the seat of development, not only of new tissue, but also of new members ; and further (p. 18), that secondary members are developed either by dichotomy or by lateral out- growth. A. Development of normal branches of the shoot_Q3L_Q£_the thallus only takes place at the growing-point. a. By dichotomy. Two modes may be distinguished accordingly as the growing-point has or has not an apical .cell : — when there is an apical cell, true dichotomous branching is A Vio. 108.— A B C successive stages in true dichotomons branching by longitudinal division of an apical cell; from the shoot of an Alga Dictyofa dichotoma (highly magni- fied; after Naegeli). effected by the longitudinal division of the apical cell into two, each of which becomes the apical cell of a branch : — when there is no apical cell, the growing-point becomes broadened, and the central portion of it passes over into condition of permanent tissue, leaving two distinct masses of embryonic tissue, which constitute the growing-points of the two branches (e.g. Marchantiaceae). b. By lateral outgrowth : — irlii-n there is a single initial cell in the growing-point, the growing-point of the branch is developed either directly__from the initial cell itself, as in some Algse, or more commonly from a seg- ment of the initial cell, as in many Algse, Mosses, Liverworts, etc. : — when there is not a single initial cell (e.g. Phanerogams), the growing-point of the branch is formed by division of cells of the periblem, including several layers, which grow and divide, form- ing a lateral protuberance with the growth of which the dermato- § 34] CHAPTER II. — THE TISSUES. 133 gen. keeps pace ; the primary meristem of the branch vindergoes differentiation into tissue-systems corresponding to those of the parent members, and continuous with them. Normal branches, however the details of their development may vary, agree in this, that they are, with rare exceptions^ exogenous origin. B. Development of Leaves only takes placeji^the^rowing-poiut of a stem, andjdways by lateral outgrowth (see p. 28). When the growing-point of the stem has a single initial cell, the growing-point of the leaf is developed either from the apical cell itsetf,l>r,'more commonly, from the whole or a part of a segment of the apical cell. When the groicing-point of the stem has not a single initial cell, as in Phanerogams, the growing-point of the leaf is formed by the division of _cells belonging to one or more of the superficial layers of the periblem, accompanied by growth and division of the cor- responding cells of the dermatogen. The primary meristem of the leaf becomes differentiated into tissue-systems corresponding to, and continuous with, those of the stem which bears it. In the developing leaves of those vascular plants which have common bundles (see p. 127), the differentiation of the protoxylem begins at the point of junction of leaf and stem, extending outwards in the procambium-strands of the leaf, and in- wards in those of the stem. The development of secondary branches of the leaf takes place in essentially the same manner as that of the leaf from the stem. Dichotomous branching of the leaf (see p. 34) takes^ place in the same way as dichotomous branching of the stem. It will be seen that the development of a leaf on any stem takes place in essentially the same way as the development of a lateral branch on that stem ; it is only later that leaves and branches assume their distinctive characters. C. Development of Branches of the Root. It has been pointed out that the only normal secondary members produced by the root are root-branches or secondary roots ; these may be developed either by dichotomy or by lateral outgrowth. a. By dichotomy. This has only been observed in certain sporo- phytes among the Pteridophyta (Lycopodium, Isoetes). Here the growing-point broadens, under the root-cap, the central portion passing over into permanent tissue, whilst the two sides remain merismatic and form the growing-points of the two secondary 134 PART II. — ANATOMY AND HISTOLOGY. [§34 roots; the old root-cap is exfoliated, and each growing-point forms a new one for itself. The successive dichotomies take place in planes at right angles to each other. b. By lateral outgrowth. It has been already stated (p. 9) that the lateral development of secondary members does not take place at the growing-point of the root, but at a considerable distance behind it, where the tissues have already assumed their permanent differentiation. The lateral roots are developed endogenously from a layer of this tissue which remains embryonic longer than the FIG. 107. —Illustrating the development of a secondary root in a Phanerogam. A trans- verse, D longitudinal, section ; ep epiblem ; en endodermis ; pe pericycle ; w protoxylem and b phloem of the parent-root ; re root cap ; c periblem, and 2^ plerome, of the de- veloping lateral secondary root. (Teesdalia nudicaulis ; x about ZOO ; after van Tieghem). adjacent tissues. This layer may be either the pericycle, as in Phanerogams, or the endodermis, as in most Vascular Cryptogams. In_the Phanerogams (Fig. 107), the growing- point of a lateral root is formed by the growth and division of a group of pericycle- cells, lying usually just externally to the outer end of a xylem- bundle ; hence there are as many longitudinal rows of lateral roots produced as there are xylem-bundles in the parent root, and cor- responding with them in position. But to this rule there are some exceptions; for instance, when, as in the Grasses and Cyperacese, § 34] CHAPTER II.— THE TISSUES. 135 the pericycle is wanting opposite to the xylem-buudles, the lateral roots are developed, not opposite to the xylem-bundles, but opposite to the phloem-bundles. Again, when there are only two xylem- bundles in the parent root, four rows of lateral roots are produced, each root being developed on one side of a xylem-bundle of the parent root : a similar displacement occurs in Umbelliferse, Araliacese and Pittosporacese, where the pericycle is interrupted opposite to each xylem-bundle by an oil-duct (see p. 119). In most Vascular Cryptogams (except Lycopodium and Isoetes, where secondary roots are produced only by dichotomy), the apical cell of a secondary root is formed from one of a row of large endodermal cells, the rhizogentc cells, lying just externally to each xylem-bundle of the parent root. In Equisetum, where the endodermis consists of two layers (see p. 115), the secondary roots are developed from cells belonging exclusively to the inner layer, which are adjacent to the xylem-bundles. It will be understood that, in order to reach the surface, the lateral secondary roots must penetrate the external tissues of the parent root. This is nq^effected by purely mechanical means, but by chemical action, leading to solution and absorption, exerted on the tissues, either by the rootlet itself, or, more commonly, by a digestive sac which invests the root- let, and is formed in Phanerogams by the growth and division of the cells of the endodermis (and sometimes one or two layers of cortical cells), in Vascular Cryptogams, by the growth and division of one or more of inner layers of cortical cells just external to the endodermis of the parent root. D. Development of Hairs. These structures are m_all cases developed from the superficial cells of the parent member, that is, from dermatogen-cells in those parts in which this layer is differ- entiated ; in the great majority of cases each hair arises from a single superficial cell. Hairs are generally developed in acropetal succession, but considerable irregularity is not uncommon, and they are frequently developed on members in which the tissues have already acquired their permanent characters (see p. 46). E. Development of Emergences. When exogenous they are developed from the superficial and from one or more of the sub- jacent layers of tissue of the parent member, that is, from the dermatogen and periblem of those members in which this differen- tiation of the primary meristem obtains. When they are endo- genous (e.g. haustoria of Cuscuta, see p. 49), they are developed exclusively from the periblem. F. Development of Reproductive Organs. The question as to 136 PART II.— ANATOMY AND HISTOLOGY. [§ 34 the relation of these members to the primary meristem, only arises with reference to those plants, the bodies of which consist of many layers of tissue ; their origin in plants, the bodies of which consist either of filaments, or of flattened expansions of a single layer of cells, need not be considered here. In the cases under consideration, the reproductive organs may be developed either from the superficial layer alone, or from that and one or more of the subjacent laj'ers. Organs developed from the superficial layer alone Jderinatogen when differentiated) : these may be developed each from a single cell, as all sexual organs, and the sporangia of all leptosporangiate Ferns and Rhizocarps (Hydropteridese) ; or they may be developed from a group of superficial cells, as the sporangia of the Ophio- glossacese and Marattiacese (eusporangiate Ferns), of Equisetum, and of Lycopodinse. Organs developed from the superficial and deeper layers. In most cases the organ is developed from the superficial and one or more of the subjacent layers, e.g. microsporangia (pollen-sacs) and macrosporangia (ovules) of most Phanerogams. The primitive sporogenous tissue (archesporium, see p. 53) is, in the sporangia of all Vascular Plants, derived from the hypoclermal layer of the young sporangium ; it may consist of a single cell, or of_a row of cells, or of a layer of cells. In the Mosses the archesporium is more deeply seated, arising from the external layer of the endothecium (rudimentary plerome) as in most Mosses, or from the innermost layer of the amphithecium (rudimentary periblem) as in Sphagnum and in the Liverwort Anthoceros. G. The Development of Adventitious Secondary Members (see p. 9). 1. On the stem. The most common case is that of the develop- ment of roots, but occasionally shoots (buds) are developed ad- ventitiously. The adventitious development of roots on the stem takes place most commonly by the division of a group of pericycle-cells to form a growing-point, in the way described on page 134 with reference to the development of normal lateral roots on the parent root. In any one plant the two processes are similar in every detail. The adventitious development of biids on the stem may take place either exogenously or endogenously. In the former case__the buds may be developed each from a single epidermal cell (e.g. §35] CHAPTER II. — THE TISSUES. 137 Begonia prolifera, underground shoots of Psilotum), or from the epidermis and subjacent layers (e.g. Linaria vulgaris}. In the latter case the adventitious bud arises from the pericycle (e.g. Cuscuta, hypocotyl of Convolvulus arvensis). 2. On the root. Adventitious buds may be formed either exo- genously or endogenously on the root ; in the former case they arise from the superficial .layers (e.g. Aristolochia Clematitis) ; in the latter, from the pericycle (e.g. Alliaria ojftcinalis, Anemone sylvestrisj etc.). B._0n the leaf. Adventitious buds developed on leaves are oj exogenous origin, the epidermis being more especially concerned in their production (Begonias). Adventitious roots are usually of endogenous origin, being derived from cells of the pericycle. Adventitious buds and roots are also developed from the callus (see § 36) formed on the injured surfaces of stems, roots, and leaf-stalks : the former may be endogenous or exogenous, the latter are endogenous. § 35. The Formation of Secondary Tissue. In addition to the formation of primary tissue from the primary meristem of the growing-point, as above described, a for- mation of secondary tissue takes place in many plants, which is in most cases asso- ciated with a growth in thickness. A. The Normal Formation of Second- ary Tissue, in_the stem takes place in most Gy mnosperms and Dicotyledons, and is Affected by the continuous merismatic activity of the cambium of their open collateral bundles. These are arranged in a circle in a transverse section (Fig. 108 A) : the commencement of growth in thickness is preceded by tan- gential divisions in the conjunctive tissue (Fig. 105) which lies between the bundles ; this gives rise to cambium which becomes continuous with that of the vascular bundles. A closed hollow cylinder is thus formed, which appears, in a transverse section, as a ring, the cambium-ring (Fig. 108 B c), completely separating FIG. 108. — Diagrammatic transverse sections of a normal dicotyledonous stem which grows in thickness. A Very young : there are five isolated bundles ; m pith ; r cortex ; V primary bast ; V primary wood ; c cambium. -B After growth in thickness has commenced ; Ji2 secon- dary wood ; b2 secondary bast. 138 PART II.— ANATOMY AND HISTOLOGY. [§35 the pith from the cortex : it consists of two portions corresponding to its mode of origin ; fascicular cambium, i.e. the cambium be- longing to the vascular bundles, and the inter •fascicular •cambium, i.e. that which is formed between the bundles in the primary medullary rays (see Fig. 97). A cambium-ring is likewise formed in the root of these plants (Fig. 109). The first indication of the formation of ji cambium- layer is the division of the cells of the conjunctive parenchyma on the inner surface of each bast-bundle : then those on the flanks of the bast-bundles begin to divide ; and thus a number of^ arcs of cambium are formed, extending from the inner surface of each bast-bundle to the pericycle. The pericycle-cells lying externally to Jhe outer ends_(protoxy- lem) of the wood-bundles now divide, and connect the arcs of cambium. Thus a con- tinuous cambium -layer is formed, which has at first a wavy outline, as seen in transverse section, but which becomes circular as the de- velopment of the secondary tissue proceeds. The cambium-layer of the primary root is continuous with that of the primary stem; hence, in a plant in which stem and root grow in thickness, there is a continu- ous layer of merismatic tissue extending from one end of it to the other ; for the cambium of the branches of both stem, and root is continuous with that of the primary members ; and further, the cambium is continuous with the merismatic tissue of the growing-points of the primary stem and root and of their branches. The cells of the cambium-ring, in the stem and root alike3 con- stantly undergo both tangential and radial division, so that the number of the cells increases in the radial direction as well as in the circumferential ; the growth of these cells produces an exten- sion of the organ in both these directions. Of the cells formed by B. FIG. 109.— Transverse section of the stela of the root of Sambucus nigra, where secondary growth in thickness is commencing, r Cor- tex ; ed endodermis ; pc pericycle ; xxx the three groups of protoxylem ; p p p the three groups of phloem ; c dividing cells of the con- junctive tissue forming part of the developing cambium-ring. §35] CHAPTER II.— THE TISSUES. 139 tangential division, those lying on the inner side of the cambium, jtre transformed into the elements of the wood (Fig. 108 B /i2), those on the outer side, into the elements of the bast, while the cells of the intermediate zone continue to be capable of dividing. The activity of the cambium thus gives rise t6 secondary wood and secondary bast, as distinguished from the primary con- stituents of the bundle, which existed previously to, and indepen- dently of, the activity of the cambium. The primary wood of the bundle is thus the innermost part of it, and the primary bast the most external. Not only does the fascicular cam- bium add secondary wood and bast to the primary bundles of the stem, but the interfascicular cambium generally forms (except Cucurbita- cese, Aristolochia, and some other plants, where it only forms conjunc- tive tissue) new secondary bundles between the primary, and in this way a compact ring of wood and of bast is formed. These secondary bundles are of course destitute of protoxylem and protophloem. In roots the secondary vascular tissue is developed in essentially the same manner as in the stem; the wood inwards, the bast outwards, from the cambium-layer ; and the same forms of tissue are produced. It is, however, only in certain cases (e.g. Taraxacum, Rubia, Taxus, Cu- pressus, etc.) that the cambium of the root produces Wood internally, bundles. B Transverse section of an and bast externally, over its whole older root of *be *ame plant' which J ' is growing in thickness : surface, so that a complete ring of secondary vascular tissue is formed : in most cases secondary vascular tissue is formed Only Opposite to wood-bundles. (Slightly magnified; the primary bast-bundles, whereas, ' opposite to the primary wood-bundles, the cambium produces only ground-tissue, thus giving rise to broad medullary rays opposite to these bundles (Fig. 110). FlG. no.-^ Transverse sectiou of « young root of Phaseoius muiti/ion** .• secondary bast ; k periderm : the four rays ex- tendin& to near the centre consist of secondary ground-tissue, and cor- respond in position to the primary 140 PART II. — ANATOMY AND HISTOLOGY. [§ 35 The Tissues developed from the Cambium. — In stems and roots in which the growth in thickness is normal, the cambium gives rise to secondary wood, secondary bast, and secondary conjunctive tissue (medullary rays). The structure of the secondary wood differs essentially from that of the primary wood only in that it includes no_ spjirad or annular vessels resembling those of the protoxylem (see p. 126). It always includes tracheal tissue ; nearly always wood-parenchyma (see p. 91); frequently sclerenchyma: the cell-walls of all these forms of tissue are usually more or less completely lignified. The secondary tracheal tissue may consist either solely of tracheae (e.g. Platanus, Fraxinus excelsior and Ornus, Citrus, Viscum, Hydrangea) ; or solely of tracheids (e.g. Coniferse, Drimys Winter I) ; or, as is generally the case, of both tracheae and tracheids. The cell-walls of the tracheal tissue are, as a rule, marked with bordered pits; but occasionally, especially in soft wood, the walls are reticulately thickened. The secondary icood-parenchyma consists of oblong cells, which are generally so arranged that their long axes are parallel to that of the member of which they form part : they occur in short longitudinal strands, consisting commonly of a single row of cells (Fig. Ill (7), but sometimes, in the middle only, of more than one row. They are true cells, containing protoplasm and a nucleus, and other substances, such as starch (especially in perennial steins and roots in winter), tannin, etc. Their walls are generally lignified, but usually not very much thickened, and have .circular or elliptical simple pits. In many soft fleshy stems ._ and_jLQOts (e.g. Potato, Radish, Turnip, Beetroot), where this tissue is the principal product of the activity of the cambium, the cell- walls are not lignified. The secondary sclerenchyma consists of elongated prqsenchy- matous cells, with more or less thickened lignified Avails marked Avith narrow oblique bordered pits (Fig. 72, p. 93; Fig. Ill A, B). Two forms of this tissue are distinguishable: woody fibres destitute of protoplasmic contents, Avhich are connected by transitional forms with the tracheids (see p. 92) : fibrous cells, with protoplasmic cell-contents, which are allied to the wood-parenchyma ; in fact, one fibrous cell corresponds to a row of wood-parenchyma cells ; the walls of the fibrous cells sometimes remain thin, as in Viscum and some other plants, Avhere they replace the Avood-parenchyma both structurally and functionally. 35] CHAPTER II. — THE TISSUES. 141 Both the woody fibres and the thick-walled fibrous cells may eventually become chambered by the formation of delicate transverse septa. The structure of the secondary wood of the root is, in some cases (e.g. Conifers), almost identical with that of the corresponding stem ; this is the case, to a somewhat less degree, in woody Dicotyledons ; whilst in herbaceous Dicotyledons the structure may be very different in the two members, owing, FIG. 111.— Isolated constituents of the secondary wood of the Lime (Tilia parvifolia). A and B wood-fibres ; C wood-parenchyma ; D and E tracheids ; F segment of a wood- vessel (trachea). G is a bast-fibre. ( x 180 : after Stras- burger.) chiefly, to the development of more medullary ray, but less woody tissue, in the root (see above p. 139). A transverse section of a stem or a root of most coni- ferous or dicotyledonous trees or shrubs exhibits, even to the naked eye, a series of concentric layers in the secondary wood known as the annual rings (Fig. 112). These layers result from the fact that the wood formed in the spring is differently constituted from that which is formed 142 PART II.— ANATOMY AND HISTOLOGY. [§35 later in the year. The anatomical cause of the distinctness of the annual rings is the same in all cases, namely, that thejast- formed xylem-elements of an annual ring have a very small radial diameter as compared with those formed when growth is resumed in the following spring. In Conifers this distinction is emphasized by the fact that the spring-wood is formed of thin-walled tracheids (Fig. 113 /") and the autumn-wood of smaller thick- walled tracheids (Fig. 113 fi). In dicotyledonous trees the num- ber and size of the vessels diminishes in each annual ring from its inner to its outer limit. When this takes place very gradually, the eye cannot detect any conspicuous difference between the spring- and autumn-wood (as in the wood of the Beech, Lime, Maple, and Walnut) ; but some kinds of wood show a ring of FIG. 112.— Part of a transverse section of a twig of the Lime, four years old (slightly magnified) : m pith ; ms medullary sheath ; x secondary wood ; 1 2 3 4 annual rings; c cam- bium ; pa dilated outer ends of primary medul- ary rays ; b bast ; pr primary cortex ; fc cork. FIG. 113. — Transverse section of por- tion of the secondary wood of a branch of the Fir at the junction of two annual rings : m a medullary ray — all the other cells belong to the wood ; / large-celled spring- wood ; ih small-celled autumn- wood; w the limit between the autumn- wood of one year and the spring-wood of the following year ; between h and w is the flattened limiting layer ( x 250) . conspicuously large vessels in the spring-wood, while in the autumn-wood there are numerous much smaller vessels (as in the wood of the Oak, Elm, and Ash). The thickness of the annual ring varies in different plants, and even in any one plant, under different conditions of growth : and not only the thickness, but also the number and relative distribu- tion of the constituents of the wood. The secondary wood gradually becomes distinguishable into an § 35] CHAPTER II.— THE TISSUES 143 older internal portion, the heart-wood (duramen\ and a younger outer portion, the sap-wood (alburnum). This arises from the fact that^ as the wood becomes older, the cells of the wood-parenchyma and the fibrous cells die and lose their protoplasmic cell-contents ; as a consequence, the heart-wood has less water, in its composition than the sap-wood. In some cases this change is accompanied by a coloration of the cell-walls of the heart-wood, with the result that the distinction of duramen and alburnum is most marked (e.g. Pine, Larch, Oak) ; it is but rarely that this distinction is not observable (e.g. Buxus, Acer pseudoplatanus and platanoides). The structure of the secondary bast essentially resembles that of the primary bast. It always consists of sieve-tubes and of paren- chyma, and very frequently of thick-walled fibres as well. The sieve-tubes of the secondary bast have the compound sieve- plates shown in Fig. 75, p. 96 ; in Dicotyledons they have com- panion-cells developed in relation with them. The parenchyma very much resembles that of the secondary wood, except that its cell-walls are not lignified ; it is abundantly developed in certain fleshy roots (e.g. Taraxacum, Rubia, and the Carrot and Parsnip), where it constitutes the chief part of the secondary bast. Prosen- chymatous cells with unlignified walls, corresponding to the thin- walled fibrous cells of the secondary wood (p. 140), are sometimes present. The bast-fibres closely resemble the woody fibres, but their walls are not lignified (Fig. Ill G). In many cases the secondary bast contains no bast -fibres (e.g. Abietinese, Fagus, Betula, Alnus, Platanus, Cornus, Ephedra, etc.). When, as is usually the case, bast-fibres are present, their arrange- ment presents considerable variety : there may be alternating tan- gential layers of fibres (hard bast) and of sieve-tubes and paren- chyma (soft bast), as in the case of the Cupressinese and some Taxoidese, and, though with less regularity, in many Dicotyledons (e.g. Vitis, Spiraea, species of Acer, Tilia, species of Salix, etc.) ; more commonly the tangential layers of fibres are interrupted here and there by soft bast (e.g. Quercus, Corylus, Carpinus, Pyrus, Juglans, Sambucus, Rhamnus, Ulmus, Populus) ; or there may be scattered groups of fibres (e.g. Cinchona, Morus, Larix, Celtis Ficus elastica). The secondary bast does not, as a rule, attain so considerable a size as the secondary wood, nor does it exhibit annual rings : this is due to the fact that, except in some fleshy roots, it is formed in smaller quantity, and further, to the fact that the older bast be- 144 PART II. — ANATOMY AND HISTOLOGY. [§35 comes crusted and flattened by the development of the more inter- nal layers subsequently formed. The structure of the secondary conjunctive tissue (medullary rays). The cambium-ring not only adds to the existing primary medullary rays, but gives rise to new (secondary, tertiary} rays in the successive years of growth (see Fig. 112), amongst the vascular tissue. The cells of the medullary rays are typically parenchymatous, somewhat brick-shaped, with their long axes along radii from the FIG. 114.— Radial longitudinal section of the wood of the stem of a Pine, along the length of a medullary ray q p q, consisting of six horizontal rows of cells, one above the other : — t tracheids with bordered pits ; the tracheids h with smaller bordered pits are the autumn- wood of one year, those to the right with larger pits constitute the spring-wood of the next year ; q tracheidal elements of the medullary ray ; p true cells of the ray : where the cells of the medullary ray abut on the tracheids the pits are simple and large ( x 300). centre to the periphery of the member (Fig. 112); their more or Irss thickened walls are lignified, and they have protoplasmic eon- tents. Occasionally, however, some of the cells of a ray lose their protoplasmic contents and constitute tracheids (e.g. Abietinese, Fig. 114 q q) ; in some few cases the ray consists of long fibrous cells, in place of parenchyma (e.g. shrubby Begonias^. The medullary ray is, then, a strand of cells p_assing_radially § 35] CHAPTER II.— THE TISSUES. 145 among the longitudinally arranged tissues of the wood and of the bast (Fig. 112). Its size varies, even in the same member, both as regards its vertical (height) and its lateral (breadth) dimen- sions. With regard to the former, the ray may consist of only a single row of cells (as in Abietinese, Quercus, Fagus) ; the limits may be generally stated at 1—12 rows of cells, though in some cases they are considerably larger than this when they include resin-ducts (e.g. Abietinese) or other forms of secretory tissue. In any case, the secondary medullary rays, unlike the primary, do not extend throughout the whole length of an internode. The breadth of the secondary medullary rays is never nearly so great as their height : as seen in tangential longitudinal section, they are narrow above and below and broader in the middle ; it is only in the middle that they ever consist of more than one row of cells in breadth, the upper and lower margins consisting of a single row only. With regard to their radial extent, it is only the primary medullary rays which extend from pith to pericyde; the Subse- FjG. ns:.Diagrammatic repre- quently formed rays (secondary, ter- sentation of the course of the tiary, etc.) extend between the wood medulla7 ™ys in a segment cut , . out of the wood of a tree-trunk: and the bast of the year in which they Q horizontal surface; R radial were formed. surface; T tangential (external) surface of the wood; the shaded AS instances of especially large portions m are the medullary rays. secondary medullary rays should be mentioned those formed in roots (see Pig. 110, p. 139) where the cambium forms only conjunctive tissue opposite the primary xylem-bundles. Again, in some few stems (see p. 139) the forma- tion of secondary vascular tissue^ is confined to the f ascicular cam- bium> the interfascicular cambium in the primary medullary rays giving rise only to conjunctive tissue; hence the primary medul- lary rays persist as broad bands of conjunctive tissue between the bundles, and are not broken up, as is usually the case, by the formation of secondary bundles from the interfascicular cambium. The Differentiation of the Secondary Tissues. — The cells, formed as the result of division in the cambium, which are to become transformed into secondary permanent tissue have, to begin with, the same form and structure as the corresponding cambium- cells, but they gradually under- go^changes in both respects, as they become transformed into permanent tissue. 146 PART II. — ANATOMY AND HISTOLOGY. [§ 35 The development of the young cell into one or other of the various forms of permanent tissue already described, may be either accompanied or unaccompanied by cell-division. In the former case, the divisions may be transverse or longitudinal ; the cell undergoes transverse division when the product is a row of short cells (e.g. wood- parenchyma, Fig. 116 Z>, and Fig. Ill C: bast-parenchyma ; secondary medullary rays, wood-vessels with short segments) : the cell generally undergoes longitudinal division once or twice, by tangential walls, soon after it has been cut off from the cambium ; but this does not take place in the line of the medullary rays, where the radial diameter of the young cells is greater than it is near the bast or the wood : again, the young cell may undergo longitudinal division in a plane other than the tan- gential, as for instance the longitudinal division of the mother-cell, which separ- ates the sieve-tube-segment from the com- panion-cell in the bast of Angiosperms. The developing cell may retain its original form and size (e.g. small medul- lary rays ; rows of parenchyma-cells, bast or wood ; thin-walled fibrous cells) : but more commonly the mature product differs very materially from the young cell, being very much wider (e.g. tracheae), or very much longer longitudinally (wood- and bast- fibres), or very much longer radially (e.g. cells of medullary ray) ; that is to say, the development of the young cell into permanent tissue is generally accompanied by very considerable growth. The radial and tangential divisions of the cambium-cells take place in such a manner that the products are, at first, arranged in very definite radial rows. When the resulting tissue consists of ele- ments which are for the most part essentially alike, this regular radial arrangement persists in the permanent tissue ; for instance, in the wood of Conifers (Fig. 113), which consists almost exclu- FIG. 116. — A Developing vascular cells, derived from the cambium, seen in tan- gential section. JB Tracheid seen from outside. C Woody fibre ; and J) vertical row of wood-parenchyma-cells seen in section, from the Oak ; isolated by maceration. § 35] CHAPTER II.— THE TISSUES. 147 sively of tracheids ; but where some of the elements (as generally in the wood of Dicotyledons) attain a much greater size (as seen in transverse section, Fig. 105), the original radial arrangement is lost. In those cases in which the permanent tissues consist of very long or very wide fibres or vessels, it is evident that the relative position of the original cells must have undergone considerable change in the course of development ; the long fibre is in contact, longitudinally, with a greater number of cells than was originally the case ; and similarly, the wide trachea touches, at its circum- ference, a larger number of cells than did the cell, originally, from which the segment of the vessel was developed. This gradual change of relative position constitutes what is termed sliding- groicth ; it is the expression of the independent growth of each cell in the course of its development into the particular element of the permanent tissue which it is destined to form. This process is by no means confined to the vascular tissues, but takes place wherever a young developing cell grows more actively, in any dimension, than the cells with which it is at first in contact ; a notable example is the growth of the laticiferous coenocytes of Euphorbia (see p. 100). • Whilst undergoing these changes of form, the developing cells undergo, as already indicated, changes in the structure and chemical composition of their cell-walls in accordance with the particular kind of tissue to which they are to give rise ; and, in some cases (tracheae, tracheids, fibres) they lose their protoplas- mic cell-contents ; the walls become more or less thickened, not spiral or annular as in primary wood, but pitted (with simple pits ; or circular bordered pits ; or oval bordered pits, either small and numerous, or large extending across a whole face of the wall, giving it a scalariform appearance, see p. 74) ; and then the absorption, more or less complete, of the septa takes place, which leads to the formation of the vessels. Glandular tissue is frequently developed in the secondary wood and bast, in the form, sometimes, of sacs containing crystals, in the paren- chyma (including medullary rays) of the wood (e.g. Vitis, and some leguminous trees) or more commonly in that of the bast : of resin-ducts which occur in the secondary wood of certain Abietineae, running hori- zontally in the medullary rays and vertically in the wood, but rarely found in the secondary bast, whereas in other plants which possess these structures, they are rare in the wood but abundant in the bast (e.g. Anacardiaceae, etc.) : of laticiferous vessels, rare in the wood (except the 148 PART II. — ANATOMY AND HISTOLOGY. 35 Papayacese, where the wood consists largely of parenchyma), abundant in the bast. In Monocotyledons there is no primary cambium-layer, the bundles being all closed. In some cases, however, secondary growth in thickness is effected by a ring of meristem quite external to the primary bundles; this occurs in the stems and roots of some arborescent Liliaceae, such as Yucca and Dracaena, where a rinS of meristem is usually developed in the pericycle, but in the roots of Dracaena it is formed partly from the pericycle and partly from the cortex. This meristem-ring is not termed a cambium- ring, because it does not form wood on one side and bast on the other, but it forms, centrifugal ly, entire closed concentric (with external ^j/ wood) bundles, together with intervening ground-tissue (Fig. 117). The development of secondary vascular tissue takes place almost exclu- sively in such stems as are monostelic and in which the primary bundles are common. It is clear that the additions to the prim- ary bundles in the older internodes of the stem, as well as any secondary bundles which may have been formed from the cam- bium are not common, but cauline ; they are, however, in communi- cation with the primary common bundles of the young unthickened internodes which are bearing leaves ; in fact, the newly-formed secondary vascular tissue of the lower internodes of the stem is in communication, on the one hand with the root, and on the other with the leaves ; and the channels of communication between root and leaf are maintained year by year by the annual formation of young conductingrtissue, both wood and bast, in the older parts of the stem and of the root. F:o. 117.— Portion of a transverse section of the stem of a Dracaena: e epidermis; k periderm ; r primary cortex, with a leaf-trace-bundle b; x merismatic zone in which new bundles g-g are in course of development ; m primary, and st second- ary, ground tissue. (Magnified : after Sachs.) § 35] CHAPTER II.— THE TISSUES. 149 It will be remarked that the development of secondary vascular tissue takes place in those plants the stems of which branch more or less (e.g. an Oak), while it usually does not take place in those plants the stems of which do not branch (e.g. the Palm), or do so only slightly. It is obvious that, wlien the ste'm is of branching habit, the number of leaves must increase year by year, whereas when the stem does not branch the number of leaves does not vary materially. Hence the whole matter may be summed up thus, that the development of secondary vascular tissue in a stem is directly correlated with an increase in the area of leaf -surf ace : as in each year the leaf-surface of a tree increases in consequence of repeated branching, so does the annual ring of secondary vascular tissue become larger in circumference and possibly also of greater thickness ; when, however, the tree begins to grow old, and its branches, instead of increasing in number, begin to die off, then the annual growth in thickness becomes arrested. Some further explanation of this is given in Part III. B. The formation of Secondary Tegumentary and Cortical Tissue. It is clear that the more or less considerable development of secondary tissue in the interior of a young stem or root, must have a very considerable effect on the primary cortex, and on the primary tegumentary tissue. This effect will be one of pressure and tension ; the radial growth of the stelar tissue will exert a radial pressure upon the external tissues, while the tangential growth of the stelar tissue will exert a tangential tension on the external tissues. The radial pressure of so firm a structure as is usually that of the secondary vascular tissue tends to cause more' or less rapid obliteration of the softer cortical tissue ; whilst the tangential tension stretches the cortical cells and tends to cause them to grow tangentially, and to multiply by radial division. According to the predominance of the radial pressure or of the tangential tension, the primary cortex is either rapidly destroyed, or it persists for a very considerable period. It may be stated generally that the epidermis and the primary cortical tissue of herbaceous dicotyledonous stems keep pace by growth with the formation of new tissue in the interior. This is- true also of most woody shoots during the first year of their- growth and in certain cases (e.g. Mistletoe, Holly, Acer striatuirij- etc.) of woody shoots during their entire existence ; in some cases (e.g. Euonymus) the epidermis persists and grows for several years, but is at length disorganised. These primary tissues per- 150 PART II. — ANATOMY AND HISTOLOGY. [§ 35 8\st also in some roots (e.g. Vicia Faba, Alchemilla vulgar is), in which the development of secondary vascular tissue is not very active. The extension of the tissue is effected by tangential growth and radial division of the cells. The secondary tegumentary and cortical tissue is formed by a layer of merismatic cells, which is known as the Phellogen. In fhc stem the place of origin of the phellogen is by no means uniform. It is sometimes formed by the epidermis becoming mer- ismatic (e.g. Pomese, Salix, Viburnum Lantanctj Euonyrnus, So- larium, etc.) ; most commonly it is the hypodermal layer of cells, the outermost layer of the cortex, which becomes merismatic and constitutes the phellogen (e.g. Platanus, Acer, Fagus, Quercus, Castanea, Betula, Alnus, Ulmus, Populus, Ailanthus, Abies pec- tinata, etc.) : in other cases the phellogen is formed at a greater depth from the surface, being developed from a more internal layer of cells of the cortex, sometimes even from the endodennis (e.g. Cojfea ctrabica ; subterranean shoots of some LeguminosaB such as Lotus corniculcttus, Trifolium alpestre) ; or, finally, it is stelar, being formed from a layer of cells belonging to the peri- cycle (e.g. Hypericum, Erica, most Caryophyllacese, Lonicera, Vitis, Clematis, Berberis, Rosa, Spiraea, Ribes, etc.). The development of tissue from the phellogen follows the same law as in the case of the cambium. Generally speaking, a tissue, the periderm, is formed on the outer side of the phel- logen by repeated centripetal division ; whilst on the inside of the phellogen a tissue, the phettoderm, is formed by repeated centrifugal division. The periderm constitutes the secondary tegumcntaiy tissue of the stem or root ; the phelloderm consti- tutes the secondary cortex. The developmental relations between the two tissues are not constant. In some cases the formation of phelloderm only begins after a considerable mass of periderm has already been produced ; but in others, the formation of the two tissues goes on almost simultaneously. The relation between the amount of periderm and the amount of phelloderm formed by one and the same phellogen is by no means uniform : whilst the development of periderm is most marked in subaerial stems with superficial phellogen, there is little or no phelloderm in these stems ; again, in subaerial stems with a deeply-placed (e.g. pericyclic) phellogen, periderm and phelloderm are developed about equally ; finally, in subterranean stems with a pericyclic phellogen, the well-developed phelloderm may exceed the periderm. § 35] CHAPTER II.— THE TISSUES. 151 In the root, as in the stem, the position of the phellogen, and the products of its activity, are various. The phellogen is developed but rarely (e.g. Solidago) from the epiblem ; more commonly from the exodermis, or from the next internal layer of the primary cortex, as in a few woody Dicotyledons (e.g. Jasminum) in which the formation of secondary vascular tissue takes place relatively late ; and in the Cycads among Gymnosperms. In the great ma- jority of Dicotyledons and Gymnosperms the phellogen of the root is stelar in origin, being derived from the pericycle. As in the stem, so in the root, the phelloderm is more highly developed when the phellogen is deeply placed than when it is superficial ; but even with a uniform position of the phellogen, the relation between the periderm and the phelloderm developed, varies considerably : thus, among plants with a pericyclic phellogen, whilst the development of periderm and phelloderm is sometimes about equal (e.g. Willow), no phelloderm but only periderm is developed in Nerium, whilst in some others (e.g. Vicia Faba, Alcliemilla vulgaris), where the primary cortex persists (see p. 149), only phelloderm is developed. It frequently happens in both stems and roots that the first- formed primary phellogen has but a limited period of merismatic activity ; this is always the case when the primary phellogen is of deep origin (pericyclic in roots), whereas when it is of superficial origin (e.g. epidermal or hypodermal phellogen in stem of Beech, Hornbeam, Silver Fir, Cork-Oak, Cork -Elm), the primary phellogen is frequently persistent. In the former case, however, when the primary phellogen has passed over into some form of permanent tissue, a new secondary phellogen, also of limited duration, is developed internally to the first, and this process is repeated at intervals ; hence the phellogen-layers become successively more and more deeply seated, penetrating at length into the bast-tissue of the stele. The periderm, or secondary tegumentary tissue, the tissue formed externally from the phellogen, consists of parenchymatous cells more or less cubical in form, though sometimes somewhat elongated tangentially (Fig. 118) ; the cell-walls may be thin or considerably thickened ; generally speaking, the walls are completely suberised (see p. 76), whence the tissue is often termed Cork; the cells gradually lose their protoplasmic contents, and become filled with air ; moreover, no intercellular spaces are formed in the tissue. In view of its structure, it is clear that the periderm is a tissue 152 PART II.— ANATOMY AND HISTOLOGY. 35 which offers an obstacle to the passage of water ; hence all the tissues, in a stem or root, lying externally to the periderm can receive no supplies of water, and must dry up, and are eventually exfoli- ated. The more deeply seated the phellogen, the greater is the amount of primary tissue thrown off; thus, when the phellogen arises in the inner layers of a heterogenous pericycle (see p. 119), as in Berberis, Lonicera, etc., where the outer portion of the pericycle is fibrous, the epidermis, the primary cortex, and the outer portion of the pericycle are exfoliated. The cells of the periderm are not always completely suberised. In some cases (roots and stems of Onagracese, Hypericacese, some Rosacese, etc.) some layers of the peri- derm consist of cells with a suberised zone like that of the cells of the endodermis (see p. 116), though these cells usually become completely suberised eventually. In other cases (e.g. stem of Poteriiim, Alchemilla, Agri- monia, Epilobium) the periderm con- sists mainly of cells with cellulose-walls, FIG. 118.-Periderm of one-year's shoot of Ailanthus V0+™PAT1 whir}i m- glandulosa (trans, sect. ; x 350) : e the dead epidermis ; k cork ; D the inner shaded layers are merismatic, the innermost being tercellular Spaces are the phellogen, those external to it being young periderm fnrTnp^ together cells; r primary cortex. with occasional com- pact layers of cells with a suberised zone. When the' primary periderm is of superficial origin, it forms for many successive years the external investment of the branch ; it may attain considerable thickness, as in the Cork-Oak, and at the same time exhibit an alternation of dense and loose layers (e.g. the Birch, in which the layers may be peeled off in thin white sheets) ; sometimes (as in Acer campestre and the Cork-Elm) it forms wing- like projections from the angles of the branches. In a few trees, as the Silver Fir, the primary periderm persists for some years, or, as 35] CHAPTER II.— THE TISSUES. 153 in the Beech, during the whole life of the tree ; the outer cork-cells split off as the trunk of the tree increases in thickness, while the phellogen, growing and extending in a tangential direction, gives rise to new layers of cork. When, as in most cases, new layers of phellogen arise after a few years in the deeper -tissues, leading to the development of corresponding layers of periderm, an external investment of a more or less complicated structure comes to be formed. In consequence of the impermeability to water of these secondary layers of periderm, all the tissues lying externally to them become dried up. These dried-tip tissues, which may belong to different tissue- systems and include the most various forms of cells, constitute what is known as Bark. When the primary periderm is superficial, the new secondary layers of periderm are only arcs of the circumference, and as their margins- are in contact with the periderm which has been previously formed (Fig. 119), a scaly bark is formed, that is, isolated patches of tissue are transformed into bark. This bark is stretched and torn by the increasing size of the trunk, and the scales of it may be shed, as is the case in the Plane, or they may adhere one upon the other, as in the Pines and Larches, or remain connected by the bast-fibres in long strips as in Robinia. When, on the other hand, the primary periderm has been formed in the deeper layers of the cortex, the secondary periderm often forms complete concentric rings ; thus hollow cylinders of the cortex are transformed into bark (ringed bark). The longitudinal rupture of this kind of bark is effected by the bast-fibres enclosed in it (e.g. Vine, Clematis, and Thuja). There are frequently in the periderm of both stems and roots Fig; 119.— Formation of Scaly Bark in a Larch, as seen in a piece of the outer portion of the stem cut both transversely and longitudinally (nat. size) ; r the secondary cortex ; fe plates of cork ; b the scales of bark cut off by the cork. 154 PART II. — ANATOMY AXD HISTOLOGY. [§ 35 organs corresponding to the stomata of the epidermis, serving, like them, to admit air to the living internal tissues ; these are the Lenticels. They are usually circumscribed circular areas of the periderm where the cork-cells formed in the course of the summer are not arranged closely together, but are separated by intercellular spaces. In winter the lenticels are closed by a layer of ordinary periderm. They are most easily detected in branches of one year's growth, where they are to be seen in the summer in the form of brownish or whitish specks. When the periderm of the stem is superficial, the lenticels are developed under the places where the stomata occur in the epidermis, and these spots are commonly the starting-points of the formation of the periderm; but this is not the case in stems with a deep periderm, nor is it ever the case in roots. In many trees, as the Birch, the lenticels become much extended in width by the growth of the branch in circumference. When the periderm is very thick, as in the Cork-Oak, the len- ticels form deep canals filled with a pulverulent FIG. 120.-Lenticel in the transverse section of a ™aSS of cells. Sometimes twig of Eider (x 300): c epidermis; q pheiiogen; i lenticels are not formed ; cells, and pi the pheiiogen of the lenticel ; Ic cortical , , , parenchyma containing chlorophyll. they are not PreSent m the stems of some plants which have a pericyclic pheiiogen (e.g. Vitis, Clematis, Rubus, Lonicera). The phcllodcrm or secondary cortical tissue, the tissue formed internally from the pheiiogen, consists of cells which have essentially the same structure as those of the primary cortex : the secondary cortex can, however, be distinguished from the primary by the regular radial rows in which, like those of the periderm, its cells are arranged. The cells have protoplasmic cell-contents, and, when developed near the surface of aerial stems, they contain chloroplastids ; their walls are usually thin and consist of cellulose, but, like those of the cells of the primary cortex, they may become more or less thickened and eventually lignified. Just as the periderm replaces the disorganised epidermis as a tegumentary tissue, sa the phelloderrn replaces the primary cortex as a nutritive (metabolic) tissue when the primary cortex becomes obliterated under the conditions explained on p. 149. §3G] CHAPTER II.— THE TISSUES. 155 § 36. Formation of Tissue in consequence of Injury. When the internal tissues of most parts of plants are laid bare by injury, they are gradually covered by a formation of cork taking place in the outermost layer of cells which remain uninjured and capable of growth. This is easily seen in injured fruits, leaves, and herbaceous stems, in which the wounds that have been covered by a layer of cork are distinguished by a grey-brown colour. The process is very easy to observe in potato-tubers, for each portion of living tissue taken from one, if only prevented from drying too quickly, will soon be covered over the whole surface by a layer of cork precisely similar in structure to the ordinary rind. In plants in which the wood is well developed, cork is not immediately formed — particularly when the cam- bium is wounded or laid bare — but all the living cells which border on the wound become merismatic and give rise to a homogenous j>aren- chymatous tissue known as tlic Callus. If the wound is small, the callus-cells pro- ceeding from the different sides soon come into contact and close up into a single mass of tissue, which then gives rise to cork on its outer surface, and, joining the old cambium at the margins, forms a new layer of cambium which fills up the cavity. If the wound is a large one, cork and new cambium are formed in the callus at the margins of the wound, and it is not wholly closed till after repeated rupture of the approach- ing cushions of callus. The wood exposed by the wound, which usually assumes a dark colour tinder the influence of the air, does not grow with that formed from the new cambium of the callus : hence inscriptions, for instance, which are cut in the cortex so as to reach the wood, though subsequently covered by a number of annual layers of wood corresponding to the number of years, FIG. 131. — Diagrammatic longitudinal section of a woody stem : A a short time after t.£e amputation of a lateral branch s; JB when the wound is completely closed ; r cortex ; c cam- bium ; 7i wood ; c' position of the cambium- layer at the time of amputation ; fc' wood formed since the amputation ; w the cushion of callus formed over the surface of the wound. 156 PART II.— ANATOMY AND HISTOLOGY. [§ 36 may easily be found. A similar explanation accounts for the fact that the surfaces of the stumps of cut-off branches become over- grown : the callus first appears as a ring from the cambium ex- posed in the transverse section, and afterwards closes like a cap over the old wood (Fig. 121). Foreign bodies — nails, stones, and stems of other plants — may thus become enclosed in the wood of a tree and be over-grown by it ; the cortex, being forced against the foreign object by the pressure of the growing wood, splits, and the callus formed in the rent grows round the object, enclosing it and producing new cambium. Steins of plants of the same species will grow together if they are in close contact : the callus formed by the cortex of both, coalesces and gives rise to a common cambium. On this depend the various modes of artificial grafting, in which branches orjbuds with a portion of the cortex are taken from a variety or an allied species and placed so that their cambium is in contact with that of a stem which serves as the stock, and subsequently they grow together. In conclusion, the mechanism by which deciduous members (see p. 10). are detached has to be considered : the fall of the foliage- leaf may be taken as the illustration. In some cases (e.g. Palms ; some Ferns, as in the section Phegopteris ; the Oak) the leaves simply wither on the stem, when they are non-articulated, and are gradually destroyed and removed ; but in most cases they are thrown off by a vital act before they wither, when they are said to be articulated. The fall of the articulated leaf depends upon the growth and division of all the living cells lying in a trans- verse layer near its insertion ; by this means several (3 — 6) layers of compact tissue are formed. A median layer of this tissue, the abscission-layer, becomes disorganised, and then the leaf is held in position only by the vascular tissue which enters it from the stem. This soon breaks under the weight of the lamina, especially if it be agitated by the wind, and the leaf falls. The disorgan- isation of the median layer is often accelerated by the action of -frost. The scar on the stem (leaf-scar, p. 10) either simply dries up, or a layer of cork is formed over it by the merismatic tissue which remains : in any case the vessels become sealed with mu- cilage. PART III PHYSIOLOGY § 37. Introductory. The province of physiology is the study of those phenomena which, taken together, constitute the life of the plant ; in other words, whilst morphology is concerned with what plants arc, and histology with their structure, physiology deals with what they do. These phenomena may be classified, according to their nature, into functions, or different kinds of physiological work. The body of the plant, whether it be unicellular or multicellular, is one physiological whole. In the lower and simpler plants the various functions are equally discharged by all parts of the body ; but in more highly-organised plants the functions are distributed among the members and tissues, that is, there is physiological division of labour. In these higher plants each member, and each tissue, is adapted to the performance of one or more functions, and is the organ (p. 1) by which these special kinds of physio- logical work are done. The performance of their functions by the organs of the plant is, however, materially affected by various external conditions. For instance, the activity of the assimilatory function of green leaves is altogether dependent upon exposure to light of adequate intensit3r. Hence the object of physiology is not only to distinguish and study the various functions, and to demonstrate the relation between them and the internal structure and the external form of the organs per- forming them, but also to determine what are the external conditions by which the performance of the various functions is affected, and the modes in which these conditions exert their influence. CHAPTER I GENERAL PHYSIOLOGY § 38. The Functions. In entering upon the consideration of the vital phenomena of plants, it must be clearly understood that these phenomena all depend upon the living protoplasm ; that the 157 158 PART III.— PHYSIOLOGY. [§ 38 vital functions are performed by the protoplasm, though the other cell-contents and the cell-walls are not without their physiological importance. With regard to the functions themselves, it is apparent, in the first place, that the outcome of the physiological activity of the plant is the maintenance of itself, and the production of new individuals resembling itself. Hence a distinction may at once be drawn between the nutritive and the reproductive properties of protoplasm. Moreover, during its life, the plant responds, in a more or less marked manner, to the action of external forces, such as light, gravity, etc. This is a manifestation of another property of the protoplasm, namely irritability or sensitiveness. Very commonly the response to the action of the external forces is of the nature of movement ; but movements may also be performed spontaneously. It is clear that nutrition necessarily depends upon the absorption of food from without ; hence the plant is capable of performing the function of absorption. From the food absorbed, protoplasm is ultimately formed; the building up of protoplasm out of the food is termed assimilation, and the property by means of which this function is performed is termed the metabolic property of proto- plasm. But the metabolic processes going on in the protoplasm are not only such as lead to its maintenance or increase in bulk ; on the contrary, the protoplasm is continually undergoing decomposition. It is to be clearly apprehended that there are two sets of chemical processes continually and simultaneously going on in living proto- plasm. Of these, which together constitute the metabolism of the plant, one set includes those processes which lead to the formation of more complex substances from simpler ones ; the other, those processes which lead to the formation of simpler substances by the decomposition of more complex ones. The former are designated the constructive metabolism, or more shortly the anabolism, of the protoplasm ; the latter are designated the destructive metabolism, or the catabolism, of the protoplasm. It must also be clearly understood that these two sets of processes affect not only the state of the matter or substance of which the plant consists, but also the state of the energy in the plant ; for the anabolism is accompanied by a conversion of kinetic into potential or latent energy ; and the catabolism, by a conversion of potential into kinetic energy. The products of metabolism may be classified as plastic products and waste-products : the former are such as can be further worked up in anabolism ; the latter are not so used, but are withdrawn § 39] CHAPTER I. — GENERAL PHYSIOLOGY. 159 from the sphere of the metabolic activity, by being either excreted, 6r~~iecreted in the insoluble form in special receptacles (see p. 96}7~ Of the products of catabolism, carbon dioxide is' the most constant. There is one property of living plant-cells which is of such funda- mental importance, particularly in connexion with movement, that it requires special mention. It is this, that the cells tend to take up such large quantities of water, that a considerable pressure is set up in the cell between the cell-sap, on the one hand, and the elastic cell-wall on the other. This state of tension is known as iuryidity or turgescence, and a cell in this state is said to be turgid. The conditions upon which turgidity depends are three : first, the presence of substances in the cell-sap which attract water ; second, the presence of a layer of protoplasm lining the cell- wall ; third, the presence of an elastic cell-wall. With regard to the first of these conditions, the necessity for it is obvious. It appears that the substances in question are especially the organic acids or acid salts, which are abundantly produced in the metabolism of plants. The significance of the second condition is, that the layer of protoplasm prevents, at least within certain limits, the escape of the cell-sap as the pressure in the cell increases, and it is on this account that a high tension can be attained. Finally, the presence of an elastic cell-wall is a necessary factor, for without resistance there can be no tension . § 39. The External Conditions. The functions of the plant can only be carried on under a certain coincidence of favourable external conditions. Thus, an ordinary green plant will onlv flourish when the conditions are such that it is supplied with appropriate food, with water, and with free oxygen for its respir- ation, and is exposed to a suitable temperature and to sufficiently intense light. The importance of a supply of food and of water is sufficiently obvious to need no further explanation here. The importance of a supply of oxygen is that without it the normal catabolic processes would either cease, or be SD far suppressed that the plant would no longer manifest its vital phenomena ; for instance, it would cease to grow, and would eventually die. Inasmuch as the influence of heat and light is so comprehensive, it may be generally considered now, the detailed consideration of these and other external conditions being relegated to the discussion of the functions which they especially affect. 160 PART III.— PHYSIOLOGY. [§ 39 HEAT. Every function of the plant can only take place within certain limits of temperature : that is, between a certain minimum and a certain maximum degree. Between these limits there Js for each function a degree of temperature, the optimum, at which that function is carried on with the greatest activity ; any fall of temperature from this optimum, or any rise above it, leads to a diminished activity of the function. These general laws have been arrived at by observation of such processes as movement, absorp- tion by the roots, assimilation, etc. It may be stated generally that all the functions of plants inhabiting temperate climates begin to be carried on at a tempera- ture a few degrees above the freezing-point ; as the temperature rises to 25°-30° C. the activity of the functions is increased and the optimum attained ; with a further rise the activity of the functions is diminished, and at 45°-50° C. they commonly cease altogether. In the case of plants which naturally grow in warmer climates, the minimum-temperature is somewhat higher than that stated above. Thus a pumpkin-seed will not germinate at a temperature below 13° C. The power of withstanding the injurious effect of exposure to too high a temperature depends mainly upon the proportion of water which the plant, or any particular part of it, contains. Thus, dry peas can withstand exposure for an hour to any temperature up to 70° C., whereas, when they have been soaked in water, exposure to a temperature of 54° C. proves fatal. Most parts of plants are killed by prolonged exposure to a temperature, in air, of about 50° C., and in water, of about 45° C. Injury or death by exposure to cold, is only induced when the temperature falls — in some cases many degrees— below freezing- point. Some plants — just those, namely, such as Lichens, and some Fungi and Mosses, which can undergo dessication without injury — are not killed by exposure to low temperature. Here, also, the liability to injury depends upon the amount of water contained in the tissue. Thus, dry seeds and the winter-buds of trees can readily withstand low temperatures ; but when they contain a considerable quantity of water, as when the seeds are germinating or the buds unfolding, they are very susceptible to injury. When a part of a plant, which contains a large proportion of water, is ex- posed to a low temperature, a portion of the water contained in the cells escapes from them and becomes frozen on their surface, the whole tissue at the same time contracting ; the water does not §39] CHAPTER I. — GENERAL PHYSIOLOGY. 161 freeze in the interior of the cells. The water which has thus escaped and frozen forms an incrustation (Fig. 122), consisting of a number of elongated ice-crystals arranged side by side. This ice is very pure, for the substances in solution in -the cell-sap remain behind in a more concentrated form. The effect on the trunks of trees of exposure to cold is to cause radial splits, which close up again as the temperature rises, the which actually heal only in the cortex. The splitting is due to the unequal contraction of the wood, which is greater in the external more watery portion, than in the interior. JLiGHT. The influence of light may be considered under t_wp_ heads : (1) the chemical effects, pro- duced for the most part by the less refrangible rays of the spectrum ; (2) the mechanical effects, produced mainly by the highly refrangible rays. The most conspicuous chemical effects are manifested in plants which normally contain chlorophyll. They are : — a. The formation Of chlorophyll : Pls' ^--Transverse section of a *- frozen leaf-stalk of Cynara Scolymus: in Phanerogams the Colouring-matter 6 the detached epidermis ; g the paren- in which lie the transverse sections of the vascnlar bundles (left white) ; K K the incrustation of ice Of Conifers and SOine Other plants), consisting of densely-crowded prisms , , - — v — -,1 , .. ,. -, ' (the cavities of the ruptured tissue but remains yellow (etiolm), unless are iett black in the figure), exposed to light of not too great in- tensity. This effect is not confined to the rays of low refrangi- bility, but is produced (with equal intensity of light) also by those of high refrangibility. The formation of chlorophyll is also de- pendent on temperature, and will not take place if it be too low ; hence the shoots of plants developed in the early spring remain yellow if the weather is cold. b. The assimilation of carbon dioxide by the chloroplastids will only take place in the presence of light of considerable intensity ; it is especially a function of the rays of low refrangibility, as will be subsequently explained. This is also true of the first steps in the assimilation of mineral nitrogenous food (nitrates). The most conspicuous mechanical effects, exhibited by plants of all kinds are : — of the chloroplastids cannot acquire cb5[r its green hue (except in the seedlings 162 PART III.— PHYSIOLOGY. [§ 40 a. The paratonic effect. All parts of plants grow more rapidly in feeble than in strong light, as is shown by the excessive length attained by the shoots of plants grown in the dark ; hence, light exercises a retarding influence on the rate of growth ; it Jikewise inhibits the spontaneous movements of motile leaves. b. The phototonic effect. Dorsiventral leaves, when growing, generally cease to grow, and when motile lose the power of move- ment, if long kept in darkness ; but they_ soon regain the power of growth and of movement on being again exposed to light ; this condition of motility induced by light is known as phototonus. c. The directive effect. The direction of the incident rays of light effects the position of growing and other motile members : these phenomena are designated by the general term heliotropism. The various influences of light are well illustrated by plants grown in darkness, or etiolated plants. For instance, an etiolated potato-shoot has a stem with excessively long internodes, a result of the absence of the paratonic effect of light ; very small leaves, in consequence of the absence of the phototonic effect ; no chloro- phyll, in consequence of the absence of the chemical action of light. Etiolation can, however, be induced, not only in plants which normally possess chlorophyll, but in others as well ; jor instance, Fungi grown in darkness exhibit the characteristic excessive elongation. Again, plants grown in light of low re- frangibility (yellow or red) show the elongation characteristic of etiolation ; chlorophyll is formed, and the leaves are fairly well developed, but there is no heliotropic curvature : grown in light of high refrangibility (blue), the stem is stunted and the leaves very small, though chlorophyll is developed, and heliotropic curvature is well marked ; they soon die. § 40. The Functions of the Tissues. In dealing with this subject, it is important to distinguish between the vital and the physical functions; to distinguish, that is, the functions which depend upon the activity of the living protoplasm, from those which depend upon some chemical or mechanical property of the cell-sap, or of the cell-wall, of the constituent cells. The following remarks apply especially to the higher terrestrial plants. a. The Tegumentary Tissue (pp. 106, 149), whether primary (epidermis) or secondary (periderm), has as its primary function : the mechanical protection of the underlying tissues : but it has the further functions of absorption and of preventing excessive loss of water by transpiration. § 40] CHAPTER I.— GENERAL PHYSIOLOGY. 163 The absorptive function is confined to the primary tegumentary tissue : it is by means of this tissue that absorption is carried on by subterranean roots, either with or without root-hairs (see pp. 109, 110), as also by the general surf ace- of submerged parts of aquatic plants (p. 109). The prevention of excessive transpiration is effected by the more or less well-marked cuticularisation of the walls of the epidermal and peridermal cells of sub-aerial parts. Since these walls, though more or less pervious to gases, are almost or altogether impervious to watery vapour, the watery vapour evolved in the interior of the plant has to escape through special apertures, namely the stomata and the lenticels : and the tran- spiration is further regulated (see p. 108) by the opening and closing of the stomata. The importance of the tegumentary tissue in preventing desiccation is directly established by the fact that parts of plants deprived of their tegumentary tissue quickly dry up : and indirectly, by the relation between the degree of develop- ment of this tissue and the conditions of life of the plant. Thus, this tissue is highly developed in plants which grow in dry situations, whereas in the submerged parts of aquatic plants it is imperfectly differentiated, and there are usually no stomata or lenticels : hence, the more the conditions of life tend to pro- mote transpiration, the more highly-developed is the tegumentary tissue. b. The Parenchymatous Tissue (see p. 90), consisting as it i typically does of cells which contain living protoplasm, is the seat, not only of the metabolic processes, but also of the movements and irritability of plants. Different nutritive functions are discharged by various regions of this tissue. For instance, the parenchymatous tissue of sub- aerial parts, lying near the surface and exposed to light, contains chlorophyll, and carries on the assimilation of carbon : this applies especially to the leaves. Again, the cells of this tissue are frequently glandular (see p. 96), containing or excreting various waste-products : or they serve as depositories of reserve plastic substances (e.g. starch, etc.), or as conducting-tissue for organic substances. Further, the cells of this tissue, having usually extensible walls, are capable of becoming turgid and of varying in bulk : hence they are the seat of the movements of those members, or parts of them, in which movement is a mechanical possibility ; and when 164 PART III.— PHYSIOLOGY. [§ 40 turgid, they give a considerable degree of rigidity to the member of which they form part. The intercellular spaces (p. 89) of this tissue, which are es- pecially large in submerged parts of aquatic plants, are of great importance in connection with transpiration and the distribution of gases in the plant : they communicate with the interior by means of the stomata and the lenticels. c. The Sclerenchymatous Tissue (see p. 92), more especially the prosenchymatous or fibrous form of it, has the purely mechanical function of giving firmness to the members in which it is present. Whilst it is true that a considerable degree of rigidity is afforded by turgid parenchymatous tissue, and that many members con- taining little or no sclerenchymatous tissue can grow erect (e.g. conidiophores of Moulds, and succulent stems of herbaceous annuals), yet this source of rigidity is precarious, as it is so largely dependent upon external conditions, and is therefore insuf- ficient in the case of perennial plants. In these plants rigid tissue (stereom) is developed, and it is distributed in the body in just such a manner as most adequately meets the mechanical require- ments in each particular case (p. 120, Fig. 98). Stereom is most perfectly developed in the stems of land-plants which grow erect and have to support the weight of many leaves and branches ; whereas in water-plants the development of stereom is rudi- mentary, for their stems, being supported by the water, do not need to be highly rigid. When it is developed in the walls of fruits or in the seed-coats, the sclerenchymatous tissue serves to protect the seed from being eaten or digested by animals. d. The Tracheal Tissue of the Xylem (see p. 93). It is clear that when a plant-body is massive, partly subterranean and partly sub-aerial, there must be some means for readily distributing the water and other substances absorbed by the root. This distribu- tion may take place by diffusion from cell to cell ; and, as a matter of fact, this mode of distribution suffices in some plants in which the seat of absorption is not far from that of consumption (e.g. larger Fungi and Algae). But when these points are widely sepa- rated, special conducting-tissue, in the form of the tracheal tissue of the xylem, is differentiated. The function of this tracheal tissue is demonstrated by the fol- lowing experiment. If a cut be made all round the stem of a dico- tyledonous tree, to such a depth as to penetrate far into the xylem, § 40] CHAPTER I. — GENERAL, PHYSIOLOGY. 165 the effect is that the leaves borne on the stem and its branches above the incision, will soon droop and wither. This is due to loss of water, in consequence of which the cells of the leaves lose their turgidity, and the leaf-blades and petioles are.no longer sufficiently rigid to maintain their position of expansion." The loss of water is the result of the continuance of transpiration in the absence of a supply of water to meet it. The incision which has destroyed the continuity of the young wood has also cut off the supply of water from the root. The relation between the development of the xylem and the activity of transpiration is well illustrated by the com- parison of the vascular bundles of a land-plant with those of an allied submerged aquatic species. The former transpires actively and has well-developed xylem : the latter does not transpire at all, and has quite rudimentary xylem. Conduction takes place in dicotyledonous tree-trunks only through so much of the peripheral portion of the wood as includes living parenchymatous cells. The thickness of this conducting region varies widely ; it is relatively small where the wood is sharply differentiated into alburnum and duramen (see p. 143), and in such trees (e.g. Oak) section of the alburnum is soon fol- lowed by the withering of the leaves above the wound ; it is more considerable in trees like the Beech, in which the transition from alburnum to duramen is gradual ; and it is most extensive in those, such as Birch and Maple, in which there is no differentiation of alburnum and duramen. The dead portion of the wood does not conduct, but at most only serves as a reservoir of water. The tracheal tissue of the xylem discharges a purely mechanical function in connexion with the conduction of water ; it is incapable of any vital action inasmuch as it contains no protoplasm. The liquid conducted from the roots to the leaves by the tracheal tissue is not pure water, but holds in solution substances absorbed by the roots from the soil ; hence this tissue plays an important part in the distribution of food-materials in the plant. e. The Sieve-Tissue (see p. 94). The function of the sieve-tubes or phloem-vessels is to convey proteids from the organs in which these substances are deposited or are being formed, to other parts in which they are either being consumed or deposited as reserve plastic material. This is demonstrated by the following experi- ment : — If a ring of tissue, extending inwards as far as the cam- bium, be removed from the trunk of a young dicotyledonous tree, the sieve-tubes will all be cut through, and their continuity inter- 166 PART III.— PHYSIOLOGY. [§ 40 rupted. The effect of this upon the tree is that the portion of the trunk below the wound, and the roots, cease to grow, and slowly die, whei'eas the trunk and branches above the wound remain healthy and continue to grow until the roots are no longer able to absorb water, etc., from the soil with sufficient activity. Inasmuch as the cortical tissue, through which the sugar travels, is necessarily also cut through, the operation deprives the lower parts of the body of the whole of their supply of organic plastic material from the leaves, but does not interfere with the conduc- tion of water from the roots to the leaves. The sieve-tubes differ from the vessels of the xylem in that they contain living protoplasm ; their function is therefore probably not purely mechanical, but it is vital, though the relation of the protoplasm to the conduction of proteids in the sieve-tubes is not clear. The companion-cells, and in their absence the cells of the bast- parenchyma, which abut on the sieve-txibes, apparently serve in the leaves as the means by which the nitrogenous products of anabolism are brought to the sieve-tubes, and in other parts as the means by which the proteids of the sieve-tubes are distributed to the adjacent tissues ; there is some evidence to show that these cells themselves actually carry on the formation of the proteids which form the characteristic contents of the sieve-tubes. /. The Glandular Tissue (p. 96). The essential function of the glandular tissue is to secrete, and the secreta are either plastic substances or waste-products. It may be stated generally that the excretion of plastic sub- stances on the surface of plants has special reference to their relation with insects. Thus, the excretion of sugar by floral nectaries is to attract insects to visit the flowers, and thus to ensure the advantages of cross-pollination at a certain, though relatively inconsiderable, cost. The excretion of sugar by extra- floral nectaries is an expense incurred by the plant with the object of attracting to it insects of a kind which will keep off noxious insects or other animals ; these organs are especially char- acteristic of mymnecopMlous (ant-loving) plants, which by this means provide themselves with a police of ants to keep off either other injurious (e.g. leaf-cutting) species of ants, or insects of other kinds (e.g. boring bees, etc.), or even herbivorous mammals. The secretion of waste-products has, as its immediate object, the removal of these substances from the sphere of metabolism ; but § 41] CHAPTER I. — GENERAL PHYSIOLOGY. 167 their deposit at or near the surface serves the purpose of protection in various ways. For instance, the secretion of wax on the sur- face is an obvious protection against wet. Similarly there can be little doubt that when the system of resin-ducts, in plants which contain them (e.g. most Conifers, etc.), is opened by a wound, the resin serves to protect the raw surface both mechanically and antiseptically ; and this doubtless also applies to the latex present in many plants. Further, these waste-products, by their bitter, acrid, or astringent taste, by their frequently poisonous properties (e.g. alkaloids), or by their hardness, serve to protect the plants from being eaten by animals ; for instance, the presence of raphides, or of strongly acid sap, in the cells of leaves, etc., has been proved to protect them against the attacks of snails. The secretion of mucilage by the glandular hairs (colleters) often developed near the growing-points of stems and leaves, serves to keep the young tissues moist. The special functional importance of the laticiferous tissue is not fully understood. There is no doubt that it is, in the first place, a reservoir of waste-products, since the latex generally consists largely of such substances (e.g. caoutchouc, as in Siphonia elastica ; alkaloids, as in the opium of the Poppy, etc.). But the latex has also been found to contain plastic substances, such as proteids and carbohydrates, and in one case (the Papaw) a proteolytic enzyme, and it has hence been inferred that this tissue may serve to con- duct plastic substances throughout the plant ; but this inference has not been satisfactorily established. § 41. The Functions of the Members. It has been pointed out (p. 3) that, in its highest development, the plant-body con- sists of the following members : root, stem, leaf. These members will now be considered from the physiological point of view. a. THE ROOT. The most general of the functions of the root is that it absorbs the solid food of the plant in solution from the substratum, whatever it may be, on which the plant is growing ; and that, at the same time, it acts as an organ of attachment : in submerged plants the latter is its main use. In some few cases the plant is rootless (p. 44): under these circum- stances other members become modified to perform the absorbent function of the root ; in Salvinia, the aquatic leaves ; in Psilotum, the subterranean shoots. In the " carnivorous " plants (e.g. Drosera, Dionsea, Nepenthes), though they possess roots, the leaves are adapted for the absorption of organic food in solution. 168 PART III.— PHYSIOLOGY. [§41 In a typical land-plant the development of the root-system is such as to ensure an adequate supply of food from the soil, and a supply of water sufficient to maintain the general turgidity of the plant in spite of continued loss of water by transpiration. The root of such a plant is adapted for the performance of its functions both in its structure and in its properties. The most striking structural adaptation is that the walls of the superficial cells of the younger parts are not cuticularised, but remain per- Fi». 123.— A. Root-hairs (7i) on the primary root (ic) of a seedling grown in water of Buckwheat (Polygonum Fagopyrum) ; he hypocotyl ; c cotyledons. B (after Sachs) Ends of root-hairs showing their intimate connexion with particles of soil which adhere to the mucilaginous external layer of the cell-walls. vious to water. Generally speaking, the absorbent area of the root is increased by branching ; and, in many cases, also by the growing-out of the superficial cells of this region into root-hairs (see p. 46). It appears that the development of root-hairs is de- termined by the difficulty of obtaining water, on the one hand, and by the relative activity of transpiration on the other : thus § 41] CHAPTER I. — GEXERAL PHYSIOLOGY. 169 root-hairs are usually not developed by aquatic plants, the roots of which, at least, are habitually immersed in water ; nor by plants in which the transpiring surface is relatively small in proportion to the root-system (e.g. small-leaved Conifers ; saprophytes, such as Monotropa and Neottia). The root-hairs not only promote the absorption of water, but also the absorption of salts from the soil, coming, as they do, into very intimate relation with the minute particles of the soil. They thus give the root a firmer hold on the soil, and render it more serviceable as an organ of attachment. In many cases the root becomes adapted to serve as a depository of reserve plastic materials : such are the tuberous roots (p. 45) of various plants, in which secondary growth in thickness (see pp. 140, 143) produces a large amount of parenchymatous tissue, in the cells of which the plastic substances (starch, etc.) are deposited. The physiological adaptation of the root is even more remark- able in its properties than in its structure, as is shown by its irritability to the action of various stimuli. Thus the action of the force of gravity causes roots (at least primary roots) to grow towards the centre of the earth (positive geotropisni) : the action of light, as a rule, causes the growing root to curve away from the source of light (negative heliotropism) : a moist body causes the root to curve towards it (positive hydrotropisni) : contact with hard substances produces curvatures by which the direction of growth of the root is altered. These various kinds of irritability are of great importance in ensuring the due performance of its functions by the subterranean root. Positive geotropism causes it to penetrate into the soil, and this is also promoted by negative heliotropism : positive hydro- tropism causes it to grow towards the moister parts of the soil, and thus tends to ensure an adequate supply of water. Its sen- sitiveness to contact enables the root to get round obstacles which it may meet with in the soil. b. THE STEM. The function of the stem is essentially this : to bear the foliage-leaves and the reproductive organs, and to bear them in such a way that they shall occupy the most favourable position for the performance of their respective functions. Further, it is the means of communication between the roots and the leaves. Occasionally it is specially modified to subserve other functions. It has been already pointed out that the form of the stem varies widely in plants, and the most characteristic forms have been de- scribed (p. 27). The general physiological meaning of this variety ] 70 PART III. — PHYSIOLOGY. [§41 of form is that different plants attain the most favourable position of their foliage-leaves and reproductive organs in different ways which depend upon the particular combination of external condi- tions under which they severally have existed. The internal structure of the stem varies to some extent with its general habit, and mainly in the arrangement and relative degree of development of the sclerenchyma ; thus, the sclerenchyma is more largely developed in an erect than in a trailing perennial stem. There is one point in connexion with the relation of the vascular tissue of the stem to the leaves which require special considera- tion. It has been pointed out (p. 149) that vascular tissue is formed secondarily in the stems (and roots) of most Dicotyledons and Gymnosperms, whereas it is not so formed in those of most Mono- cotyledons and Vascular Cryptogams. A consideration of the general habit of the plants in question at once affords a clue to this remarkable diversity. In the plants of the former groups, the stem, as a rule, branches considerably, and consequently there is every year an increase in the area of the leaf-surface of the plant ; whereas in the plants of the latter groups, the stem branches but little if at all, and the area of leaf-surface remains approximately constant in the adult plant. It is clear that, in the former case, the increase of leaf-surface necessitates an increase in the conducting vascular tissue, a demand which is met by the annual formation of an ever-widening ring of vascular tissue by the cambium. Hence, in a plant of this kind, the vascular bundles in the leaves of any one year are continuous, in the stem, with the new vascular tissue formed in that year by the cambium. Stems may be specially modified both in external form and internal structure for the performance of special functions. Thus, in leafless plants, in which the stem or its branches have to dis- charge the functions of the leaf, they may become phylloid ; that is, it may assume a flattened, leaf-like appearance (p. 28). The cortical ground-tissue of the stems of such plants resembles the mesophyll of foliage-leaves, not only in that the cells contain chlorophyll-corpuscles in abundance, but also in the more or less complete differentiation of a superficial palisade-layer from a more deeply placed spongy tissue. Again, stems may be specially modified to serve as depositories of reserve materials (e.g. tubers of potato), or of water (e.g. stems of Cactaceae), when they are much thickened by the development of a large quantity of parenchymatous ground-tissue, in the cells § 41] CHAPTER I. — GENERAL, PHYSIOLOGY. 171 of which the water or the reserve-materials are deposited. Or they may be developed into thorns (p. 27) as a protection against being eaten. The physiological adaptation of stems is such that the move- ments which they perform in response to the action of external stimuli are always such as shall place the foliage-leaves and the reproductive organs in the most favourable position. Most stems, for instance, grow away from the centre of the earth (negative geotropism) and towards the light (positive heliotropism) ; these stems consequently grow up into the air, and take up such a posi- tion with regard to the direction of the incident rays of light that the leaves may be adequately exposed to them. Others, again, grow horizontally under the influence of gravity (diageotropism) and of light (diaheliotropism), and in this way spread out their leaves to the sun's rays. In some cases stems which tend to grow erect into the air are unable to do .so in consequence of being insufficiently rigid to main- tain their own weight, and that of their leaves, etc. Such stems are enabled to obtain the necessary support by becoming attached to foreign bodies, such as other plants, rocks, etc. This attach- ment is sometimes purely accidental, as in the case of the hook- climbers, such as the Bramble, where the stem is covered with prickles which become fixed as the swaying shoot is blown about by the wind. But in other cases the attachment is the result of the mode of growth of the stem or its branches, in virtue of which they twine round any suitable foreign body with which they may come in contact. In some cases the stem and its branches are sensitive to contact, e.g. Dodder ; in others, this sensitiveness is restricted to certain specially modified branches, termed tendrils (see p. 27, e.g. Vitis, Passiflora), and it is possessed by them in a very high degree. c. THE LEAF. In the discussion of the morphology of the leaf it was pointed out that the forms of leaves are very various ; so much so that it was necessary to classify them into a number of categories. Each of these will now be briefly considered with regard to its functions. (1) Foliage-leaves. It may be stated generally with reference to land-plants that the two great functions subserved by the leaf are, first, the construction of organic substance from the raw materials of the food ; and second, the exhalation of watery vapour, or transpiration. 172 PART III.— PHYSIOLOGY. [§41 The internal structure of the leaf is in direct relation to these two functions (see p. 114). The particular significance of the form and arrangement of the cells of the mesophyll is made clear by the following considerations. The palisade-layers occur always, be- neath the epidermis, at those surfaces which are directly exposed to the sun's rays. Further, if a plant which, when grown exposed to sunlight, has well-marked palisade-layers in its leaves, be grown in the shade, it will be found that the palisade-layers are imper- fectly differentiated, even if they can be detected at all. The development of the palisade-layers is clearly a peculiarity of leaves which are exposed to sunlight. One explanation is this, that bright light not only promotes the assimilatory function, but also pro- motes the oxidation and decomposition of the chlorophyll. The palisade-tissue affords a means of protection from the latter effect. When a leaf-surface is exposed to diffuse daylight, the position of the chlorophyll-corpuscles in the palisade-cells is such as to expose them as fully as possible to the light ; they are disposed on the surface-wTalls, both upper and lower, of the palisade- cells (epistrophe). WThen, however, diffuse daylight is replaced by direct sun- light, the position of the corpuscles is changed (see Fig. 124) so that their margin, and not their sur- face, is presented to the sun's rays ; they are removed to the lateral walls and towards the inner end of the cell (apostrophe). It is clear that the elongated form of the cells facilitates this withdrawal of the corpuscles from too intense light, to light of a degree of intensity which promotes the assimilatory function to the utmost extent compatible with a due economy of the chlorophyll. The spongy portion of the mesophyll is the tissue especially adapted to the transpiratory function. By means of the large intercellular spaces which form a system of channels throughout this tissue communicating with the external air by means of the stomata, a very large cell-surface, from which transpiration can FIG. 121.— (After Stahl). Sections of the thalloid stem of LenvL tnsulca, illustrating epistrophe and apostrophe of the chloroplastids : A position in dif- B position in intense § 41] CHAPTER I.— GENERAL PHYSIOLOGY. 173 readily take place, is brought into direct relation with the external air. Transpiration takes place from the cells of the spongy mesophyll into the intercellular spaces, and the watery vapour then escapes from the leaf by the stomata. Leaves are adapted not only structurally, but also by their irritabilities, to the performances of their functions. They are sensitive to the directive action of light and of gravity and, in the course of their growth they take up a definite position termed, on account of the predominating influence of light in determining it, the fixed light-position. The response of the dorsiventral leaf to the directive action of gravity, is generally one of diageotropism, that is it places its blade horizontally, with the ventral surface uppermost ; and similarly, its response to light is to expose the upper surface of its blade at right angles to the direction of the incident rays (diaheliotropism). The response of the isobilateral and of the radial leaf to the action of gravity is one of negative geotropism, so that they grow erect ; and to light, one of positive heliotropism, as they tend to direct their apices towards the source of light. Changes in the external conditions act as stimuli, which, in many cases, in- duce a movement of the FIG. 125. -Leaf of Oxalis by day (T) and by foliage-leaves involving ni«ht <»>• Inthe l«ter,each leaflet is folded inwards at right angles along its midrib, and is Change of position : most also bent downwards. frequently these movements are performed by growing leaves, but also sometimes by adult leaves with a permanent motile mechanism. They have been observed in the growing leaves (and cotyledons) of many plants (e.g. Chenopo- dium, Impatiens, Polygonum, Linum, etc.), and in the adult leaves of many Oxalidacese and Leguminosae. The common feature of these movements is that they serve to vary the area of surface presented to the sky by the leaf. They are commonly known as " sleep-movements," or nyctitropic movements, because they are usually associated with the alternation of day and night. With a falling temperature and a diminishing intensity of light the leaves assume the "night-position," presenting a diminished surface, generally only the edge, to the zenith, the leaflets of compound 174 PART III. — PHYSIOLOGY. [§41 leaves at the same time approaching each other, with the result that they are protected from injury by cold in consequence of excessive radiation of heat : with a rising temperature and an in- creasing intensity of light, the leaves assume the "day-position," presenting their upper surfaces to the zenith. But the day-position is frequently liable to modification, with a view to the reduction of transpiration and to the protection of the chlorophyll from the action of too intense light, by movements which diminish the leaf- area exposed to the direct rays of the sun ; — and so, in some cases, the edge, and not the upper surface, is presented to the sun : these movements are designated "diurnal sleep" or parahcliotropism. Some foliage-leaves, but only such as have a special motile mechanism, respond by movement to the stimulus of a touch. FIG. 126 (after Duchartre). — Leaves of Mimosa pudtca: A normal diurnal position ; B position assumed on stimulation. This is the case in the " sensitive plants," such as Mimosa pudica and some other species : the leaflets of the pinnate leaves of these plants close together when touched, or when the plant is shaken, and they are thus protected to some extent from injury by hail, rain, or even wind. Other instances of movement in response to touch are afforded by the "carnivorous" genera, Dionsea and Aldrovanda, in which, when an insect alights on the upper sur- face of the expanded leaf and touches the sensitive hairs, the two lateral halves of the blade suddenly close together, like a hinge, with the midrib as the axis. Sensitiveness to long-continued contact is manifested by the 41] CHAPTER I.— GENERAL PHYSIOLOGY. 175 petioles of various plants (e.g. Tropaeolum, Clematis) ; sometimes by the whole phyllopodium (Lygodium) ; in many cases leaves possessing this sensitiveness are modified into leaf-tendrils (see p. 41 ; as in Cucurbitacese, etc.) ; leaves of this kind serve as organs of attachment for climbing. Foliage-leaves are sometimes modified into pitchers or ascidia (p. 41) : these serve the purpose in some cases (e.g. Nepenthes) of capturing insects and of digesting and absorbing them: in other cases (e.g. Dischidia) they collect water and organic detritus ; in Dischidia adventitious roots are developed, which lie in the pitchers and absorb water, together with dissolved substances, therefrom. Leaf-spines, like stem-thorns, appear to be exclusively protec- tive against the attacks of her- bivorous animals. (2) Cataphyllary or Scaly Leaves (p. 42) serve to protect growing-points and young leaves of buds, and in this they are assisted by the secreting- hairs (colleters, p. 101) which they frequently bear : they sometimes serve as depositories of reserve plastic materials (e.g. scales of Onion-bulb). (3) Floral Leaves. a. Hypsophyllary Leaves. The leaves included under this head are the bracts (and bracteoles) and the perianth-leaves (p. 57). When green, the bracts perform the ordinary functions of foliage- leaves ; but when they are collected around a flower (epicalyx) or an inflorescence (e.g. involucre of Compositse, Euphorbia, etc.) they serve to protect the floral organs during their development. When highly-coloured (e.g. in Aracese, Euphorbiacese, Nyctaginacese), they serve to attract insects to visit the otherwise inconspicuous flowers. The sepals, like the bracts, are commonly green, and then they perform the ordinary functions of foliage-leaves, and also serve to protect the other floral organs : when petaloid (e.g. many Ranun- FIG. 127 (after Darwin).— Petiole of Solomon jasminoides clasping a stick. 176 PART III. — PHYSIOLOGY. [§ 41 culacese and Liliales), they attract insects for the purpose of cross- pollination. The petals are brightly-coloured in most flowers, and it is their special function to attract insects. Not uncommonly they are specially modified as nectaries (e.g. Helleborus), and thus further contribute to ensure the visits of insects. The perianth-leaves (and sometimes also the bracts), are often capable of performing movements leading to the opening and closing of the flower or inflorescence : thus the flowers of the Crocus, Tulip, and Poppy, and the inflorescence of the Daisy, open under the influence of rising temperature and increasing intensity of light, closing under the contrary conditions : the closing is a protection of the essential floral organs against cold and wet ; it is essentially similar to the nyctitropic movements of foliage- leaves. b. Sporophyllary Leaves. As already stated (p. 55) the sporo- phylls are the essential organs of the flower, when they are aggre- gated on a special shoot, and have, in any case, the function of asexually producing the spores. They are more or less generally modified in form and structure in connexion with this function ; and in the many different forms of flowers these leaves present remarkable special adaptations which mainly refer to the process of pollination, to the distribution of the seed, etc. It is impossible to enter upon a further consideration of the biology of the flower, but the phenomena of movement presented by the essential floral organs deserve special mention. Thus the two lobes of the stigma (e.g. Mimulus, Bignonia, Martynia), close together on being touched : the movement doubtless ensures the adhesion of the pollen brought by an insect. The stamens are irritable in many plants. For instance, in Berberis, when an insect touches the irritable base of one of the nearly horizontal stamens, the stamen rises up on its point of attachment as on a hinge, and strikes the insect with the anther, thus dusting it with pollen. Again, the syngenesious stamens of Centaurea shorten on stimulation by touch : the flower is protandrous ; consequently, as the filaments contract, the pollen shed by the coherent anthers is pushed out of the open end of the anther-tube by the style within, and is re- moved by the insect. § 42] CHAP. II. — PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 177 CHAPTER II. SPECIAL PHYSIOLOGY OF THE NUTRITWE FUNCTIONS. § 42. Absorption. The food of plants is absorbed, generally speaking, either from the soil or from the air. t Plants which do not possess chlorophyll (e.g. Fungi) usually obtain the whole of their food from the soil ; whereas plants which do possess chlorophyll absorb from the air one of the most im^ portant constituents of their food, namely carbon dioxide, though in exceptional cases it is obtained from other sources; for instance, parasitic plants absorb their food from the hosts upon which they live, and the " insectivorous " plants absorb a portion, at least, of their food, from the insects which are caught by their specially adapted leaves. Submerged aquatic plants absorb their food entirely or mainly from the water in which they live. The food of plants is always absorbed in the fluid form ; either as a liquid or as a gas. The liquid food, consisting of a watery solution of various substances, is absorbed from the soil most commonly by the roots, or, in the absence of roots, by other members (shoots, leaves) which have become specially adapted for the performance of this function; the gaseous food (C02) is absorbed from the air by the green parts of plants, and, in the more highly differentiated forms, more especially by the leaves. It not infrequently happens that chemical elements are found in plants which are known to be present in the soil in the form of compounds which are insoluble in water. These compounds are brought into solution by various means. For instance, the soil usually contains carbon dioxide, which is evolved from the decomposing animal or vegetable matter which is commonly present, and from the absorbent organs them- selves ; and it is well known that various substances, such as cal- cium carbonate and certain silicates, which are insoluble in pure water, are soluble in water charged with carbon dioxide. Again, the sap which fills the vacuoles and saturates the walls of root- hairs, has an acid reaction, due to the presence of organic acid, and this acid sap will dissolve many substances which are insoluble in pure water. The solvent effect of this acid sap is well demonstrated by means of the familiar experiment with a slab of marble. If a highly-polished slab of marble be fixed in the bottom of a flower- M.B. N 178 PART in:— PHYSIOLOGY. [§ 42 pot, and a plant be grown in the soil above it, the roots will come into contact with the slab and will apply themselves to its surface. On subsequently examining the slab of marble, it will be found to have become corroded where the roots had been in contact with it. The corrosion is due to the solution, by the acid sap of the roots, of particles of the marble. Absorption of Liquids. The main idea connected with this function is the taking up of water and other substances into the plant from without ; but it must not be overlooked that, in a multicellular plant, the cells absorb from each other. In any case, the function of absorption depends upon the physical process of diffusion through membrane of substances in solution, or osmosis. Tor instance, supposing two adjacent cells, one of which has its cell-sap charged with sugar, whereas that of the other has none ; the sugar will diffuse through the intervening cell-wall until the sap in both cells holds the same proportion in solution. This being the mode of absorption, it is clear that the food-materials can only be absorbed in the fluid form, either as liquids or gases. So far the function of absorption would appear to be a simply physical process. It must, however, be borne in mind that the cell-wall is lined by living protoplasm which modifies the purely physical diffusion through the cell-wall, both as regards the nature and the relative quantity of the substances which pass into or out of the cell ; so that the physical laws of osmosis, as determined by experiments with dead membrane, are not directly applicable to the osmotic phenomena of a living cell. Absorption of Gases. The absorption of gases depends, like the absorption of water and substances in solution, upon diffusion. Supposing an absorbent cell, the sap of which holds, to begin with, no gases in solution, to be brought into relation with a mixture of gases, these gases will be dissolved at the surface in proportion to their solubility and to the amount of each gas present in the mix- ture ; that is, the amount absorbed of each gas depends, in the first instance, upon its solubility and its partial pressure. The relative amount of each gas absorbed over a period of time will further depend upon the extent to which it undergoes chemical alteration after absorption. Land-plants absorb gases, in the manner described above, at all points of their surface ; by their shoots from the air, by their roots from the gaseous mixture in the interstices of the soil ; the stomata § 43] CHAP. II. — PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 179 of the sub-aerial parts are of great importance in connexion with this process. Submerged water-plants absarb, in solution, the gases dissolved in the water. The absorbed gases remain in solution in t-he cell-sap, so that living cells do not contain bubbles of gases. Moreover, gases travel in the plant mainly by diffusion from cell to cell, though their distribution may also be effected by means of the intercellular spaces. § 43. Transpiration. Every part of a plant which is exposed to the air, except such as are covered by a thick layer of cork, is continually exhaling watery vapour. This may be demonstrated by placing a leafy branch under a cold bell-glass, when it will shortly be observed that the internal surface becomes covered with drops of water, the watery vapour exhaled by the branch having condensed upon the cold glass. Again, the drooping of cut flowers or herbaceous branches is due to the loss of water by transpira- tion. It must be clearly understood that transpiration is not simply evaporation. If it were so, then clearly equal amounts of water should be evaporated in a given time by equal areas of water- surface and of living plant-surface. But this is not the case. All observations show that the amount of water transpired from a given area of living plant-surface in a given time, is only a small , fraction of that evaporated in the same time from an equal surface of water. On the other hand, the evaporation from dead plant- j surface is as active, or even more so, than from a free surface I of water. Transpiration, whilst ultimately depending upon the purely physical process of evaporation, is essentially evaporation modified by the living substance, protoplasm, from and through which it takes place, and is therefore a vital function. Inasmuch as most aerial leaves and stems have a more or less well-developed and cuticularised tegumentary tissue, the transpira- tion from the external surface is insignificant. In such cases the transpiration takes place mainly through the thin uncuticularised walls of the cells of the ground-tissue into the intercellular spaces, and the watery vapour escapes from the intercellular spaces into the external air by means of the stomata and the lenticels. The stomata, especially, are organs for the regulation of transpiration. As already mentioned (p. 163), the stomata open and close, their opening and closing being dependent upon variations in the tur- gidity (p. 159) of the guard-cells. When the guard-cells are highly 180 PART III.— PHYSIOLOGY. [§ 43 turgid, that is, when they are tensely filled with cell-sap, they curve so as to separate from each other in the middle line, thus opening the stoma ; when they are flaccid, their free surfaces are brought into contact, and the stoma is closed. The opening or closing of the stomata is a function of transpiration as affected "by the hygrometric condition of the air, and by the supply of water in the plant : so that when the transpiration is normal, as determined by a certain relation existing between the hygrometric condition of the air and the supply of water to the transpiring leaf, the stomata are open ; but when transpiration becomes excessive, by the air becoming drier, or by a diminution in the supply of water to the leaf, the stomata close, even before any trace of flagging is shown by the leaf. Thus the stomata act as regulators of transpiration, and their opening or closing depends partly on external and partly on internal conditions. The water lost by transpiration is supplied to the transpiring organs from the roots. If the loss by transpiration is compensated by the absorbent activity of the roots, the transpiring organs remain fresh and turgid. But if, as is frequently the case on a hot summer day, the loss of water by transpiration is greater than the supply from the roots, the transpiring organs, more especially the leaves, become flaccid and droop, and they are only restored to the turgid condition in the evening when the temperature of the air falls and the intensity of the light diminishes ; in a word, when the external conditions become such as to lead to a diminution of the transpiration. There is, however, besides the flaccidity of the herbaceous members of the plant, another means of observing the effect of transpiration upon the amount of water contained in the tissues. If the stem, or a branch, of an actively transpiring plant be cut through under mercury or some other liquid, it will be observed that the liquid will at once make its way for a considerable distance into the woody tissue of the cut stem or branch. This is due to the fact that, in consequence of the withdrawal of water from them, the gases in the vessels are at a lower pressure than that of the atmosphere. This is termed the negative pressure in the vascular tissue. These various points can be readily observed in low-growing plants, such as the Cabbage. On a hot summer day the leaves become flaccid, and the existence of a negative pressure in the vessels of the stem can be ascertained. In the evening, when the § 44] CHAP. II.— PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 181 activity of transpiration is diminished, but active absorption of water from the warm soil by the roots continues, the leaves become turgid, and water gradually accumulates in the vascular tissue. During the night this accumulation of water in the vascular tissue goes on until it becomes quite full, so that there comes to be not only no negative pressure, but a positive pressure. This positive pressure, were there no means of re- lieving it, might become injurious to the tissues ; but it is re- lieved by the filtering of drops out of the closed terminations of the vascular bundles in the leaves, these drops making their way to the surface through openings over the ends of the bundles, which are either the ordinary stomata, or the specially-modified water-stomata (p. 108). A row of such drops on the margin of the leaves may be observed in many plants in the early morning. It : appears, then, that during the day the loss of water by transpira- ; tion is greater than the supply by absorption, whereas during the I night the contrary is the case. With regard to the physiological significance of transpiration, it is important in that it causes a rapid current of liquid, the transpiration-current, to flow through the plant from the roots to the transpiring organs, more especially the leaves. This ensures the distribution, not only of the absorbed water, but also of the substances absorbed in solution from the soil. It will be noticed that the conditions which promote transpiration, namely, light and warmth, are just those which are most favourable to the per- formance of their anabolic processes by the organs which contain chlorophyll. Thus, when the leaves are actively producing organic substance, they are actively transpiring, and they are therefore constantly receiving supplies of the substances absorbed from the soil, substances some at least of which are essential to the chemical processes in operation. Transpiration has, then, an important bearing upon nutrition. There seems to be, in fact, an optimum activity of transpiration, that is to say, a certain activity of transpiration which promotes to the utmost the formation of organic substance ; so that if the average activity of transpiration falls short of, or exceeds, this optimum, the nutrition of the plant suffers, as shown by a diminished formation of organic sub- stance. § 44. Distribution of Water and other Substances. It is clear that, when the plant-body is so far differentiated that only certain parts of it are in a position to absorb water and substances 182 PART III.— PHYSIOLOGY. [.§ 44 in solution from without, there must be a distribution of the ab- sorbed substances from the absorbent surfaces to the other parts. Further, when the plant-body is differentiated into parts which do, and others which do not, contain chlorophyll, there must be a distribution of the produced organic substance from the former to the latter. In plants of relatively low organisation, the distribu- tion takes place entirely by diffusion through the cell-walls, that is by osmosis, when the plant is multioellular : and even in the highest plants diffusion plays an important part. With regard to the distribution of water and substances absorbed in solution from without in the more highly organised plants there is a special conducting tissue, the wood or xylem of the vascular bundles, extending from the roots, the absorbent organs, to the leaves, the transpiring organs (see p. 164.) The Root-Pressure. The existence of the root-pressure can be easily ascertained. It is manifested spontaneously by that exuda- tion of drops on the margin of the leaves of low-growing plants during the night, to which allusion has already been made (p. 180). An artificial manifestation of it is induced in stems which are cut across at a time when, owing to active absorption and feeble transpiration, the plants are rich in water ; drops exude from the xylem-vessels at the cut surface of that part of a stem which is still in connexion with the root. A familiar case of this is the " bleeding " of certain shrubs and trees when pruned in the spring. It is possible, in this way, to estimate both the activity and the force of the root-pressure. By collecting the water which exudes from the cut surface of the stem, the amount of water absorbed by the root in a given time is determined ; and by attaching a mercurial manometer to the cut surface of the stem the force of the root-pressure can be measured. For instance, 3,025 cubic millimetres of liquid were collected from a Stinging Nettle in 99 hours ; and the root-pressure required a column of mercury 354 millimetres in height to counterbalance it : in other words, the root-pressure of the Nettle was nearly half an atmos- phere, and was capable of supporting a column of water about 15 feet high. The essential point in the mechanism of the root-pressure is the forcing of liquid by filtration under pressure from the paren- chymatous cells into the xylem. The process is probably to be explained somewhat in this way. When a certain degree of turgidity is attained in the parenchymatous cells abutting on the § 44] CHAP. II.— PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 183 xylem, their protoplasm undergoes a molecular change, in conse- quence of which it becomes permeable and ceases to offer resistance to the escape of the cell-sap ; consequently, under the elastic con- traction of the distended cell- walls, a portion of the cell-sap is forced out of the cell into the vascular tissue. From this point of view, the root-pressure of a plant is simply the expression of the force of the elastic contraction of the cell-walls of the parenchymatous cells abutting on the xylem-bundles in the root. With regard to the external conditions which affect the root- pressure, the most important is the temperature of the soil ; a rise of temperature up to the optimum increases the root-pressure, bat any further rise causes it to diminish, and if the soil be heated so as to kill the roots, the root-pressure altogether dis- appears. The liquid forced into the tracheal tissue is by no means pure water; it holds various substances in solution, such as mineral salts absorbed from the soil ; in the spring it is relatively rich in organic substances, such as proteids, sugar, acids, colouring- matters, etc., derived from the reserves stored in the parenchy- matous cells of the root, which are being conveyed to the opening buds. The Transpiration-Current. The mechanism by which, after the liquid has been forced into the xylem of the root, a sufficient current is maintained through the stem of a lofty tree to supply the actively transpiring leaves, is still one of the incompletely solved problems of physiology. It might be assumed that the transpiration-current is main- tained simply by the root-pressure. There is no doubt that, in low-growing plants (see p. 180), the root-pressure is sufficient to force liquid to all parts of the plant ; and this is probably true also of lofty trees. The objection is that no root-pressure can be detected in any plant at the time when transpiration is active, when, on the contrary, there is negative pressure (p. 180) in the vessels. Moreover, a transpiration-current is maintained for a time by entire plants whose roots have been killed by heat, as also by cut-off shoots. The present position of the question as to the mechanism of the transpiration-current in lofty trees, may be stated as follows. In the spring the wood is full of water forced into it by root-pressure. When the leaves unfold and begin to transpire, water is gradually withdrawn from the conducting tracheal tissue, and the tissue is 184 PART III.— PHYSIOLOGY. [§ 44 at any rate for the most part, occupied by a system of short columns of water with intervening gas-bubbles, the columns of water being in communication by delicate films along the cell- walls. If the whole of the tracheal tissue be in this state, it is suggested that as water is withdrawn from the upper part of the wood by the transpiration of the leaves, a current is set up, the water travelling along the cell-walls, between them and the gas- bubbles. But it may be that a continuous system of tracheids completely filled with water is maintained, and that it is through this that the current travels. The conducting-tissue is supplied with water, in the first instance, from that which fills the non- conducting tissue of the wood (and the old wood or duramen, if present), and ultimately by the root. It may be thought that the suction due to transpiration would be incapable of maintaining the current ; but this difficiilty is met by the consideration that the water is held in position by the capillarity and the cellular structure of the tracheidal tissue, and that the system of columns of water and gas-bubbles does not move as a whole, since the latter cannot pass the pit-membranes of tracheids. Moreover the force of transpiratory suction is considerable, thoxigh it has not been accurately measured. The Distribution of Organic Plastic Substances. These sub- stances may be generally stated to consist of organic substances of two kinds, nitrogenous and non-nitrogenous, and these are dis- tributed through different channels. 1. The nitrogenous substances travel, in plants or in parts of plants which are not supplied with vascular tissue, in the form of amides (see p. 186) by osmosis from cell to cell. But in vascular plants it is known that they also travel in the sieve- tissue from one member of the plant to another, in the form of indiffusible proteids. There is no evidence that the very slow movement of the contents of the sieve-tubes is effected by any special mechanism; it appears to be simply induced by the de- mand for these substances at any point, and it is doubtless promoted by the swaying of the stem and branches. 2. The non-nitrogenous substances travel through the plant in the form of glucose and maltose (see p. 187), in solution ; they travel by diffusion from cell to cell, and more especially in the elongated parenchymatous cells, forming the conducting-sheath, which, in the leaf, consists of mesophyll-cells closely investing the vascular bundles, and, in the stem, belong to the inner cortex. § 45] CHAP. II. — PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 185 This layer is not the endodermis, but lies externally to it ; the endodermis frequently contains starch-grains, and is sometimes termed the starch-sheath, but it is rather a depository than a con- ducting-tissue. The direction in which organic substances travel in the plant seems to be determined simply by the demand for them. Just as the water and the substances in solution absorbed by the roots travel to the transpiring and assimilating organs, so the organic substances produced in the assimilating organs travel in the plant to those parts in which organic substance is either being used in growth, or is being stored up as reserve material. In a Potato-plant, for example, part of the organic substance formed in the leaves travels to the growing-points of the roots and of the shoots, where it is required for the development of new leaves, flowers, branches, etc., whilst the residue travels to the under- ground shoots which are developing into tubers and are storing up quantities of starch. Similarly, these organic substances travel apparently by the same channels and in the form of the same chemical compounds, from organs which serve as depositories of reserve material, when these stores are drawn upon to supply the growth of developing parts. For instance, when a Potato-tuber begins to sprout, the starch, which is the principal reserve material, is drawn upon, being gradually converted into sugar, in which form it travels to the growing-points of the young shoots and supplies a large proportion of the plastic material necessary for their growth. § 45. Metabolism. This subject will be subdivided into : 1, Chemical Composition ; 2, Food of Plants ; 3, Anabolism ; 4, Cata- bolism ; 5, Products of Metabolism. 1. Chemical Composition. As a preliminary, a general account of the chemical composition of plants will be given. All parts of living plants contain a considerable quantity of water : this forms not merely the principal constituent of the cell- sap, but also saturates the cell-walls, the protoplasm, in short, all organised structures ; it is, in fact, one of the peculiarities of or- ganised structures that minute particles of water are interposed between the particles of solid matter of which they consist. By heating to 100° or 110° C., all the water contained in any part of a plant is expelled, and in consequence it will naturally lose weight. The amount of this loss, that is, the quantity of contained water, is very different in various plants ; ripe seeds dried in the 186 PART III. — PHYSIOLOGY. [§ 45 air contain from 12 to 15 per cent, of water, herbaceous plants 60 to 80 per cent., and many water-plants and Fungi as much as 95 per cent, of their whole weight. The residue, which gives off no more water at a heat of 100° C., the dry solid, consists of a great variety of chemical compounds ; these are partly organic, that is to say, combinations of carbon writh other elements, and partly inorganic. The organic sub- stances which occur in the living plant (with the exception of salts of oxalic acid) all contain hydrogen. Some of them, such as many oils, consist of these two elements only (carbon and hydro- gen), but by iar the greater number, including cellulose, starch, and sugar, as well as the vegetable acids and certain oils, contain oxygen also. The proteid substances consist of carbon, hydrogen, oxygen, nitrogen, sulphur, and sometimes phosphorus ; in other bodies which contain nitrogen, as asparagin and many alkaloids, there is no sulphur or phosphorus ; from certain other alkaloids, for instance nicotin, oxygen is also absent. The commoner organic substances of which the plant-body consists may, in the first instance, be divided into those which do and those which do not contain nitrogen in their molecule. The most important nitrogenous substances may be classified as follows : — 1. Proleids : these are substances with a large molecule of complex con- stitution, to which no chemical formula has yet been assigned ; they may be soluble or insoluble in water, and when soluble are mostly indiffusible ; they are generally of a viscid nature (like white of egg) and are rai-eiy crystal] isable. 2. Amides (or Amido-acids): these substances are soluble in water, not coagulated on boiling, diffusible, and crystallisable. Those commonly occurring in plants are Asparagin (C4H8NnO3), Leucin (C12H26N2O4), Tyrosin (C9HnN03). 3. Alkaloids: these substances are, chemically, organic bases, occurring in plants in combination with organic acids ; they are insoluble or but slightly soluble in water, soluble in alcohol ; most of them are solid at ordinary temperatures, and are crystalline, whilst others are liquid (Coniin, Nicotin) ; they are generally poisonous. The more familiar alkaloids are Coniin (C8H15N) from Conium ; Nicotin (CIOH14N2) from Tobacco ; Morphin (C17H19NO3), and other opium-alkaloids from the Poppy ; Strychnin (C2iH22N2O2) from Strychnos Nux vomica; Quinin (C2oH24N2O2) from the Cinchona; Thein (C8Hi0N4O2) from Tea; Theobromin (C7H8N4O2) from Theobroma Cacao. Some colouring-matters are also nitrogenous (e.g. chlorophyll, and indigo C8H5NO), as also some glucosides (see below). The principal non-nitrogenous substances are : — 1. Carbohydrates : substances consisting of C, H, and O, the H and O be- § 45] CHAP. II.— PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 187 ing present in the same proportions as in water (H_,O) ; of these there are the following classes : a. Amyloses : general formula n (C6H10O5) ; of these cellulose and starch are the most common, the former entering largely into the composition of cell-walls, the latter occurring as a reserve material in the form of ' • starch-grains; they are neither of them soluble in water under ordinary circumstances : dextrin or amylin, a product of the action of diastase on starch, is soluble in water but not crystallisable : inulin (see p. 83) occurs in many Composites and all ied orders (Cam- panulacese, Lobeliacese) in solution in the cell-sap; it is slightly soluble in cold water and is crystallisable. The gums and mucilages also belong to this group. b. Sucroses: C^H^On : soluble in water and crystallisable: cane-sugar occurs in many plants (esp. Sugar-cane and Beet-root) ; maltose is the chief product of the action of diastase on starch. c. Glucoses: C6H]2O6 : soluble in water and crystallisable : they occur in fruits (grape-sugar). The sucroses and glucoses are commonly known as sugars. A substance termed Mannite (C6H14O6) occurs in the cell-sap of Fraxinus Ornus and some other plants: though not a carbohydrate, it is closely allied to this group ; crystallisable, but not readily soluble in water. 2. Organic Acids: these occur in the plant either free or, more commonly, as neutral or acid salts in combination with organic or mineral bases ; some are constituents of the fats and fixed oils (e.g. palmitic and oleic acids ; see below) : the more common are oxalic acid (H2C2O4) malic acid (H2C4H4O5), tartaric acid (H2C4H4O6), citric acid (H3C6H5O7). 3. Glucosides : substances of complex constitution which owe their name to the fact that they give rise, on decomposition, to glucose among other products : such are amyrjdalin, C2>H27NOn (seeds, etc., of many Rosacese) ; coniferin, Ci6H22O8 (coniferous wood) ; myrosin, or myronate of potash, KCjoH-sNS^o (seeds of Mustard); salicin, C12H18O7 (in bark of Willows and Poplars) ; yallo-fannin, C34H28O22 (in Oak-bark). Though some of these substances (e.g. amygdalin and myrosin) contain nitrogen, it is more convenient to classify them with the more numerous non-nitrogenous glucosides. 4. Fats and Fixed Oils : these substances, as they occur in the seeds and fruits of plant?, are mixtures of free fatty acids with glycerin-compounds (glycerides) of fatty acids ; thus palm-oil is a mixture of palmitic and oleic acids with their glycerides palmitin, C3H5(Ci6H31O)3O3, which is a solid fat, and olein, C3H3(C18H33O)CO3, which is a fluid fat or oil : olive-oil consists chiefly of olein with some palmitin : castor-oil, of ricinolein (the glyceride of ricinoleic acid) and stearin (the glyceride of stearic acid) : linseed-oil, of linolein (the glyceride of linoleic acid) and palmitin. The organic compounds can for the most part be resolved into volatile products — chiefly carbonic acid, water, and nitrogen — by exposure to great heat with free access of air, that is, by combus- 188 PART III.— PHYSIOLOGY. [§45 tion. The inorganic residue is a white, or, if the combustion is imperfect, a grey powder, the ash. As the result of chemical processes attending the .combustion, the sulphur and phosphorus previously contained in the organic compounds appear as sulphates and phosphates in the ash, and the carbonic acid formed during combustion combines with some of the inorganic substances. These, therefore, must not be included in an accurate estimate of the constituents of the ash. The ash usually constitutes but a small percentage of the whole dry solid of the plant. The amount of ash increases with the age of the plant, or of any part of it, inasmuch as there is no consider- able excretion by the plant of the mineral substances absorbed. The percentage of ash in the dry solid of the plant, or of any organ, may vary widely at different times. The following analyses of various portions of plants will give an idea of its amount and composition : — 1000 PAKTS OF DEY SOLID MATTER CONTAIN : rf •g •c . cS a 4 •< 1 | 1 1 11 |2 r 02 S O Clover, in bloom 68-3 21-96 1-39 24-06 7-44 0-72 6-74 2-06 1-62 2-66 Wheat, grain . 19-7 6 14 , 0-44 0-66 2-36 0-26 9-26 0-07 0-42 0-04 Wheat, straw . 53-7 7-33 0-74 3-Oa 1-33 0-33 2-58 1-32 36-25 090 Potato tubers . 37-7 22-76 0-99 0-97 1-77 0-45 653 245 0-80 1-17 Apples . . . 14-4 5-14 3-76 0.59 1-26 0-20 1-96 0-88 062 — Peas (the seed) 273 11-41 0-26 1-36 2-17 0-16 9-95 0-95 0-24 0-42 2. The Food of Plants. The constituents of the ash do not form a merely accidental mixture ; it has been proved by experi- ment that certain inorganic compounds are absolutely necessary to the life of the plant. Those chemical elements which the plant requires for its nutrition, and which must therefore be regarded as part of its food, are : — I. Non- metallic Elements : — Carbon, hydrogen, oxygen, nitro- gen, sulphur, phosphorus, and perhaps chlorine. These elements exist in the plant, for the most part, as organic compounds ; but they also occur to some extent as inorganic compounds, carbonates, nitrates, phosphates, sulphates, of the metals mentioned below. II. Metallic Elements : — Potassium, calcium, magnesium, iron. § 45] CHAP. II.— PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 189 Besides these we find in the ash of many plants — though they cannot be regarded as essential to nutrition — the following ele- ments : sodium, lithium, manganese, silicon, iodine, bromine, and in rare cases, also aluminium, copper, zinc, cobalt, nickel, stron- tium, and barium. Fluorine must also exist in vegetables, for it is found in a perceptible quantity in the dentine of animals which feed directly or indirectly on vegetables. The essential elements of the food will now be severally dis- cussed. Carbon. Plants which possess chlorophyll obtain their carbon mainly from the air (or, in the case of submerged plants, from the water) in the form of carbon dioxide. The absorption of carbon dioxide is, however, limited to those cells which actuall}' contain chlorophyll, and it can only go on even in those cells so long as they are exposed to sufficiently intense light. Although plants possessing chlorophyll can and do use cai'bon dioxide as carbonaceous food, yet there is reason to believe that they may supple- ment this by absorbing more complex carbon-compounds. In certain cases (e.g. Drosera, Dionsea, Utricularia, etc.), green plants are provided with a special mechanism, in the form of modified leaves, for obtaining a supply of organic carbon-compounds. Such plants are said to be insectivorous. The case of Drosera may be selected for illustration. The upper surface and the margin of a leaf of this plant bears numerous glandular appen- dages, the tentacles (see Fig. 33, p. 48). The glands at the ends of the tentacles continually excrete a viscid liquid. When an insect comes into contact with one of the marginal glands, it sticks to it ; this stimu- lates the tentacle, and it moves, curving inwards to the centre of the leaf, and gradually the other marginal tentacles incurve over the insect (Fig. 33 B). The glands then secrete an acid liquid containing a digestive enzyme which acts upon and dissolves the soft parts of the insect, and the products of this digestion are absorbed. Plants which do not possess chlorophyll are incapable of using carbon dioxide as carbonaceous food, but require more complex carbon-compounds. Such plants are, all Fungi, and among the higher plants, Cuscuta (Dodder), Orobanche (Broomrape), Neottia, etc., though in some of these latter, a small, but altogether insig- nificant quantity of chlorophyll has been detected. These plants absorb the complex carbon-compounds which they require, either from living animals and plants, or from the organic substances formed by animals or plants : in the former case they are termed parasites, in the latter saprophytes. In some cases plants destitute of chloro- phyll obtain their carbonaceous food from green plants, without^ 190 PART III. — PHYSIOLOGY. [§ 45 however, being strictly parasitic upon them since they do not destroy or injure them. This association of two distinct plants is termed symbiosis. The best instance of it is afforded by the Lichens, where a Fungus and an Alga are associated symbiotically. It is remarkable that certain plants which possess chlorophyll are never- theless parasitic in habit; for instance, Viscum (the Mistletoe) which is parasitic on various trees, Rhinanthus (the Eattle) and other Scrophu- lariacete. also Thesium (Bastard Toad-flax,) which are attached to the roots of other plants by their haustoria. The nutritive processes of these green parasites are not yet fully understood, but it seems probable that they absorb from their hosts the substances which they should normally obtain from the soil, though in a somewhat modified form. The great majority of the saprophytes are Fungi, such as the various Agarics which grow in the soil of woods (humus) which is formed by de- cayed leaves and is rich in organic matter ; the Moulds and Yeasts which grow in saccharine juices, or fruits, etc. ; and Saprolegnia which attacks the corpses of animals. Some of these Fungi, notably the Yeasts and the various kinds of Bacteria (Schizomycetes), are peculiar in that they not only decompose the amount of organic substance which they require for their nutrition, but they give rise to widespread decompositions which are known as fermentation and putrefaction. Amongst the higher plants there are many saprophytes which grow in soils rich in humus : they may be almost destitute of chlorophyll (e.g. Monotropa ; Neottia and some other Orchids) : or they may possess it in considerable quantity (e.g. some Orchids ; Pyrola ; Ericaceae), in which case they are probably only partially saprophytic ; plants of this kind grow mostly in the leaf- soil of forests, or in peat on moors. Hydrogen. The hydrogen of the plant is mainly absorbed in the form of water (H.,0), but it may also be absorbed in combination with nitrogen as ammonia-compounds (NH3), and also in combin- ation with carbon when complex carbon-compounds are absorbed by the plant. Oxygen is absorbed in combination with carbon, as C02, and with hydrogen, as H.,0, and in many of the inorganic salts of the food, such as sulphates, phosphates and nitrates, as well as in more complex carbon-compounds. Oxygen is also absorbed uncombined, in connexion with the catabolic processes, in respiration. Nitrogen, which is an essential constituent of proteid substances, is only exceptionally assimilated in the free form ; although it is present in large quantities in the atmosphere, most plants perish if the soil in which they grow contains no compounds of nitrogen. Nitrates and compounds of ammonia are widely distributed, and it is in this form that nitrogen is mainly taken up by plants ; it seems § 45] CHAP. II.— PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 191 probable that plants possessing chlorophyll absorb their nitrogen in the form of nitrates only. Nitrogen may be also absorbed, at any rate, by parasites, sapro- phytes, and insectivorous plants, in the form of "nitrogenous carbon- compounds. Although it is generally true that plants cannot assimilate un- combined nitrogen, nevertheless certain plants (Papilionese, such as Peis, Beans, etc.) will grow and flourish in a soil from which all traces of nitrogen-compounds have been carefully removed. The nature of the means by which this result is attained is not yet completely determined, but the principal facts are briefly as follows. In the first place, the roots of these papilionaceous plants have been found to bear peculiar gall-like outgrowths termed tubercles. The tubercles are the result of the attack of a Fungus which penetrates into the root through the root-hairs. The green plant and the Fun- gus appear to exist in a state of symbiosis, with the result that the green plant is adequately supplied with combined nitrogen although growing in a soil from which such compounds are originally absent. In explanation of these facts there can, first, be no doubt that the supply of combined nitrogen obtained by the green plant is ultimately derived from the free nitrogen of the atmosphere ; and, secondly, that the supply is not obtained from the atmosphere directly by the leaves, but indirectly by the roots through the soil. Nor can there be much doubt that the tubercles are associated with the process of the assimilation of the free nitrogen, and that it is effected by the Fungus. The tubercles are structures formed by the hypertrophy of the cortex of the root : their cells are rich in sugar and starch : the branches of the fungus-mycelium penetrate most of the cells, and there bud off innumerable gemmules (sometimes called bacterioids). The tubercle eventually becomes disorganised ; the gemmules are then set free into the soil, and are doubtless the means by which other roots become attacked by the Fungus. Sulphur, which is a constituent of proteids and a few other sub- stances occurring in plants, such as oil of Mustard, is derived from the sulphates of the soil. Phosphorus is absorbed from the soil in the form of phos- phates, and enters into the composition of some of the proteid substances ; phosphates constitute a large proportion of the ash of seeds. As regards Chlorine, it has been experimentally proved so far to 192 PART III. — PHYSIOLOGY. [§ 45 be indispensable in the case of one plant only, the Buckwheat (Polygonum Fagopyrum). Iron, though it is met with in very small quantities, is absolutely necessary for the formation of chlorophyll. The leaves produced by plants which are not supplied with iron during their growth, are white so soon as their own store of iron is exhausted ; these leaves, which are said to be chlorotic, become green in con- sequence of the formation of chlorophyll if the soil be supplied with iron, or even if their surface is washed with a very weak solution of iron. Potassium. Unless the soil contains potassium-compounds, the assimilation of carbon dioxide by plants possessing chlorophyll does not go on, as is shown by the fact that, under these circumstances, the plant does not increase in dry weight. Potassium-salts are especially abundant in those parts of the plant which are rich in carbohydrates such as starch and sugar, as in potatoes, beet-roots, and fruits. Calcium and Magnesium have been shown to be necessary to the normal development of plants : they are absorbed as nitrates, phos- phates and sulphates, and thus serve as bases for the absorption of these other important elements. Little is known as to their direct use : they may be of importance in neutralising the organic acids (especially oxalic) formed in the plant. The distinction of the essential from the non-essential elements has been arrived at by the method of water-culture, which consists in growing plants from the seed with their roots in a solution of various salts in distilled water. By varying the salts in the solution, and observing the effect of the change on the health of the plant, the relative importance of the different elements can be ascertained. The following are examples of solutions containing all the essential elements : — 1. 2. Potassium nitrate Calcium nitrate Calcium phosphate Potassium sulphate Magnesium sulphate Magnesium phosphate Ferric chloride Ferric chloride. In these two mixtures, as well as in others of the same acids and bases which might be formulated, all the essential elements are in- cluded in forms suitable for absorption ; the proportion of mixed § 45] CHAP. II. — PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 193 salts should not, however, exceed about '3% by weight of the liquid. The following is a brief account of the non-essential mineral constituents of the food. Silicon, is absorbed from the soil as silica (Si02) or as silicates. It cannot be regarded as of nutritive importance, since plants which are usually rich in silica can be brought to an apparently normal development under conditions which render the absorption of silica impossible. It is usually deposited in the cell-walls, as in Diatoms, Equisetum, many Grasses, etc. Iodine and Bromine are found in the many marine plants, especially in Algse, and are prepared from them ; it is not known that they are of any value in the economy of the plant. Sodium, being universally distributed, is found in plants. Lithium occurs in the ash of several plants, particularly in Tobacco. Zinc, Copper, and other metals, though they are not commonly present in the ash of plants, are nevertheless taken up by plants from soils which are rich in them ; from this it appears that plants may absorb substances which are not necessary and may be even injurious. 3. Anabolism. Under this term are included all the chemical processes going on in the plant which lead to the formation of complex substances from simpler ones (p. 158). Of these, those which are undergone by the food of the plant constitute assimila- tion. In the case of plants which contain chlorophyll, the first step in the assimilation of the food is the construction from carbon dioxide and water of an organic molecule which contains carbon, hydrogen and oxygen. The process may be represented by the following equation : — That some process of the kind takes place is proved by the fact that when green plants are placed under the necessary conditions, that is, when they are supplied with carbon dioxide, with water and with salts from the soil, and are exposed to light, they gain in weight in consequence of an increase in the amount of their dry organic substance, and they give off oxygen. Moreover, the volume of the free oxygen evolved is actually equal to that of the carbon dioxide absorbed, as indicated in the equation. There are three points connected with the performance of this M.B. - O 194 PART III.— PHYSIOLOGY. [§ 45 process which require special notice : the part played by the mineral food, the action of light, the function of chlorophyll. With regard to the first point, it appears that the process in question cannot be performed unless potassium-salts are supplied to the plant. There is no reason to believe that this metal takes any direct part in the process ; but it has an indirect, though none the less well-marked effect upon it (see p. 192). The importance of exposure to light is briefly this. The chem- ical process represented in the foregoing equation is one which involves the doing of work ; for, from the simple and stable mole- cules C02 and H20, a more complex and less stable molecule CH20 is produced. Work cannot be done without energy, and the plant cannot evolve in itself the energy necessary. It avails itself, there- fore, of the kinetic or radiant energy of the sun's rays. Hence the importance of exposure to light is that the plant, by absorbing the light-rays, obtains the energy required for the chemical work which has to be done. Next, as to the function of chlorophyll. The function of chloro- phyll is to serve as the means by which the rays of light are absorbed, and their energy made available for the performance of the chemical work by the protoplasm with which the chlorophyll is associated. When light which has passed through a solution of chlorophyll is examined with a spectroscope, the spectrum is seen to present certain dark bands, known as absorption-bands, in the red, yellow, green, blue, and violet, the band in the red being the most conspicuous. These bands indicate that certain of the rays of the solar spectrum do not pass through the chlorophyll, but are arrested and converted into another form of energy. It is this energy which, in the living plant, the chlorophyll places at the disposal of the protoplasm for the construction of an organic molecule out of carbon dioxide and water, as expressed in the fore- going equation. Protoplasm without chlorophyll is incapable of making use of the kinetic energy of the rays of light for the per- formance of this chemical work. The product of this process of carbon-assimilation is (as indi- cated in the foregoing equation) a non-nitrogenous organic sub- stance having the composition of a carbohydrate. A leaf which is actively assimilating carbon under the influence of light is generally found to contain relatively large quantities of carbo- hydrate, in the form either of sugar or starch. The performance of this process can be readily demonstrated. § 45] CHAP. II. —PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 195 If a water-plant (e.g. a leaf of Potamogeton natans, or a portion of the shoot of Elodca canadensis) be placed in water which holds carbon dioxide in solution, and be exposed to sunshine, it will be seen that from the cut surface of the leaf or stem bubbles" of gas are given off at regular intervals (Fig. 128). These consist of oxygen. The relation of light and of chlorophyll to the formation of organic substance by a green plant can be demonstrated by the starch-method. For instance, if a leaf of a starch-forming plant, which has been exposed to bright light for some hours, be removed, decolourised by alcohol and tested with iodine, it will assume a dark blue colour, showing an abundant accumulation of starch. If a leaf, still on the plant, be exposed, not to white light, but to a spectrum, the starch will be found to have accumulated in those portions of the leaf upon which have fallen the rays of light which correspond to the principal absorption-bands of the chlorophyll- spectrum. It is, generally speaking, only plants possessing chlorophyll which can create organic sub- stance. Inasmuch, therefore, as organisms, whether plants or animals, which do not possess chlorophyll require for their nu- trition more or less complex . , Fie. 128. — Evolution of oxygen from a organic substances, they are water.plant (Elodea canadensis) : a the cut entirely dependent for their food stem ; g a weight that keeps the stem in its upon organisms which do pos- *££*"** sess chlorophyll. This process is also of great importance in another direction. All living organisms, speaking generally, absorb free oxygen and evolve carbon dioxide in respiration. Those organisms which possess chlorophyll prevent the excessive accumulation of carbon dioxide in the atmosphere, and keep up the supply of free oxygen, in that, under the influence of light, they absorb the former gas from the air, and replace it by an equal volume of the latter. The characteristic difference between the anabolic capacity of plants which do and of those which do not possess chlorophyll is then this, that the former can produce, from carbon dioxide and water, organic substances containing the elements C, H, and 0, whereas the latter cannot produce these, but must be supplied with them as food. From this point onwards the anabolic 196 PART III.— PHYSIOLOGY. [§ 45 processes in the two cases are, as far as is known, identical. From the simpler plastic substances containing C, H, and 0, whether they have been formed from CO., and H20 in the one case, or have been absorbed as organic food from without in the other, other more complex substances such as sugar, etc., are formed, probably by the polymerisation or condensation of the simpler molecules. Further, the nitrogen of the food, absorbed either as nitrates or salts of ammonia, is worked into the anabolic processes, so that nitrogenous organic substance is produced. Probably the first formed nitrogenous substances are comparatively simple crys- tallisable substances, such as asparagin and leucin, which belong chemically to the amides (see p. 186). The next step is doubtless the formation of those more complex nitrogenous substances, the proteids, and here sulphur, and phosphorus in some cases, is introduced into the molecule ; and finally the series of assimila- tory processes concludes with the formation of molecules of protoplasm. These various assimilatory processes are not, however, carried on simultaneously with equal activity. In plants which contain chlorophyll, when under conditions favourable for carbon-assimi- lation, the construction of non-nitrogenous organic substance from C0;j and H20 appears to be the most active process, for an accumu- lation of non-nitrogenous organic substance can be detected in the green parts of these plants when assimilation is being carried on. Most commonly this excess of non-nitrogenous organic sub- stance is accumulated in the form of starch-granules which are formed in the chloroplastids ; less commonly in the form of sugar which is held in solution in the cell- sap (e.g. leaves of Onion). This excess of non-nitrogenous organic substance in the green parts soon disappears, however, when, by withdrawal from the influence of light, its further formation is arrested. For instance, if a plant which has been exposed to light and whose leaves are rich in starch, be placed in the dark for some hours, the starch will then ba found to have almost or entirely disappeared. The organic substance resulting from the anabolism of the plant, is partly used in the growth of the plant, in forming new protoplasm, cell-walls, etc., and is partly stored up, in various organs, in the form of reserve materials which serve either for the growth of the plant itself at a subsequent period (roots, tubers, etc.), or for the nutrition of new individuals in the early stages of their growth (spores, seeds, etc.). § 45] CHAP. II.— PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 197 4. Catabolism. Under this term are included all the chemical processes going on in the plant which lead to the formation of simple substances from more complex ones. The chief physiological importance of the catabolic processes is this : that, inasmuch as they consist in the decomposition of relatively complex and unstable substances into others which are relatively simple and stable, they necessarily involve a conversion of potential into kinetic energy ; and it is by means of the kinetic energy so evolved that the plant exhibits those phenomena, such as growth, movement, etc., which characterise it as a living organism. The degree of activity of life depends directly upon the degree of catabolic activity ; when catabolism ceases, life ceases ; the organ- ism is dead. A good illustration of this is afforded by the scarcely perceptible catabolism of seeds, bulbs, etc., when quiescent, and their very active catabolism when they begin to germinate. The catabolic processes of the plant are carried on either by the living protoplasm itself, or by means of certain substances formed by the protoplasm, which are termed enzymes. The catabolic processes carried on by the protoplasm are mainly such as depend upon the absorption of free oxygen from without, and are accompanied by an evolution of carbon dioxide ; in fact this gaseous interchange between the plant and its environment, termed Respiration, is the external manifestation of the performance of these catabolic processes. The seat of these processes is the protoplasm, and it is mainly the molecules of protoplasm that are decomposed ; in other words, just as the construction of the protoplasm-molecule is the ultimate result of anabolism, so the- decomposition of the protoplasm-molecule is the central fact o£ catabolism. The reason, then, why most plants die when they are deprived of free oxygen, is that they are unable to carry on, under these- circumstances, those catabolic processes by which the energy essential to the maintenance of life is evolved ; just as a fire goes out, that is the oxidation of the coal stops, under the same con- ditions. Though it may be generally stated that living plants at all times absorb free oxygen, and that the maintenance of life depends upon a constant absorption of free oxygen, yet there are excep- tions. There are, for instance, certain Fungi, such as Yeast and Bacteria, which can live in the absence of free oxygen. Under these conditions they carry on other processes of decomposition into 198 PART III.— PHYSIOLOGY. [§ 45 which free oxygen does not enter, provided that suitable material is accessible ; these processes are termed fermentations, and th6 plants organised ferments. Thus, Bacteria cause putrefaction and other similar fermentations in the most various organic substances with which they happen to come into contact. Similarly Yeast is the cause of the alcoholic fermentation of sugar, which may be re- presented by the equation C6H1206 = 2C2H60 + 2C02. The chief kinds of enzymes which have been found in plants are: — 1. Those that act on carbohydrates, converting the more complex and less soluble carbohydrates into others of simpler composition and greater solubility. 2. Those that act on fats, decomposing them into glycerin and fatty acid. 3. Those that act on glucosides, glucose being a constant product. 4. Those that act on the more complex and less soluble proteids, converting them into others which are more soluble and probably less complex, or decomposing them into non-proteid nitrogenous substances (amides, etc.). The chemical action of some of these enzymes are illustrated by the following equations : — 1. Conversion of starch into sugar (amylolytic enzyme, commonly termed diastase) : — Starch. Maltose. Dextrin. 3 (C6H1005) + H30= C12H2,On + C6H1005 2. Conversion of cane-sugar into grape-sugar (invert enzyme) : — Cane-sugar, Dextrose. Lsevulose. C18H220U + H20-C6H1206 + C6H1S06 8. Action of fat-enzyme : — OleYn. Oleic acid. Glycerin. CjjH^Oe + SHsO- 3C16H3402 + C3H8O3 It will be noted that, in every case, the action of the enzyme involves the taking up of one or more molecules of water. The action of the enzymes which act on proteids (protedytic enzymes) cannot be represented by equations, inasmuch as no formulae for the various proteids have at present been arrived at. It may be generally stated that their effect is, like those of the other forms, to induce de- composition with the assumption of water. The proteolytic enzymes, acting some in an acid medium, others in an alkaline, convert the more complex proteids, such as globulins, into simpler ones, such as peptone ; § 45] CHAP. II.— PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 199 and further cause the decomposition of peptone into amides, such as asparagin, leucin, and tyrosin. The chief importance of the enzymes in tb.e economy of the plant is that by their means the reserve materials, which are accumulated to such a large extent in the form of substances, such as starch, fat, cellulose, proteids of aleuron-grains, which are either not soluble in water, or if soluble are only slightly diffusible, are converted into substances, such as amides and cer- tain sugars, which are both readily soluble and diffusible, and which can therefore travel osmotically from one part to another. For instance, as mentioned above, the excess of carbohydrate formed in the leaves when they are actively assimilating, is com- monly stored up in the form of starch. This carbohydrate is eventually conveyed to other parts of the plant ; but, since starch is insoluble, it cannot be conveyed in that form ; it is, in fact, con- verted into maltose by an amylolytic enzyme present in the leaves, and it is in this form that non-nitrogenous organic substance is conveyed away from the leaf where it has been produced. Other striking illustrations of the importance of enzyme-action are to be found in the chemical changes going on in germinating seeds, bulbs, tubers, etc. When a starchy seed, or a starchy tuber like the potato, germinates, the starch-grains are gradually dissolved, the starch being converted into maltose. When the tuber of the Dahlia or Artichoke, which contains inulin as the non-nitrogenous reserve material, germinates, the inulin disappears and is gradually replaced by grape-sugar. When an oily seed germinates, the oil-drops become less and less apparent, as the oil is gradually decomposed by enzyme-action into glycerin and fatty acids ; the next step is the formation of carbohydrate (sugar or starch), pro- bably from the products of the decomposition of the oil, a process which involves the absorption and fixation of oxygen, since carbohydrates contain a higher percentage of oxygen than does any form of fat or oil ; and then, finally, any starch so formed is converted into sugar. Similarly, the aleuron-grains in a germin- ating seed gradually disappear, the indiffusible proteids composing them being decomposed by the action of a proteolytic enzyme into peptone, and then into amides, in which form they are conveyed osmotically to the growing embryo. Finally, it is obvious that the indiffusible proteids which are conveyed from part to part in the sieve-tissue of vascular plants (see p. 165) must eventually be 200 PART III.— PHYSIOLOGY. [§ 45 distributed osmotically in the form of diffusible compounds, pro- bably amides, to the adjacent parenchyinatous tissues, and it is probable, though not yet ascertained, that here again a proteolytic enzyme is involved. Respiration. This term is applied to the gaseous interchange, consisting in the absorption of free oxygen and the evolution of carbon dioxide, which takes place (with but few exceptions) be- tween the living plant and the atmosphere, and which may be re- garded as the external expression of the oxidative catabolic processes going on in the tissue of the plant. This gaseous interchange goes on over the whole surface of the body ; but in those parts jvvhich possess stomata or lenticels, it is mainly conducted through these apertures. Respiration seems to be somewhat diminished under the in- fluence of bright light ; but its activity is promoted by a rising temperature, and to some extent by greater moistness of the air. The relation to temperature is such that respiration takes place at temperatures even slightly below 0°C. ; that it increases in intensity with a rise of temperature, but in greater proportion, up to an optimum of 40°-45° ; and then sinks as the temperature further rises until the fatal degree is reached. The relation of the volume of the gases absorbed and evolved in respiration, that is, of oxygen and carbon dioxide, is a matter of importance. It may be generally stated that the relation is de- finite and constant for any given plant, or for any part of it, at a given stage of development, all other conditions being constant : the proportion £2? may be unity, or less or more than unity, ac- cording to the nature of the plant under experiment, and is not affected either by temperature or by light. Respiration can be demonstrated by placing a quantity of germinating seeds, or opening flower-buds, in an air-tight glass receiver (somewhat as in Fig. 129), through which a current of air is drawn previously freed from CO2 by passing through solution of caustic potash. On examining the gas drawn from the receiver, by passing it through a clear solution of lime-water, it will be found that the lime-water becomes turbid in con- sequence of the formation of calcium carbonate, the CO2 in the gas with- drawn combining with the lime. 5. The Products of Metabolism. The relation between the anabolism and the catabolism of the plant may be generally stated thus, that the construction of organic substance in the former is greater than the decomposition of it in the latter, so that on the § 45] CHAP. II.— PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 201 whole there is an accumulation of organic substance in the body of the plant. The organic substance is accumulated to some extent in the actual structure or fabric of the plant, as protoplasm and cell-wall, and to some extent in the form of "compounds which may be present in some or all of the cells, but which do not constitute any portion of the fabric. These compounds may or may not be of nutritive value ; in the former case they are termed jrtasfic products, in the latter icaste-products, of metabolism (see p. 158). The most important of the plastic products are enumerated be- low. They are all found accumulated as reserve materials iu various parts of plants. Non-nitrogenous reserve mattriaJs: — a. Carbohydrates; in solid granules, starch; in many seeds, and tubers, in thickened cell-walls, cellulose ; as in Date- seed, Coffee-seed, Vegetable Ivory, dissolved in cell-sap ; grape-sugar, as in the Onion and in fruits ; cane-sugar, as in the Sugar-cane and the Beetroot ; inulin, as in the Jerusalem Artichoke and Dahlia. b. Fats ; in drops in many seeds (Rape, Linseed, Castor-oil, Palm. etc.). Nitrogenous reserve materials : — a. Proteids ; in solid granules (aleuron ; p. 80), in seeds, more espe- cially oily seeds. 6. Amides; asparagin, etc., in solution in the cell-sap of bulbs, tubers, bulbous roots, etc. The icaste-products are most probably all formed as the result of catabolic processes ; though their formation is often associated, both as to time and place, with active anabolism. They may be classified into nitrogenous and non-nitrogenous. The principal nitrogenous waste-products appear to be the alkaloids (see p. 186). They are probably products of the nitro- genous catabolism of plants ; and it is suggestive that they prin- cipally occur deposited in "the cells of deciduous parts, such as leaves, seeds, bark, etc. The principal non-nitrogenous waste-products are, water; free oxygen (green plants in light) ; carbon dioxide, and some other highly oxidised carbon-acids, such as the oxalic ; resins and ethereal oils, tannins, aromatic substances, etc. Of these waste-products, some are retained in the cells of the 202 PART III.— PHYSIOLOGY. [§ 45 plant, whereas others are thrown off or excreted. The nitrogenous waste -products are deposited either in cells or in the laticiferous tissue : there is practically no excretion of such waste-products by plants. Similarly, those of the non-nitrogenous waste-products which are not gaseous at ordinary temperatures, are retained by the plant. For instance, oxalic acid is deposited in the form of crystals of calcium oxalate either in the cavities or in the walls of the cells (see pp. 78, 81) : the crystals may have either six mole- cules of water of crystallisation, when they are quadratic ; or two molecules, when they are prismatic (raphides). The resins and ethereal oils are usually excreted by the cells in which they are formed, into intercellular spaces (resin-ducts, oil-glands, see p. 97) : the tannins are mostly stored in cells, dissolved in the cell- sap. The oxygen which is set free in connexion with the decomposi- tion of C02 in the green parts under the influence of light, is exhaled in the gaseous form ; this is also the case with the carbon dioxide produced in catabolism. In some cases, however, some portion of the carbon dioxide forms calcium carbonate, which is either deposited in the solid form (e.g. cystoliths, see p. 78), or is excreted by means of the chalk-glands (p. 96). In some cases, substances of nutritive value are excreted by plants, as for instance, the sugary liquid known as nectar by special glands, the nectaries (see p. 26), of flowers, and the di- gestive liquid poured out by the glands of the insectivorous plants. This loss of substance is, however, compensated for by the advan- tages gained by the excretion. The nectar attracts insects, and so ensures cross-fertilisation, and the excretion of the insectivorous plants results in the digestion of the entrapped insects (see p. 189). The mechanism of excretion may be generally illustrated by reference to two cases : to the nectaries, and to the chalk-glands. The former afford an example of that mode of excretion in which the necessary force is supplied by the excreting cells themselves : the latter, of that mode in which the necessary force is derived from another source. Excretion by nectaries can be well observed in the case oiFritillaria imperialis (Fritillary, or Crown Imperial). At the base of each of the petals of the flower, there is an oval depression which is the gland or nectary and is seen to be occupied by a large drop of nectar. If the flower be cut off, and the drop be removed from the nectary by means of blotting-paper, it will be shortly replaced by a fresh drop. It is therefore clear § 45] CHAP. II. — PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 203 that in this case the excretion of the liquid is effected, not by the root-pressure, for the flower is no longer in connexion with the root, but by the cells themselves. The mechanism of excretion seems to be this, that the cells of the nectary become turgid, and when a certain degree of turgidity has been attained, nitration under pressure takes place, and liquid is pressed out. Excretion by chalk-glands can be well observed in some of the Saxifrages. The chalk-glands are here situated at the end of the finer vascular bundles round the margin of the leaves, each gland being at the bottom of a depression in the surface, and communicating with the surface by two or three water-stotnata (see p. 109). So long as the leaf is in connexion with the rest of the plant, and provided that transpiration is not too active, drops of water holding chalk in solution are poured out by these glands on to the surface through the water-stomata. The excretion stops, however, directly the leaf is removed, or the stem is cut through. In this case the excretion clearly depends upon the root-pressure ; the gland itself has no excreting power, but it simply accumulates the chalk which is then washed out by the current of water forced through the gland by the root-pressure. In connexion with the catabolic processes there is an evolution of energy constantly going on in the plant, which is for the most part lost to the plant, or dissipated, most commonly in the form of heat, in a few cases in the form of light, and also commonly in the form of movement. The evolution of heat by plants is not usually sufficient to cause the temperature of the plant-body to be higher than that of the surrounding air. This is partly due to the fact that the catabolic processes of plants are not generally very active, and partly to the continual loss of heat by radiation and in con- nexion with transpiration. It is however easy, under appropriate conditions, to demonstrate the evolution of heat. If a quantity of seeds be made to germinate in a heap, they will be found to be distinctly warm (Fig. 129). This happens on a large scale in the process of malting barley. When a large quantity of barley -grains are germinating on a malting-floor, they become quite hot : they have, in fact, to be continually turned to prevent overheating. The conditions are here most favourable : for the catabolic pro- cesses are extremely active in germinating seeds, and there is but little loss of heat by radiation and transpiration. Similar observa- tions may be made with opening flower-buds, the opening of the bud being also a period of great catabolic activity. In some cases, 204 PART III.— PHYSIOLOGY. [§ 45 as in the Aracese, where the inflorescence consists of a great num- ber of flowers which open simultaneously, and which are protected by a large leaf, the spathe, a rise of temperature as much as 18° O has been observed. The few plants in which an evolution of energy in the form of light has been clearly established are all Fungi. It is commonly termed phosphorescence. The so-called phos- phorescence of decaying wood is due to the presence of the mycelium of Agar- icus melons, and that of putrefying meat and vegetables to Schizornycetes of the nature of Micrococci. Various other species of Agaricus have been found to be luminous. Movement of some kind is manifested by all plants. All plants exhibit that slow movement which is termed growth : in many, there is a more or less well-marked movement of the pro- toplasm in the cell or cells of which the plant-body consists, which is known as cydosis, circulation, or rotation :. some are capable of locomotion during the whole or a portion of their life, a peculiarity which is shared by many reproductive cells, such, as zoospores and spermatozoids : in some cases, the floral or the foliage-leaves of the plant can perform movements, as the foliage- leaves of the Sensitive Plant, of the Telegraph-plant, of Dioncea muscipiila (Venus' Fly-trap), the stamens of Ber- beris and of the Cynareae, or portions of leaves as the tentacles of Drosera (Sun-dew, see p. 189). These movements are considered in detail in the next chapter. The connexion between these various forms of dissipation of energy and the catabolic processes, is clearly demon- Fio. 129.— Apparatus for de- tecting the rise of temperature in 8mp.ll opening flowers or ger- minating seeds. The seeds are heaped as closely as possible in the funnel r which is inserted into the mouth of a bottle con- taining a solution of caustic potash. This absorbs the car- bon dioxide produced by respi- ration. The whole is enclosed in a glass vessel, and a delicate therm oraeter is inserted through the cotton wool which closes the mouth. The bulb of the thermometer is plunged in among the seeds. The tempera- ture in this apparatus will be higher than in another arranged in the same way forcompnrison, and in which the flowers or seeds have previously been Ic.lled. § 46] CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 205 strated by the fact that any change which prejudicially affects the activity of catabolism, similarly affects the dissipation of energy. For instance, in the absence of free oxygen, a condition which" diminishes catabolic activity in most cases, germinating seeds or opening flowers cease to evolve heat ; the luminous Fungi cease to emit light ; growth, and the other more conspicuous movements are arrested : similar effects are produced by exposure to a low temperature. CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. § 46. Introductory. The movements to be specially con- sidered here are such as may be characterized as vital ; that is, they are essentially manifestations of the life of the protoplasm. This statement is rendered necessary by the fact that movements do occur in plants which are dependent upon purely physical causes ; instances of these are afforded by the rupture of pollen- sacs and other sporangia, the twisting and untwisting of awns (as in the fruits of Erodium and Stipa), the bursting of fruits (as in the Balsam, Impatiens Noli-me-tangere, and the Squirting Cucumbers, such as Ecbalium, Momordica, and Elaterium). These movements may be due, in the simpler cases, either to expansion and contraction of hygroscopic cell-walls resulting from variations in the moisture of the air, or to the imbibition with water and the consequent swelling-up of mucilaginous substances in the cells ; in the more complicated cases the movement depends upon tensions set up between different layers of tissue in consequence of unequal expansion. The vital movements are either spontaneous or induced. In the former case they are the result of causes operating in the organism itself ; in the latter, they are the result of causes acting upon the organism from without. The following are the principal phenomena of movement ex- hibited by plants ; the streaming movement of protoplasm (cyclosis) ; the expansion and contraction of contractile vacuoles ; the locomotion of entire organisms ; the movements of cellular members. 206 PART III.— PHYSIOLOGY. [§ 47 § 47. The Spontaneous Movements may be conveniently considered under the two heads of movements of protoplasm, and movements of cellular members. A. Movements of Protoplasm. Under this head are included such spontaneous movements as can be directly observed in the protoplasm. The first to be noted is the streaming movement, which can be frequently observed either in naked protoplasm (e.g. plasrnodia of Myxomycetes), or in the protoplasm of ccenocytes clothed by a cell-wall (e.g. hyphse of Fungi), or in that of cells (e.g. leaf of Elodea and Vallisneria, internodal cells of Characeae, root-hairs of Trianea bogotensis, hairs of the stamens of Trades- cantia, etc.). The movement takes place in the more fluid por- tion of the protoplasm, and is made evident by the granules of various kinds which are carried along by the current. The direction of the movement varies somewhat according to circum- stances : the current travels in one direction, and this simple longitudinal movement is all that can be observed in plasmodia and in hyphse ; but in cells, owing to their shortness, it can be observed to travel up one long side, across the end, and down the other side ; and when the cytoplasm forms not merely a parietal layer, but forms strands traversing the vacuole (e.g. Fig. 36 Z>), currents can be observed in these strands also. The contractile vacuoles are small, more or less nearly spherical, cavities which make their appearance in the protoplasm and then suddenly disappear. In their relatively slow expansion (diastole), they become filled with cell-sap, which is forced out on the sudden contraction (systole). They have been exclusively found in motile organisms, such as the Volvocineae, the plasmodia of Myxomycetes, the zoospores of many Algae and of some Fungi. In the second place the protoplasmic movements which involve locomotion have to be considered. The simplest case of this is the amoeboid movement exhibited, among plants, by the zoospores of the Myxomycetes and of some Algae, and by the naked masses of protoplasm which constitute the plasmodia of the Myxomycetes. There is here no specialised motile organ, but any part of the protoplasm may be protruded as a pseudopodium into which the remainder of the protoplasm gradually flows, and thus locomotion of the whole is effected. The locomotory movements of most zoospores, of spermatozoids, and of entire organisms such as Volvox, Pandorina, etc., is effected by means of specialised motile organs, which are delicate proto- § 47] CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 207 plasmic filaments termed cilia (see p. 51) ; each cell may have one, two, four, or many cilia (see Figs. 1, 62, 63). Locpmotion is also exhibited by other Algae, such as Diatoms, Oscil- latorias, etc., but the mechanism is not fully understood. B. Movements of Cellular Members. Instances of the move- ment of parts of plants consisting of one or more cells having a cell-wall, are afforded by all growing members, and by some specially modified mature members ; the movements of the latter are termed movements of variation, those of the former, movements of growth. These two kinds of movements can be readily distinguished from each other, inasmuch as the movements of variation are rapid and can be easily observed, whereas the movements of growth are slow and can only be followed by means of special apparatus. a. Movements of Variation. The majority of the movements of variation are induced, a few only being spontaneous. An instance of spontaneous movement is afforded by the rising and falling of the lateral leaflets of the trifoliolate leaf of Desmodium gyrans, the Telegraph-plant. It must, however, be pointed out that the power of spontaneous movement may be possessed by plants though they do not manifest it under ordinary circumstances. Thus the leaves of the Sensitive Plant (Mimosa pudicd) move spontaneously in darkness, but they will not do so in the light. This is also true of various Leguminosce and Oxalidacese. b. Movements of Growth. Before entering upon a description of the movements of growth, a clear idea must be formed of what growth really is. By growth is meant change of external form, which is usually, though not necessarily, accompanied by increase in bulk ; the change of form being rendered permanent by the deposition of new substance : it is a function of embryonic proto- plasm (see p. 8). The growth of the plant-body takes place to a greater or less extent in all three dimensions of space. For instance, when it takes place equally in all three dimensions, a spherical body is produced, as in Protococcus and Volvox. Occasionally it takes place especially in two dimensions, the result being a flattened body, such as a Fern-prothallus or an Ulva. More commonly, however, it takes place especially in one direction, so that the plant-body assumes an elongated form. An extreme case of this 208 PART III.— PHYSIOLOGY. 47 is afforded by Spirogyra and other filamentous Algae. It is this growth in length which has been more especially studied physio- logically, and in what follows, " growth " may be taken to mean " growth in length," unless there is some definite statement to the contrary. The growth in length of the plant-body takes place at first throughout its whole extent ; but at a later period it is limited, as a rule, to particular regions (see p. 8). In the growing portion of any member two regions may be distinguished : the formative region, which is the growing-point proper: and the region of elongation adjacent to it (Fig. 130). In the formative region the construction of the new tissue from plastic substances takes place, as is specially manifested in the formation of cell-walls accom- panying the cell-division going on in this region of a multicellular growing-point; but the amount of elonga- tion is slight. In the region of elongation, the formative processes have ceased : in multicellular plants little or no cell-division takes place in this region ; the cells are here fully formed, and they simply require to increase in bulk, to grow in fact, in order to attain the mature form. Beyond the region of elongation comes the portion of the member which has already ceased to grow. It must be clearly understood that each portion of the growing-point passes through these three phases. For instance, in a multicellular apical growing-point, each cell is produced in the formative re- gion ; and as in consequence of the con- tinued formation of younger cells in front of it at the apex, it comes to lie at an increasing distance from the apex, it passes through the stage of growth, to become an adult tissue-element. The movement of growth in length is altogether spontaneous. It may be generally described as the travelling of the organic apex in a line which is the continuation of the longitudinal axis of the growing member. Both the rate and the direction of growth are liable to variation. Variations in the Rate of Growth. When a member begins to KIG. 130.— The growing primary root of the Pea in two stages. .4 The root is marked by lines atequal dis- tances. In B the differences in rapidity of growth are perceptible : the uppermost lines have not been sepa- rated ; the root has ceased to grow here. The lowest likewise are still close toge- ther ; at the apex elonga- tion has not taken place. In the intermediate zones the elongation has been very great. CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 209 grow, its rate of growth is at first slow ; it then accelerates, until a maximum rapidity is attained ; after which it diminishes until growth ceases altogether. This gradual rise -and fall in the rate of growth, extending over the whole of one period of growth, is termed the grand period of growth. This periodicity is manifested also in each cell of the growing region. A young cell grows but slowly ; as it becomes older, and is gradually removed from the growing-point, its rate of growth increases up to a maximum ; as it becomes still older and is still more remote, the rate of growth sinks, until finally the adult stage is reached, and growth ceases. Careful observation of growing members has shown that, in addition to the spontaneous variation constituting the grand period of growth, small irregular variations are constantly taking place, which, since they are apparently spontaneous, are termed irregular spontaneous variations. Another point which must be taken into account is the energy of groicth ; that is, the relative capacity of different members for growth in length. The differences in the energy of growth in growing members manifest themselves in differences either in the length of the grand period, or in the rate of growth ; in other words, members may grow for a longer or shorter time, or they may grow more or less rapidly. In any case the result is that members attain different lengths. Tor instance, it is easy to observe that the lower internodes of most stems remain short; that those above them are longer ; that those of a certain part of the stem are the longest ; and that the upper ones again are short. In the same way the size of the leaves attached to these various parts of the stem increases from below to about the middle, and then diminishes. Variations in the Direction of Growth. Although it is true, as stated above, that the result of growth is, generally speaking, that the apex of the growing member is moved onwards in a line which is the continuation of the axis of the growing organ ; yet, during the actual process of growth, this relation of position is not maintained, because the rate of growth is at no time uniform throughout the transverse section of the region of elongation. Suppose a radial stem rising vertically from the soil ; the longi- tudinal axis of the fully grown portion of this stem is vertical, but this is not true of the growing portion. If the apex be looked down upon from above it will be seen to travel in an orbit round M.B. P 210 PART III. — PHYSIOLOGY. [§47 the prolongation of the longitudinal axis of the fully grown portion. "When the stem is radially symmetrical, the orbit is approximately circular ; but in cases in which the member tends to be bilaterally symmetrical, one diameter of the orbit becomes proportionally elongated, the orbit being then oval, or elliptic, until, finally, when the bilateral symmetry is strongly marked, the orbit becomes a straight line, the growing-point simply oscil- lating from side to side. Whilst the growing-point is travelling Fie. 131.— Illustration of the epinastic growth of the leaves of the Sunflower (Helianthus animus). A represents the position of the leaves when the plant is exposed to light; J5 represents the position of the leaves when the plant has been kept in darkness for twenty- four hours. In A the leaves are expanded in consequence of the directive (diaheliotropic) action of the incident rays of light. In B the leaves, in the absence of light, have become recurved in virtue of their inherent epinastic growth. in its orbit, it is at the same time being raised upwards ; so that it describes a path which is, according to the form of the orbit, a circular spiral, an elliptical spiral, or a zig-zag. These changes of position are, however, not permanent ; for example, though the growing-point may be travelling upwards in a spiral, the fully- grown stem does not resemble a corkscrew, but is straight. § 48] CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 211 These spontaneous changes in position of growing-points are designated generally by the term Nutation. All growing members nutate in a more or less marked manner ; but the most conspicuous instances are afforded by slender struc- tures, such as tendrils, and the internodes of twining stems. A peculiar form of nutation is commonly exhibited by dorsiventral members, such as leaves. In the early stages the one surface of the leaf grows much faster than the other, thus leading to certain peculiar forms of vernation and aestivation (see p. 43) ; in the later stages the other side grows the faster, and so the expansion of the leaf is brought about. When it is the upper surface which is growing the faster, whether along the transverse or the longitudinal axis of the leaf, it is said to be a case of ejrinasty (Fig. 131) ; when the lower surface, it is said to be a case of hyponasty. A striking example is afforded by leaves having circinate vernation, as many Ferns, Drosera, etc. ; this form of vernation is due to the growth of the leaf being at first longitudinally hyponastic. The convolute, involute, and conduplicate forms are all the result of trans- verse hyponastic growth in the early stages of development of the leaf, whereas the revolute form is the result of transverse epinastic growth. § 48. Induced Movements. All parts of plants which can exhibit movement are also irritable ; that is, they respond to the action of external agents either by a movement or by a change in the rate or the direction of their movement. The following are the principal causes, or stimuli, of movement, or change of movement : — a. Mechanical ; contact or pressure ; b. Variations of temperature ; c. Variations in the intensity of light ; d. Changes in the direction of incidence of the rays of light ; e. Changes of position with regard to the line of action of gravity (vertical); /. Differences of degree of moisture in the surrounding medium. a. Irritability to Mechanical Stimuli. This form of irritability is most strikingly manifested by motile mature members, and less markedly by certain growing members. The following are instances of irritability to contact manifested by mature motile members : by the leaves of the sensitive plants (see p. 174), and by those of Dion sea and Drosera ; by the stamens of Berberis, Mahonia, the Cynarese, and the Cistaceae ; by the lobes of the stigma of Miinulus, Martynia, and Bignonia (p. 176). 212 PART III. — PHYSIOLOGY. [§ 48 The most familiar case is that of Mimosa, pudica, the Sensitive Plant. The leaf of this plant is bipinnate, consisting of a primary petiole bearing at its free end four secondary petioles, upon which the leaflets or pinnae are inserted (see Fig. 126). The primary petiole is articulated to the stem; each secondary petiole to the primary petiole ; and each pinna to the secoridar3- petiole, by a pulvinus. When stimulated, the pinnae fold together forwards and upwards : the secondary petioles move sideways so as to come closer together and to lie almost parallel; and the primary petiole sinks downwards ; the pulvini act as hinges upon which the various parts move. It is only a few growing members which react perceptibly to mechanical stimulation ; such are tendrils, the petioles of leaf- climbers (e.g. Tropseolum, Clematis, Solanum jasminoides], the stem of at least one stem-climber, namely that of Cuscuta (Dodder), and roots. In these cases the contact must be of relatively long duration, becoming, in fact, pressure. The irritability of growing members to mechanical stimulation is, however, less marked than that of the mature motile members mentioned above. Even in the most sensitive growing members, such as tendrils, the resulting movement is comparatively slow. The movement induced in these members is that they tend to curve round -the object with which they have come into contact. The result of this is that fresh portions of the member come into contact and are stimulated to curve, so that the member forms coils round the object, and thus becomes firmly attached to it. In the case of roots, when the growing-point is more or less injured by pressure or otherwise, a curvature is induced of such a kind that the injured side becomes convex, with the result that the growing-point, and consequently the direction of growth, is de- flected from the obstacle or other cause of injury. 1). Irritability to Variations of Temperature. Movement, like the other functions (see p. 160), is affected by temperature, but this influence is not stimulating but tonic : it does not induce movement, but merely modifies the activity of movement. A sudden variation of temperature may, however, act as a stimulus and induce a movement. This kind of irritability has been de- tected in various leaves : for instance, a rise of temperature causes certain flowers (e.g. Tulip, Crocus) to open, and a fall of tempera- ture causes them to close : similarly a fall of temperature causes the leaves of such plants as the Sensitive Plant and the Wood- Sorrel (Oxalis Acetosetta) to fold up, whereas a rise of temperature causes them to expand (see Tigs. 125, 126). § 48] CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 213 c. Irritability to Variations in the Intensity of Light (Paratonic Effect of Light ; p. 162). This is exhibited in a marked manner by the majority of motile members, more especially leaves. When, for instance, the intensity of the light is diminished, the perianth- leaves of many flowers and the foliage-leaves and cotyledons of many plants perform movements which are termed nyctitropic or sleep-movements (see p. 173). Thus, the flowers close; and the foliage-leaves change their position in various ways, assuming what is known as the nocturnal position, so that thsy no longer present the surface, but the margin of the blade to the sky. Con- versely, when flowers or leaves which have assumed the nocturnal position are exposed to light, or to brighter light than before, they resume their normal expanded (diurnal) position. Another remarkable manifestation of this irritability is that movements of variation in some cases, and movements of growth in most cases, are retarded or arrested by exposure to light of a sufficient intensity. In illustration of the effect of light upon movements of varia- tion, it may be stated that certain members, such as the leaves of the Sensitive Plant, which perform spontaneous movements of variation, are unable to do so when exposed to bright light : under this condition the leaves become fixed, as it were, in the diurnal position. This is not, however, the case with all mature motile members : for instance, the movement of the lateral leaf- lets of the Telegraph-plant (Desmodium gyrans) continues even in bright sunlight. The paratonic action of light on movements of growth is strik- ingly exhibited in various ways. It is well demonstrated by etio- lated plants (see p. 162), that is, by plants which have been kept in darkness for some considerable time. A characteristic feature of etiolated shoots is the excessive length of their internodes, as com- pared with those of a shoot which has been growing for the same period exposed to the normal alternation of day and night. This ex- cessive elongation in darkness — which occurs as a rule in all radial and isobilateral members which usually grow exposed to light — is the result of the absence of the retarding paratonic action of light. The effect of the paratonic action of light can also be estimated by direct measurement of the growing member. As the result of a great number of comparative measurements, it has been found, in regard to members of all kinds, that the rate of growth is more rapid in darkness than in light. 214 PART III.— PHYSIOLOGY. [§ 48 An interesting demonstration of the relation of the rate of growth to light is afforded by the observation of the growth of any member at given intervals — every hour, or every two or three hours — daring an entire day of twenty-four hours. By this means it has been ascertained that a growing member exhibits a regular daily periodicity in the variations in its rate of growth, which has a direct relation to the alternation of day and night. The paratonic action of light varies with its intensity : the more intense the light the more marked the paratonic action. Exposure to very intense light may entirely arrest growth for the time being.' It has been found that the different rays of the spectrum are not equally active ; the paratonic effect of the more highly refran- gible rays (violet, indigo, blue) is far greater than that of the rays of lower refrangibility. d. Irritability to the Direction of Incidence of the rays of Light (Heliotropism). This kind of irritability is extremely common, and generally manifests itself in the most striking manner. The most active rays of light are those of high refrangibility (violet, indigo, blue). A remarkable example of this is afforded by the zoospores of various plants (e.g. Ulothrix, Hsematococcus, Botrydium, etc.). When light falls obliquely upon them, these zoospores arrange themselves in the water so that their long axes are parallel to the direction of incidence of the rays ; this phenomenon is termed Photo- taxis. Moreover, the direction of their movement is also determined by the direction of incidence of the light. They move in the line of incidence, but they may move either towards or away from the source of light ; the direction depending partly on the intensity of the light, and partly on the degree of irritability of the zoospore. When a zoospore moves towards a source of light, it is said to be positively phototactic ; when away from it, negatively phototactic. Another important case is the change of position of the chlorophyll- corpuscles in the cells (see p. 172). Motile cellular members, whether mature or growing, are, as a rule, sensitive to the directive influence of the incident rays of light. Among mature motile members, foliage-leaves are those which most markedly respond to the directive or heliotropic influ- ence of light ; among growing members, it is more especially stems and leaves which are sensitive, but roots have in many cases been found to be so. All these irritable members take up a definite § 48] CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 215 position, the light-position (p. 173), with reference to the direction of incidence of the rays of light. Members capable of performing movements of variation can, if necessary, change their light- position, whereas the light-position of other members can only be changed so long as they are growing. The particular position which the member assumes under the heliotropic influence of light, depends upon its organisation. Three classes of members, namely the dorsiventral, the isobilateral, and the radial, have therefore to be considered. It may be generally stated of dorsiventral members, that, for a certain mean intensity of light, their light-position is such that the morphologically upper surface is directed towards the source of light, and lies in a plane perpendicular to the direction of incidence of the rays : that is, they are diaheliotropic. The case of motile foliage-leaves may be taken first in illustra- tion, such as those of the Sensitive Plant, Robinia, Scarlet Runner, etc. When these leaves are exposed to light of sufficient intensity to cause them to assume the diurnal position (see p. 174), their upper (ventral) surfaces are at right angles to the direction of incidence of the rays. If, on the one hand, the light to which they are exposed becomes less intense than this, they will manifest no sensibility to its direction of incidence, but will merely assume the nocturnal position. If, on the other hand, the light becomes more intense, then the leaves will alter their position so that the blades will present their edge, instead of their ventral surface, to the incident rays (paraheliotropism, see p. 174). In the case of foliage-leaves and other dorsiventral members which cannot execute movements of variation, the light-position is- assumed in the course of development, and is fixed. Since it cannot be altered in relation to variations in the intensity of the- incident rays, the position assumed is determined by the most frequent direction of incidence of the rays of suitable intensity. Tor instance, the fixed light-position of the foliage-leaves of plants growing free in the open, is usually not such that the upper sur- face is horizontal, facing the zenith ; but such that it is directed towards that quarter of the sky from which, not the brightest sunlight, but the brightest diffuse daylight, falls perpendicularly upon it. In fact, it is not unusual to find that the fixed light- position of leaves, when the light is of high average intensity, is such that the surfaces are vertical, so that the margin is presented to the zenith. Under these circumstances both surfaces are equally 216 PART III.— PHYSIOLOGY. [§ 48 exposed to light, and the structure of the leaf becomes more or less isobilateral (see p. 114). The fact that the ultimate position of dorsiventral leaves is mainly determined by light, is demonstrated by removing them — whilst still growing, and therefore capable of a change of posi- tion— from its influence. In darkness these leaves take up an altogether different position (see Fig. 131), becoming curved in various ways ; when again exposed to light they resume their previous diaheliotropic position. With reference now to radial members, it may be generally stated that the essential feature of their response to the directive influence of light is that they tend to place their long axes in the direction of incidence of the brightest light falling upon them. Whereas in the case of dorsiventral members the important point is the relation of the morphologically upper surface to the direc- tion of the incident rays ; in the case of radial members the im- portant point is the relation of the long axis to the direction of the incident rays. An exact coincidence between the direction of the long axis of the mem- ber and that of the incident rays is, however, not always attained in .nature, on account of the antagonistic action of other directive influences. This point is more fully discussed on p. 222. It must be mentioned that, inasmuch as there are no radial members which are both heliotropically irritable and capable of performing movements of variation, all that is here said refers to growing radial members. In illustration, the case of a radial member which has been grown in the dark may be taken, and it may be assumed to be vertical. Light is allowed to fall upon it from one side ; the effect is a gradual curvature of the member, as it continues to grow, so that its long axis comes to coincide more or less nearly with the direction of the incident rays. But the curvature may be in one of two directions ; it may be .either such that the apex of the member comes to point towards the source of light, or such that it points in the opposite direction. When the former is the case the member is said to be positively heliotropic ; when the latter, it is said to be negatively heliotropic. The nature of the curvature, whether positive or negative, depends upon the specific irritability of the member. Thus, gene- rally speaking, primary shoots, including such forms as the stems § 48] CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 217 of Chara and Nitella, the peduncles of flowers, the stipes of the larger Fungi, and the conidiophores of Moulds, as also radial leaves such as those of the Onion, are positively heliotropic. Negative heliotropism has been observed in many roots, especially aerial roots, and in the root-hairs of Marchantia. With regard to shoots, the hypocotyl of Viscum, the Mistletoe, is negatively helio- tropic. Although the relation between the external symmetry of the member and its heliotropic irritability is generally that indicated above, yet there are exceptions : all dorsiventral members are dia- heliotropic ; but not all radial members are positively or negatively heliotropic, for some of them are diaheliotropic. It seems that continual exposure to intense light falling on one side induces at least physiological dorsiventrality in some radial members (e.g. shoots of Ivy and Tropseolum). The flattened, typically isobilateral, leaves of various Monocoty- ledons, such as those of Iris, appear to be positively heliotropic. e. Irritability to the Directive Influence of Gravity (Geotro- pism). The effects of the stimulating directive action of gravity must be clearly distinguished from those which are due to the mere weight of the parts. It is only the former which are referred to by the term geotropism. The geotropic curvatures are effected with considerable force, and will take place even against consider- able resistance ; for instance, it has been observed that the primary roots of seedlings will curve downward into mercury. Geotropic irritability is manifested by various members, such as stems, leaves, and roots. The phenomena of geotropism in the three categories of members, the dorsiventral, the radial, and the isobilateral, will now be studied. With regard to dorsiventral members, it appears that many leaves, both growing and motile, lateral shoots of Conifers and of many dicotyledonous shrubs, runners, etc., which are dorsiventral, take up such a position, when acted upon solely by gravity, that their longitudinal axis is horizontal — that is, at right angles to the line of action of gravity, the vertical — and that their morphologi- cally superior surface is directed upwards. If these members are moved out of this position so that their long axis is not horizontal, the}- curve until it is so ; or if they be so moved that the normally upper surface faces downwards, they twist until it faces upwards. These members behave in respect to the line of action of gravity 218 PART III.— PHYSIOLOGY. [§ 48 just as they do to the direction of the incident rays of light. They are diageotropic, just as they are diaheliotropic. It is a familiar fact that at all points of the earth's surface typi- cal radial members, such as primary shoots and roots, grow with their long axes vertical, but with this difference, that the direction of growth of the primary shoots is away from the centre of the earth, whereas that of the primary roots is towards the centre of the earth. It can be readily demonstrated (by Knight's machine) that this vertical direction of growth is due to the force of gravity, that it is, in fact, a phenomenon of geotropism. But the effect produced is precisely opposite in the two cases ; primary shoots grow in a direction opposed to that of the action of gravity, they are negatively geotropic ; primaiy roots grow in the same direc- tion as that of the action of gravity, they are positively geotropic. If these members be moved out of their normal position, they will return to it by performing geotropic curvature. The principle of Knight's machine is to expose growing plants to the action of centri- fugal force, either alone or together with gravity. The object of it is to demonstrate that gravity is the directive force which determines the re- _ . lative directions of growth of Fie. 132. —Geotropic curvature of a Pea-seedling e placed horizontally. The thicker outline indicates shoots and roots. When seed the original positions of the primary shoot and lings are grown on a rapidly root ; the shoot s has curbed upwards in the rotating vertical wheel, in course of its growth, the root v> has curved consequence of the continuous downwards. The bud at the apex of the shoot . . . . , is nutating. change in position with re- gard to the vertical, it is obvious that the directive action of gravity is eliminated, for all parts of the seedlings are acted upon by gravity for successive equal times in opposite directions: the only force in action is the centrifugal force. The result is that the primary roots grow towards the centre of the wheel, in a direction contrary to that of the line of action of the cen- trifugal force, whilst the primary shoots grow outwards, away from the centre of the wheel, in the same direction as the action of the cen- trifugal force. It is clear from these facts (1) that a purely physical force can determine the direction of growth of roots and shoots : (2) that the physical force employed (centrifugal force) affects primary roots and shoots in a precisely contrary manner : and it may be concluded that since the phenomena produced by the action of centrifugal force in these experiments are quite analogous to those observable in nature, the cause § 48] CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 219 of the natural phenomena is also a purely physical force, and the force of gravity is the one which meets all the necessary conditions. The geotropic influence of gravity is greatest when the radial member is in a horizontal position ; that is, the curvature into the normal position then takes place with the greatest rapidity. But the visible effect is the more marked, the further the member is removed from its normal position ; for instance, when a primary shoot is turned upside down, a curvature of 180° has to be per- formed in order that the apex may again point upwards. In addition to the primary shoots of seedlings, the following radial members are negatively geotropic ; the stipes of Mushrooms, the conidio- phores of Moulds, the stems of Characese, the stalks of the receptacles of Liverworts, the peduncles of many flowers, the setse of Mosses, etc. Also isobilateral leaves, such as those of Iris ; when placed horizontally in darkness, whether flat or edgeways, they curve upwards. In addition to the primary roots of seedlings, the following radial mem- bers are positively geotropic ; the hyphse of Fungi which penetrate into the substratum, the root-like filaments of Vaucheria and other Algse, the rhizoids of Muscinese, the rhizomes of Yucca filamentosa and of Cordyline rubra, etc. An instance of the absence of geotropic irritability in a growing member is afforded by the hypocotyl of the Mistletoe. The degree of geotropic irritability is not the same in all radial members. It may be generally stated that the lateral branches both of shoots and roots are less irritable than primary shoots and roots. For instance, the secondary branches of roots grow, not vertically downwards, but obliquely outwards and downwards, in the soil. It has been observed in some cases that the nature of the geo- tropic irritability of a member may change in the course of its development. For instance, the peduncle of the Poppy is posi- tively geotropic whilst the flower is in the bud, but negatively geotropic during flowering and fruiting. Again, the flowers of the Daffodil are negatively geotropic when in the bud, but they become diageotropic as they open. /. Irritability to Differences in the degree of Moisture in the surrounding Medium (Hydrotropisrn}. Irritability of this kind is especially characteristic of earth-roots which possess it in a high degree. It can be readily demonstrated by a well-known experiment. Peas or Beans are made to ger- minate in a sieve full of damp sawdust, the sieve being suspended 220 PART III. — PHYSIOLOGY. [§ 49 in a slanting position. The primary roots grow downwards through the sawdust, and escape into the air (which is kept moist). At first they grow vertically downwards in consequence of their positive geotropism, but they soon curve upwards towards the moist surface. They do this in virtue of their hydrotropic irrita- bility, and it is clear that they are positively hydrotropic. g. Irritability of other kinds. It has been ascertained by ex- periment that members of various kinds may be stimulated to curvature by other causes, such as differences of temperature on the two sides, galvanic currents, the flowing of currents of water, and by the presentation of various chemical substances ; but these various phenomena are not of such immediate importance to the well-being of the plant as those which have been described above in detail. The stimulating action of certain chemical substances (cheinio- tajcis) is, however, of some importance in connexion with the re- productive processes. It had been frequently observed that the motile male cells (spermatozoids) of plants possessing them appeared to be attracted to the female organ, fertilisation being thus en- sured, but the cause of this has only recently been ascertained, and only in certain cases. It appears that the female organ, when it is fit for fertilisation, excretes into the surrounding water a substance which attracts the male cells. In Ferns and Selaginella this substance is a compound of malic acid ; in Mosses it is cane- sugar. § 49. Localisation of Irritability. Among members which perform movements of variation, there are many instances of well- defined localisation of irritability. Thus, in the Sensitive Plant, no movement ensues when the upper side of the pulvinus of the primary petiole is touched, but only when the sensitive hairs on the under side of the pulvinus are touched ; and, in the leaflets, it is the upper side of the pulvinus which is sensitive. In Drosera, the irritability of the tentacles is localised in the terminal gland, In Diona3a, movement only ensues when the irritable hairs on the upper surface of the leaf are touched. Among growing organs, tendrils offer well-marked localisation of irritability. In most tendrils the lower or basal part is either not at all sensitive, or is sensitive only to prolonged contact. Most tendrils have their tips slightly hooked, and their irritability is localised in the concavity of this curvature. The tendrils of Cobcea scandens and of Cissus discolor are irritable on all sides ; § 50] CHAPTER III.— SPECIAL PHYSIOLOGY OF MOVEMENT. 221 in those of Mutisia the inferior and lateral surfaces are irritable, but not the superior. The irritability of the root to the pressure of obstacles (see pp. 169, 212) is localised in the.tip. The foregoing examples sufficiently prove the localisation of irritability to mechanical stimulation : and the question arises whether or not irritability to other stimuli is also localised. It has been ascertained that this is the case, in connexion with heliotropism and geotropism, at least in certain plants. Thus, the heliotropic irritability (i.e. sensitiveness to the directive influence of light) of the cotyledons of certain Grasses, though not abso- lutely confined to the tip, has been found to reside especially in that part, and the same is the case with the primary shoot of many dicotyledonous seedlings and with young shoots of. various plants. The geotropic irritability of roots also resides in the tip, and this appears to be also true of other members. § 50. Transmission of Stimuli. The most striking in- stances of this are offered by motile leaves, such as those of the Sensitive plant and of Drosera. If the terminal pair of leaflets of a pinna of the leaf of the Sensitive Plant be irritated, not only will they fold up, but each of the other pairs of leaflets of the same pinna will fold up in succession ; if the stimulus is sufficiently strong, its effect may extend to other pinnae causing their leaflets to fold up, or to the secondary petioles causing them to converge, or even to the main petiole which then sinks downward (see Fig. 126). Stimulation of one leaf, if sufficiently powerful, will cause move- ment in another. In the case of Drosera, stimulation of the central tentacles of a leaf causes the inflexion of the marginal tentacles (p. 48). In so far as heliotropic and geotropic irritability is localised in the tips of growing members, these must also afford instances of transmission of stimuli. The stimulus acts upon the irritable tip and the impulse is transmitted to the region in which the curva- ture takes place. The means by which stimuli are transmitted is a matter which is still under discussion ; but it appears that the means of trans- mission is not the same in all cases. Whilst in some, such as ten- drils and the leaves of Drosera, the stimulus is probably transmit- ted by means of the delicate protoplasmic filaments which connect the protoplasm of adjacent cells (see p. 65) ; in others, for instance Mimosa pudica^ the stimulus is transmitted as a disturbance of the hydrostatic equilibrium of the cells : it would, in fact, appear that 222 PART III.— PHYSIOLOGY. [§ 51 whilst the former means of transmission suffices for a short distance, the latter is necessary when the distance to be traversed is considerable. In Mimosa pudica there appears to be a special tissue along which the stimulus is conducted : it belongs to the bast, and consists of large elongated cells with pitted cellulose walls. § 51. Combined Effects of different Stimuli. Inasmuch as it is commonly the case that the motile members, whether growing or mature, are irritable to stimuli of various kinds, it is clear that the assumption by them of any particular position is the resultant effect of the stimuli which may be acting simultaneously. The phenomena in question are strikingly manifested by growing members, and it is to these that the following account especially refers. According to the position assumed in the course of their growth under the influence of various external directive influences, plant- members may be conveniently classified into those which have their long axis vertical, and those which have their long axis oblique or horizontal, the former are distinguished as orthotropic, the latter as plagiotropic. Most radial and isobilateral members are ortho- tropic ; all dorsiventral, and some radial members are plagiotropic. For instance, radial primary shoots and roots are orthotropic ; all dorsiventral leaves, etc., are plagiotropic ; lateral branches of shoots and roots, even though radial, are plagiotropic. The directive influences which mainly determine the direction of growth of radial primary shoots are gravity and the direction of the incident rays of light, and the shoots themselves are negatively geotropic and positively heliotropic. If only the conditions are such that each side of the shoot receives an equal amount of light, as when the plant grows quite in the open, no heliotropic curvature takes place, and the shoot grows erect. But when one side of the plant is shaded, as when it grows by the side of a hedge, the shoot in most cases curves heliotropically out of the vertical. This curvature is the resultant effect of the negative geotropism of the shoot which tends to keep it straight, and its positive heliotropism which tends to make it curve more than it actually does. Uni- lateral illumination usually causes some degree of curvature in shoots, because, as a rule, their heliotropic irritability is higher than their geotropic irritability. Exceptions to this rule have been found in the inflorescences of Verbascum and Dipsacus, which remain erect even when one side is shaded. § 52] CHAPTER III.— SPECIAL PHYSIOLOGY OF MOVEMENT. 223 Similarly, the influences which ordinarily determine the direction of growth of radial primary roots, are gravity and the distribution of moisture in the soil. If the soil is uniformly moist, the root grows vertically downwards under the influence of gravity ; but if the soil is not uniformly moist, the root will curve out of the vertical towards the moister area, because its hydrotropic is greater than its geotropic irritability. The conditions which determine the plagiotropic position of most radial lateral branches of shoots are these : they are negatively geotropic, and they are diaheliotropic, at least in intense light. In darkness they grow erect, in virtue of their negative geotropism. Some radial subterranean rhizomes are, however, diageotropic. The oblique growth of lateral roots is simply due to their feeble geotropic irritability. The conditions which determine the plagiotropic position of dorsiventral members are these : they are both diageotropic and diaheliotropic. But inasmuch as their heliotropic is higher than their geotropic irritability, their ultimate position is that in which the incident rays of appropriate intensity fall nearly or exactly at right angles upon the upper surface. It will be observed that, as a rule, in growing members wrhich are heliotropically irritable, this irritabilit}' is higher than any other ; consequently the ultimate position assumed by the member is mainly determined by the direction of the incident rays of light, and it is termed, therefore, a light-position (see p. 215), although other directive influences may have contributed to its assumption. The most remarkable case of combined effects is afforded by the growth of twining stems. A twining stem, at its first development, is straight, but after it has come to consist of two or three inter- nodes its apex hangs over to one side, for the stem is not sufficiently rigid to support its own weight. It then exhibits circumnutation in a marked manner. If once it comes into contact with a more or less vertical support of appropriate thickness, it twines round it. The commonly accepted explanation of twining is that it is due to the negative geotropism of the stem, combined with its circum- nutation modified by contact with the support ; but it is doubtful if this explanation is adequate. It has been suggested, with some show of reason, that twining stems may be irritable, like tendrils, though in a less degree, to continuous contact with a support. § 52. Conditions of Movement. Inasmuch as the move- ments under consideration are vital, they can only take place when 224 PART III.— PHYSIOLOGY. [§ 53 the external conditions are favourable to the life of the plant. The following conditions are essential ; a moderate temperature, extremes of heat and cold arrest movement ; a supply of water, all move- ments are arrested by drought ; a supply of free oxygen, in the case of most plants (p. 197) ; and, in some cases, exposure to light of a certain intensity. The importance of exposure to light as a condition of movement requires special consideration. It has been ascertained to be essential to movements of the most different kind. For instance, a Bacterium (Bacterium photometricutn) has been found to be motile only when exposed to light. Again, various movements of vari- ation, such as those of the foliage-leaves of Mimosa, etc., do not take place unless the plant either is, or recently has been, exposed to light. But the most important case is the arrest of growth of dorsiventral members when kept in darkness. For example, if a potato-tuber sprouts in a dark chamber, the produced shoots have excessively elongated internodes (see p. 162), but very small leaves ; the growth of the leaves is arrested in darkness. On the other hand, intense light retards movement (e.g. its action on growth) or altogether arrests it (e.g. arrest of spontaneous movement of the leaves of the Sensitive Plant). What is exactly the influence of light in promoting movement is not understood, but it is termed the pholotonic influence (p. 162) : it induces, that is, a particular condition, the condition of phototonus, in the protoplasm, without which movement is impossible. It appears that the rays of low refrangibility (red-yellow) are most favourable for the phototonic condition. Irritability also depends upon the above essential conditions : in fact, induced movements are more rapidly arrested by unfavourable conditions than are spontaneous movements. For instance, when a Sensitive Plant is kept in continuous darkness, the leaves first lose their power of responding to stimuli, and later their spontaneous movements cease. Irritability may also be abolished by special means. For in- stance, exposure to the vapour of chloroform or ether destroys the irritability of the leaves of the Sensitive Plant. Again, it may be abolished by repeated stimulation, the interval between the stimu- lations being very short. This has been observed in the case of the leaves of the Sensitive Plant and of Dionsea. §53. Mechanism of the Movements. The ultimate factor in the mechanism of the vital movements of plants, whether spon- § 53] CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 225 taneous or induced, is the motility of the protoplasm. With re- gard to the streaming movement of the protoplasm, it is probably due to wave-like contractions and expansions- ^of the protoplasm. The mechanism of the movements of the contractile vacuoles appears to be this : the systole of the vacuole is due to the sudden active contraction of the protoplasm, the contained liquid being expelled ; the diastole, to the active but gradual expansion of the protoplasm, the cavity of the vacuole, as it enlarges, being filled with liquid. The protrusion and retraction of pseudopodia in amoeboid movement may be accounted for in the same way ; the protrusion is probably analogous to the diastole .of the contractile vacuole, the retraction to the systole. A similar explanation may be offered of ciliary movement. The movements of cellular members take place in a definite region, which may be distinguished as the motile region ; this is, in growing members, the region of elongation (see p. 208) ; and, in mature members, is a more or less well-marked region of motile tissue which may constitute a distinct motile organ (e.g. pulvinus of a motile leaf). The movements depend essentially upon varia- tions in bulk of the cells, and these, in turn, upon variations in turgidity. It is clear that if the turgidity, that is the hydrostatic pressure of the cell-contents, increases, the cell will expand pro- vided that the wall be extensible ; and conversely, that if the turgidity diminishes, the cell will shrink, provided the wall be elastic. Movement can only take place when the cell-walls possess these physical properties: hence, the pulvinus of mature motile leaves consists mainly of parenchymatous cells with unlignified walls, the lignified tissue being reduced as much as possible : similarly, in the elongating region of growing-members the cell- walls are thin and unlignified. But whilst the movements of variation (p. 207) are the result of a sudden loss of turgidity, which is either spontaneous or the effect of stimulation, the move- ment of growth depends upon the maintenance of turgidity, and the variations in the rate of growth (see p. 208) are the expression of variations in the degree of turgidity. The following instances will serve to illustrate the foregoing considerations. A simple case is offered by the induced movement of the stamens of the Cynarese (p. 176). When at rest, the cells of the filaments are expanded in the direction of their length, and are turgid ; on stimulation, the cells suddenly shorten and become flaccid, having M.B. Q 226 PART III.— PHYSIOLOGY. [§ 53 lost a portion of their cell-sap. The expanded state is regained by the gradual expansion of the cells, tiu*gidity being restored by the absorption of water. In the foregoing case, all the cells of the motile portion are affected ; but in many cases some only of the cells are affected. Thus, in the case of the leaf of the Sensitive Plant, the primary petiole, when at rest, stands out nearly at right angles to the stem (Fig. 126, p. 174) : on stimulation, it sinks downwards so as to form an acute angle with the internode below its insertion. The mechanism is this : when at rest, the cells of the pulvinus are all turgid, and they support the petiole in its normal position : on stimulation, the cells of the lower portion of the pulvinus lose their turgidity, water escaping from them into the intercellular spaces ; these cells, being flaccid, are unable to counteract the downward pressure of the still turgid cells of the upper half of the pulvinus, and to support the weight of the leaf ; consequently the primary petiole sinks downwards. The same mechanism obtains in the movements of the leaflets and of the secondary petioles ; the only difference being that, in the pulvinus of a leaflet, it is the cells of the upper half of the pulvinus which lose their turgidity on stimulation, so the leaflet is raised upwards ; and, in the pul- vinus of the secondary petiole, it is the cells of the inner half which lose their turgidity, so the secondary petioles approach the middle line. This account is also applicable to all side-to-side movements, such as that of the leaf of Dionsea, and that of the stamens of Berberis and Mahonia. The heliotropic or other curvatures taking place in the elonga- ting region of growing cellular members, are due to the shortening of the cells on the side becoming concave, and to the elongation of the cells on the side becoming convex. The mechanism of the curvature seems to depend in this case not so much upon a differ- ence of turgidity between the cells of the two sides as upon a difference in its effect : whereas turgidity induces the usual longi- tudinal elongation of the cells of the convex side, it induces longi- tudinal shortening in the cells of the concave side in consequence of extension in the other dimensions. Turgidity is then the main factor in the mechanism of the move- ments of cellular members ; its mechanical importance is further strikingly illustrated by the great rigidity of turgid members, and by the great force, equivalent in some cases to twenty times the atmospheric pressure, which they develope in opposition to ex- § 54] CHAP. IV.— SPECIAL PHYSIOLOGY OF REPRODUCTION. 227 ternal resistance, as when the roots of tree cause the splitting of walls or of pavements. Although one essential factor in turgidity (see p. 159) is the purely physical osmotic activity of substances in the cell-sap, it must not be forgotten that it also depends upon the resistance offered by the protoplasm to filtration under pres- sure : so that the maintenance of turgidity is after all a vital act. The maintenance of turgidity appears, in fact, to depend upon a certain state of molecular aggregation of the protoplasm lining the cell-wall, in which it offers resistance to the escape of the cell-sap ; whereas in the flaccid condition the state of molecular aggregation of the protoplasm is such that it readily permits the escape of the cell-sap under the elastic pressure of the cell-wall. Whilst the fundamental mechanism of the movement of mature motile members and that of growing members is essentially the same, there is this secondary difference between the two cases. The change of position which is the result of the movement of mature members, is reversible ; they can return to their former position : the change of position, curvature for instance, of growing members is reversible only so long as it has not been rendered permanent by actual deposition of substance. Thus the changes of position due to the nutation (p. 211) of growing members are only temporary, for they are of brief duration ; but changes of position due to some directive influence acting for a considerable time become permanent, for instance, the light-positions (p. 223) assumed by growing members. CHAPTER IV. SPECIAL PHYSIOLOGY OF REPRODCUTION. § 54. Introductory. It has been already stated (see p. 49) that reproduction consists essentially in the throwing off by the individual of a portion of its protoplasm which does not merely grow but developes into a new organism ; and that two modes of reproduction, vegetative multiplication and spore-reproduction may be conveniently distinguished, though they are not absolutely distinct. Reproduction has been considered so far mainly from the morphological standpoint, and it now remains to discuss it from the physiological point of view. The most important general consideration is that reproduction is a function of embryonic, as 228 PART III.— PHYSIOLOGY. [§ 55 distinguished from adult, protoplasm. But it must not be over- looked that all embryonic protoplasm is not necessarily reproduc- tive : and it is interesting to trace the differences in this respect, presented by various kinds of embryonic protoplasm. To begin with, there is no doubt that the merismatic cells of the cambium are embryonic : but they are not at all reproductive, for they cannot give rise to a new member, still less to a new organism ; they can only add to the bulk of the body of which they form part, by the development of new tissue. Again, the protoplasm of a growing-point is embryonic, but it is only imperfectly repro- ductive ; it possesses this property to the extent that it not only contributes to the increase of the member to which it belongs, but also developes new members. Finally, the protoplasm of a reproductive cell, such as a spore, is embryonic and is completely reproductive ; for it does not in any degree contribute to the bulk of the parent-organism, but gives rise to a new individual. § 55. Vegetative Multiplication. This mode of reproduc- tion is distinguished as vegetative, because it is carried on by the vegetative organs of the plant, and, in the simpler cases, it is not distinguishable from the ordinary processes of growth ; though, in its higher forms it approximates to reproduction by spores. The simpler cases referred to are those of unicellular organisms : these, when they have reached by growth their characteristic limit of size, undergo cell-division, with the result that each new cell constitutes a new individual : here, multiplication is effected by a purely vegetative process, which, in a multicellular plant, would merely result in an increase in the number of the cells of which the individual consists. Much the same thing occurs in higher plants when (as in many Bryophyta, and in rhizomatous Pteridophyta and Phanerogamia) the main shoots die away, and the isolated lateral branches constitute new independent in- dividuals. Something of a similar kind also takes place in the artificial multiplication of plants by means of cuttings : in many plants, but by no means all, if a shoot be cut off and be kept under favourable circumstances with its cut end in suitable soil, the cutting will complete its segmentation by the development of roots, and will then be a new individual. Not uncommonly, certain parts of the body may become more or less specially modified to effect vegetative propagation : for instance buds be- come developed into bulbs or into bulbils (see p. 25), or portions of the stem or the root become tuberous. But the specialisation § 55] CHAP. IV.— SPECIAL PHYSIOLOGY OF REPRODUCTION. 229 which may be regarded as the highest of all, because it approaches most nearly to spore-reproduction, and involves the entire develop- ment of all the new members, is that of gemmae in which the vegetative reproductive body is not merely a modified member of the parent, but is a special development consisting in some cases of only a single cell (e.g. gemmae of some Algae and Liverworts ; oidium-cells of Fungi). Something of the same kind occurs amongst the higher plants, such as some Ferns, Bryophyllum, etc., where an entirely new structure, a bud, is developed on the leaf, and produces stem, leaves and roots ; it is in this way that Bego- nias are artificially propagated (see p. 136). An interesting artificial mode of vegetative propagation is that known as grafting or budding, in which a young shoot or a bud, termed the scion, of one plant is inserted into the stem of another, though allied plant, the stock (see p. 156) •, the scion and the stock grow together so as to form one plant, the scion retaining its own peculiar characters (e.g. graft- ing of fruit-trees, budding of roses). An important fact connected with vegetative reproduction is that it is associated with a rejuvenescence of the protoplasm. For example, when an adult cell of a unicellular plant, such as Pleuro- coccus (Fig. 137), divides, it gives rise, not to adult cells, but to young ones : and a cutting produces a young plant, not an old one. The relation of vegetative reproduction to the alternation of generations is of importance. In the lower plants (e.g. Bryophyta) where the gametophyte is the conspicuous generation, it is this generation which multiplies itself vegetatively, although vegetative reproduction of a somewhat different kind has been artificially induced in the sporophyte of some Mosses ; but in the Phanerogamia it is exclusively the sporophyte which thus multi- plies itself. In the Pteridophyta, whilst vegetative multiplication of the sporophyte is common, the gametophyte still retains this capacity in certain cases (some Ferns ; Lycopodium). Vegetative multiplication does not, as a rule, affect the alternation of genera- tions, each generation producing its like : the exceptions are afforded by cases in which the one generation is developed vegeta- tively from the other; that is, where vegetative propagation replaces spore-formation. For instance, in some Ferns, the pro- thallium is developed vegetatively from the Fern-plant, without the intervention of spores (apospory) ; and the Fern-plant vegeta- tively from the prothallium, without the intervention of sexual organs (apogamy}. 230 PART III.— PHYSIOLOGY. [§ 56 § 56. SpOKe-Reproduction (see p. 50). The highest degree of reproductive capacity is that possessed by spores. Though they are single cells, they are nevertheless capable, each by it- self, of giving rise to a plant-body which, as in the higher plants, may present complete morphological and histological differentiation. The advantages gained by the development of spores are, first, that they are readily scattered, so that the plants developed from them grow at a distance from each other ; this is, for instance, the meaning of the development of free swimming zoospores by plants (Algae) living in water. Secondly, spores, especially in the lower plants, are highly resistent to unfavourable conditions, such as drought and extremes of temperature ; so that they serve to maintain the species under conditions which would be fatal to the plant itself. In Phanerogams the function of maintaining the species through a period of unfavourable conditions, as also the dissemination of the new plants, is transferred to the seeds which, like the spores of lower plants, have a great capacity for endurance. Most plants, and probably all, produce spores ; and from the physiological point of view there are two modes of origin of spores : they are developed either asexually or sexually. In the lowest plants (e.g. Cyanophyceae, Schizomycetes, etc.), as also in others which have become sexually degenerate (Fungi, such as the J^cidiomycetes and Basidiomycetes), spores are only produced asexually : whereas in some sexual plants there is an exclusively sexual formation of spores (some Algae, such as the Conjugates, the Fucacese, and the Charoideae). In the higher plants (Bryophyta, Pteridophyta, Phanerogamia) spores are produced both sexually and asexually. Sexual Spore-formation. — The sexual process consists typi- cally in the fusion of two gametes, that is, of two sexual reproduc- tive cells, neither of which is capable, by itself, of developing into a new individual. The first question which naturally arises is as to the nature of sexuality ; the question, namely, as to what difference, if any, can be observed between a gamete and an asexually-produced spore. To this question no answer can at present be given ; no difference can be detected between a gamete and a spore. It must not, however, be concluded that because there is no observable differ- ence between a gamete and an asexually-produced spore, there is no difference whatever between them; on the contrary it is clear § 56] CHAP. IV. — SPECIAL PHYSIOLOGY OF REPRODUCTION. 231 that they differ widely, since the former cannot (except in rare cases), whilst the latter can, develope -by itself into a new organism. The second question is as to the nature of sex : what is the difference, if any, between a male and a female gamete ? In some cases there is a marked external difference ; for instance, in the Pteridophyta, Bryophyta, and many Algae, the female gamete- is a large motionless oosphere, whilst the male gamete is a small- actively-swimming sperrnatozoid. But this marked difference is not essential, it is merely adaptive ; it is an adaptation to a more or less aquatic mode of life or, at least, of fertilisation. Moreover, it is obviously inapplicable in explanation of the many cases in which the two conjugating gametes are externally quite similar. Nor has minute microscopic investigation brought to light any distinguishing criterion. But it must not be concluded on this account that there is no difference between a male and a female gamete ; it is obvious that there is an essential physiological' difference between them. For, were it otherwise, it would be- impossible to account for such a fact, for instance, as that even where, as in many Algae, the gametes are all extruded into the- water, fusion never takes place between two male or two female- gametes, but only between a male and a female. Brief allusion may be made to the means by which the sexual' process is ensured. It might be thought that the most effectual' means would be the development of the male and female organs- in close propinquity on the same individual. No doubt this is- the case, but the result is to ensure the less advantageous mode of the process, the mode of self-fertilisation ; in fact, in many cases in which the male and female organs are thus developed close together, as in moncBcious plants (p. 61), self-fertilisation is pre- vented by the male and female organs maturing at different times. The real problem is, then, to ensure a sexual process between two gametes derived from distinct individuals. The end is attained either directly, by bringing the diverse gametes together ; or indirectly, by bringing the spores together, and consequently also the gametophytes. The method of bringing the two gametes together is essenti- ally connected with the aquatic mode of fertilisation. It has been observed and investigated in plants in which, whilst the oosphere is motionless and remains in the female organ, the spermatozoids are free-swimming ; and it is among the most 232 PART III.— PHYSIOLOGY. [§ 56 striking phenomena of chemiotaxis (see p. 220). In various Mosses and Ferns it has been ascertained that, on the opening of the archegoninm, the mucilage which is extruded includes some substance which diffuses into the water and attracts to the archegonium any spermatozoid that may be present ; in Mosses the substance in question is cane-sugar; in the Ferns, a salt of malic acid. The method of bringing the spores together, so that they may germinate near each other, is especially characteristic of hetero- sporous plants, and more particularly of those which grow erect on dry land. It is thus most strikingly exhibited in the pollination of the Phanerogams, where the microspores are carried by the wind or by insects into such a position that they germinate in proximity to the macrospores. In order that a sexual process may take place between them, a certain relationship must exist between any two gametes of oppo- site sex ; when the limit is overstepped in the direction of either a too close or a too remote relationship, the process will either not take place at all, or the offspring will be few and feeble. The most fertile sexual process is that taking place between the gametes of different individuals of the same species. It has been proved that the offspring of such cross-fertilisation have the advantage in vigour and fertility over the progeny of one of the Bame plants when self -fertilised. , It has, in fact, been proved that in many Phanerogams the pollen of a flower is incapable of fertilising the oospheres of its own ovules ; and that the pollen from another flower of the same plant is only slightly, if at all, more potent. A sexual process may also take place between varieties of the same species ; between distinct species of the same genus ; and even between species belonging to different genera. Such a process is known as hybridisation, and the progeny as hybrids, the hybrid being distinguished as a variety- hybrid, species-hybrid, or genus- hybrid, according to circumstances. Effects of the Sexual Process. The sexual process is not always limited in its effect to the production of a spore which will give rise to a new individual. For instance, when the female cell is borne by the parent at the time of fertilisation, the act of fertili- sation induces a more or less marked growth and change in the adjacent organs and tissues of the parent, leading to the formation of & fruit (see p. 61). PART IV. CLASSIFICATION. Introductory. A systematic classification of plants may be arrived at by either of two methods. In the first, the different forms of plants are arranged according to some one given prin- ciple ; by this means order is established, and a definite position in the system is assigned to each plant. Many such systems have been devised, and are known as artificial systems. The principle of classification in such a case must be determined more or less arbitrarily and without considering whether or not, in the resulting arrangement, the plants which are nearly allied are always brought together, and those which are less nearly allied are kept apart. The best known of these artificial systems is that of Linnseus, called the sexual system, which classifies plants by the number and mode of arrangement of the floral organs. This system is, however, only applicable to Phanerogams. The natural system, to the gradual development of which a more exact knowledge of the reproduction of Cryptogams has largely contributed, has for its object the classification of plants according to their fundamental relationships ; and as these are established once for all by Nature itself, the natural system is not based upon any arbitrary principle of classification, but depends upon the state of our knowledge of these fundamental relationships. These find their chief expression in the structure and other characteristics of the reproductive organs, as well as in the peculiarities of poly- morphism presented by the life-history (see p. 2). This is more particularly true with regard to the definition of the larger groups of the Vegetable Kingdom ; within these groups relationships may be exhibited sometimes in one way and sometimes in another, so that it is not possible to lay down any universal rules for deter- mining closS affinities. As the investigation of this subject is still far from complete 234 PART IV.— CLASSIFICATION. the natural system cannot be regarded as being perfectly evolved ; the various general sketches which have hitherto been given are therefore no more than approximations to the truth. The following are the main divisions of the Vegetable King- dom :— IST GROUP. Thallophyta. Class 1. Algse. Class 2. Fungi. 2xD GROUP. Bryophyta. Class 3. Hepatic*. Sub-kingdom Class 4. Musci. ( Cryptogamia. 3RD GROUP. Pteridophyta. Class 5. Filicinae. Class 6. Equisetinaa. Class 7. Lycopodinee. 4TH GROUP. Gymnospermae. Class 8. Gymnospermae. Sub-kingdom STH GROUP. Angiospermae. Phanerogamia. Class 9. Monocotyledones. (Spermaphyta). Class 10. Dicotyledones. In considering the distinguishing characteristics of these great groups, it may be pointed out, in the first place, that whereas in the Bryophyta, Pteridophyta, and Phanerogamia, without excep- tion, the life-history presents a regular alternation of generations, in the Thallophyta the alternation is generally irregular and is, in many cases, altogether wanting. The Bryophyta differ from the Pteridophyta and the Phanerogamia, in that (a) in their life- history, " the plant " — that is, the form to which the name is attached (see p. 2) — is the gametophyte, whereas in the two latter groups it is the sporophyte ; and in (fe) the relatively rudimentary differentiation, both morphological and histological, of the sporo- phyte, whereas their gametophyte is more highly differentiated than that of the two latter groups. Finally, though resembling them in many respects, the Gymnospermae and the Angiospermse differ from the Pteridophyta in that they produce seeds : in fact, the Phanerogamia may be contrasted, as seed-bearing plants (Spermaphyta), with the three groups (Thallophyta, Bryophyta, Pteridophyta) of plants which do not bear seeds, and which are collectively termed Cryptogamia. INTRODUCTORY. 235 Furthermore, the Thallophyta are characterised by the fact that the female organ is never an archegonium, whereas in the other three groups it is never anything else than an archegonium, though it may present variations of form and structure (see p. 61). Considered with reference to plants now actually living, the above-mentioned Classes are of very unequal extent ; for while certain of them, as the Equisetinse, include few forms, and those for the most part very closely allied, others, as the Dicotyledones and the Fungi, include an enormous number of very different forms. These discrepancies arise from the very nature of the natural system, for a great diversity does not necessarily display itself within the limits of a single Class ; and it must not be for- gotten that when the living representatives of a Class, for instance the Equisetinae or the Lycopodinae, are few, they are but the sur- viving remnant of once various and numerous forms which have become in great measure extinct. Those Classes which include a sufficiently large number of forms are subdivided into subordinate divisions, as (1) Sub-classes, (2) Series, (3) Cohorts, (4) Orders, and these again, if necessary, into Sub-orders, etc. ; but these names are applied in the most arbitrary manner to the different sub-divisions. The two narrowest system- atic conceptions, viz., Genus and Species, are used to indicate an individual plant. Under the term Species are included all in- dividuals which possess in common such a number of constant characters that they may be considered to be descended from a common ancestral form. New peculiarities may arise in the course of multiplication : the individuals characterised by these new peculiarities are regarded in classification as varieties of the species. When several species resemble each other so distinctly that their general characters indicate a relationship, they are grouped together in a Genus. The limits of genera are conse- quently by no means fixed, but vary according to the views of individual botanists. In the larger genera the species are grouped into Sub-genera. The scientific name of every plant consists — on the plan intro- duced by Linnaeus — of two words, the first indicating the name of the genus, and the second that of the species. Thus, for instance, the greater Plantain, Plantago major, and the Ribwort, Plantago lanccolata, are two species of the genus Plantago. Since in early times the same plants were often described under different names, and as different plants were often designated by the same name, it 236 PART IV. — CLASSIFICATION. is necessary in systematic works, in order to avoid confusion, to append to the name of the plant the name of the botanist who is the authority for it. Thus Plantago lanceolata L., indicates that Linupeus gave the plant this name, and at the same time that the plant meant is the one which Linnseus described and to which he gave the name. Again, the Spruce Fir is called Picea excelsa Link, while the same plant was placed by Linnaeus in the genus Pinus under the name Pinus Abies L., and by De Candolle in the genus Abies (Don) as Abies excelsa DC. ; hence these names are synonymous : but Pinus Abies Duroi, or Abies excelsa Link, is another plant altogether, the Silver Fir (Abies pectinata DC). The method by which each plant has its place assigned to it in the natural system is exhibited in the two following examples — I. Plantago major ; II. Agaricus muscarius : I. Sub-kingdom : Phanerogamia. Group : Angiospermse. Class : Dicotyledones. Sub-class : Gamopetalse. Series : Hypogynse. Cohort: Lamiales. Order : Plantaginaceas. Genus : Plantago. Species : major. II. Group: Thallophyta. Class : Fungi. Sub-class : Basidiomycetes. Series : Autobasidiomycetes. Order : Hymenomycetes. Family: Agaricinse. Genus : Agaricus. Sub-genus : Amanita. Species : muscarius. GROUP I.— THALLOPHYTA : ALG.E. 237 GROUP I. THALLOPHYTA THIS group includes the more lowly-organised plants. As already mentioned, the alternation of generations is here either irregular or wanting. The morphology of these plants is such that the body is generally a thallus, though in certain cases there are more or less distinct indications of that differentiation of the body into root, stem, and leaf, which is so familiar in the sporophyte of the Pteridophyta and Phanerogamia. In those forms in which the sexual organs are differentiated, the female organ may be an oogonium, or a procarp, or an archicarp, but it is never an arche- gonium. These plants are further characterised by the simplicity of their structure : the body may be unicellular, ccenocytic and unseptate or incompletely septate (see p. 63), or it may be multicellular. One conspicuous structural feature (shared, however, with the Bryophyta), is the absence of lignified cell-walls, the cell-walls consisting generally of some form of cellulose, and being frequently mucilaginous. In the lower forms, vegetative reproduction by some mode of cell-division is not uncommon. The division of the group into the two classes Algse and Fungi appears to be artificial, inasmuch as it is based upon a single character, the presence (Algse) or absence (Fungi) of chlorophyll. But the division is really natural, since this one character is correlated with various others. It is, indeed, becoming usual to regard the Algse and the Fungi as altogether distinct groups : but it appears to be preferable to continue to regard them as classes of the group Thallophyta, inasmuch as the Fungi have doubtless arisen from the Algse, and since they possess many features in common. CLASS I.—ALGM. Many of these are plants of the simplest structure, which either live in water in the form of green, blue-green, red, or brownish filaments or masses of cells, or clothe damp surfaces, such as rocks, walls, or the bark of trees, with a covering of one or other of these colours. In the sea they attain often a very considerable bulk ; some of them are of a very beautiful red or brown colour, and 238 PART IV. — CLASSIFICATION. attract the attention of the observer, partly by their considerable size, and partly by the elegance of their form. The most important feature in which the plants of this Class differ from the Fungi is the presence of chlorophyll and the con- sequent mode of life. The Algae are able to form the organic siib- stances necessary for their nutrition, whereas the Fungi are obliged to obtain them from other organisms (p. 195). The presence of chlorophyll is obvious enough in the green Algae, but it exists also, though less evidently, in Algae which have a bluish-green, olive- green, brown, or red colouring-matter in addition in their chroma- tophores. The nature of this additional colouring-matter is usually the same throughout whole families which also resemble each other in their modes of reproduction. Hence this characteristic affords a trustworthy basis for classification, on which the Algae are divided into the following sub-classes : — Sub-class 1 : CYANOPHYCEJS (or Phycochromaceae), blue-green Algae, containing a blue colouring -matter phycocyanin ; ., 2 : CHLOROPHYCEJE, green Algae, containing only chlorophyll and its derivatives ; „ 3 : PELEOPHYCE.E, brown Algae, containing a yellow or brown colouring-matter phycophcein ; „ 4 : RHODOPHYCE.E, red Algae, containing a red or purple colouring-matter phyc.oerythrin. The colouring-matters phycocyanin, phycophsein, and phycoerythrin, can be extracted by means of water; they thus differ from chlorophyll, which is insoluble in water. The presence of chlorophyll in the Cyanophyceae, Phaeophycese, and Rhodophyceae, can be proved by ex- tracting the other colouring-matters with water ; the plants then assume a green colour. Structure. The body may be unicellular ; or coenocytic and unseptate (as in the Siphonaceae), or incompletely septate (Clado- phoraceae) ; or multicellular. The unicellular forms either exist singly, or a number may be held together in a colony by a mucila- ginous common cell- wall, either as a filament (e.g. some Desmidiese) or a mass (palmelloid Protococcaceae, Chroococcaceae). In some of the multicellular forms (e.g. Spirogyra, Pandorina, Ulva) all the cells of the body are quite similar; at first vegetative, they eventually become reproductive, so that there is no distinction between nutritive and reproductive cells: in these histologically GROUP I.— THALLOPHYTA : ALG^E. 239 uudifferentiated forms the body is termed a ccenobium. Even the most highly organised forms attain but a low degree of his- tological differentiation, amounting (as e.g. 'in the Fucacese) only to a distinction between peripheral assimilatory tissue and central conducting tissue : in some of the Laminariacese the con- ductiug-tissue has the form of sieve-tubes. Morphology. The body may be entirely undifferentiated ; this condition is most common in the unicellular forms, but it also occurs among the multicellular (e.g. Volvox) ; or it may present a distinction of base and apex (e.g. Rivularia) ; or it may be differentiated into root and thalloid shoot (e.g. Botrydium, Fucus) ; or into root, stem, and leaf (e.g. Cladostephus, Chara, Polysiphonia). The undifferentiated body (thallus), as also the thalloid shoot, A xjy. e. FIG. 133.— Growing-points of Algae. A Apical growing-point, with apical cell, of StypoeauloTi rcoparium ( x 30). B Intercalary growing-point (where the transverse lines are close together) of Desmarestia ligulata in longitudinal section (x 60). C Apical growing- point, with apical c«ll, of Cltaitopteris plumosa (x 40: afterFaulkenberg). presents great variety of form: it may be spherical, or filamentous, or a flattened expansion, and its symmetry may be multilateral, isobilateral, or dorsiventral. The growth in length of the thallus or of the shoot is effected in a variety of ways. It may be either apical or intercalary (Fig. 133.) In cellular plants the apical growth is effected either by a single apical cell (e.g. Characese, Sphacelariese, Fucacese, Dictyota, Fig. 106, most Rhodophycese) ; or by a marginal series of apical cells (e.g. Coleochseteae, some flattened Rhodophyceae) ; whereas in those coenocytic plants (Siphonoidese) wrhich grow apically, there is no apical cell, but an apical mass of embryonic protoplasm. In some •.cases of intercalary growth there is no growing-point, all the cells 240 PART IV. — CLASSIFICATION. of the body being merismatic (e.g. Spirogyra, Ulothrix, Ulva). In some few cases (e.g. Volvocoideae, Botrydium) there is no growth after the embryo-stage. The primary root is never developed in due proportion to the shoot ; consequently, in order to ensure the attachment of the plant, adventitious roots are very commonly formed on the shoot, and when the shoot is dorsiventral unicellular root-hairs are commonly developed on the surface in contact with the substratum. In some cases special organs of attachment (haptera, see p. 48), are developed on the shoot ; they may be adhesive discs borne on on the ends of branches of the shoot (e.g. Plocamium coccineum), or root-like out-growths as in Laminaria bulbosa, where at the base of the shoot, a large umbrella-shaped out-growth is formed, bearing numerous haptera on its upper and outer surface. The leaves vary in form. In a few cases they somewhat resemble the foliage-leaves of the higher plants : in others (e.g. Cladostephus, Chara) they resemble the stem and its branches, but are distinguished by their limited growth ; in others again (e.g. Polysiphonia and other Rhodophyceae), they are filamentous and hair-like. The Reproduction of the Algae is effected in various ways. Vegetative multiplication takes place in the unicellular forms (e.g. Cyanophyceae, Protococcoideae, Desmidieae, etc.) by cell-division, in some of the higher forms (e.g. Sphacelaria, Chara, Melobesia) by means of multicellular gemmae (see p. 49). Non-motile cells, with a cell- wall, which are probably gemmae, are thrown off by Vaucheria gcminata, and sometimes by other species of Vaucheria (Chloro- phyceae). Reproduction by means of asexually-produced spores occurs with but few exceptions (e.g. Conjugatse, Fucaceae, Characeae). Sexual reproduction is general throughout the class, though it has not yet been observed in all forms ; it appears to be definitely absent in the Cyanophyceae, and in some of the lower Chlorophyceae (e.g. some unicellular Protococcoideae). There are various modes of sexual reproduction in the group. The following is an enumeration of them (see also p. 58) : — I. Isoyamy : the sexual cells are similar gametes ; process, conjugation ; product, a zygospore. (a) Gametes ciliated (planogametes) ; set free ; e.g. Ulothrix, Pandorina, Ectocarpus, Cutleria. (6) Gametes not ciliated (aplanogametes) ; not set free in the Conjugates ; set free in the Diatomacese. GROUP I. — THALLOPHYTA : ALG.E. 241 II. Hcterogamy : (a) Oogamy : the female organ is an oogonium ; the sexual cells are spermatozoids and oospheres, the former ciliated and free-swimming, the latter not ciliated but sometimes free-floating; process, fertilisation ; product, an oospore ; (e.g. Volvox, Vaucheria, (Edogonium, Coleochsete, Characese, Fucacese). (b) Carpogamy ; the female organ is a procarp in which no female cell is differentiated ; male cell free, not ciliated, a sperma- tium ; process, fertilisation ; product, a fructification termed a cystocarp (Rhodophycese). The sexual cells are aplanogametes, planogametes, oospheres, spermatozoids and spennatia ; though they differ widely in various respects, they agree in being nucleated masses of protoplasm destitute of a proper cell-wall. The aplanogametes are characterised by the absence of cilia and of any defined form ; they are confined to the Conjugates and Diatomacese. The planogametes are somewhat pear-shaped, the anterior more pointed end being destitute of the chromatophores which are pre- sent in the more rounded portion. They have two cilia which are inserted, in the isogamous Chlorophyceee, at the pointed end of the cell ; in the isogamous Phseophycese, laterally at the junction of the anterior colourless portion with the posterior coloured por- tion of the cell. In conjugation, the planogametes first come into contact by their colourless anterior ends. The oospheres are spherical cells, usually containing chroma- tophores either throughout their whole substance, or leaving a colourless area on one side, the receptive spot, at which the sper- matozoid enters in the process of fertilisation (e.g. (Edogonium, Vaucheria, Sphaeroplea). The spermatozoids may be somewhat pear-shaded, resembling the zoospores of the plant, but smaller (e.g. Coleochsete, (Edogo- nium) ; or they may be more elongated and club-shaped (e.g. Sphseroplea, Volvox) ; or still more elongated and spirally twisted (Characeae). They usually bear two cilia at the pointed end ; but in Vaucheria, Volvox, and the Fucacese, they are inserted laterally ; in (Eiogonium there is a circlet of cilia round the pointed colour- less end. They are faintly coloured, in the Chlorophycese usually yellow. The male cells of the Rhodophycese are peculiar on account of the absence of cilia, and are distinguished by the special name M.B. R 242 PART IV.— CLASSIFICATION. spcrmatium :— the spermatia surround themselves with a proper wall at the time of fertilisation. The sexual organs. In those Algse in which the sexual cells are similar, and the sexual process is isogamous, the sexual organs are gametangia. In many cases they are unicellular and undifferen- tiated : thus, when the gametophyte is unicellular (e.g. Desmidiese, Diatomacese) the cell itself constitutes the gametangium ; and in some multicellular or coenocvtic forms (e.g. Zygnemeee, Hydrodic- tyon, Confervoidese) the gametangia are simply ordinary vegetative cells or coenocytes. In some isogamous Algse, however, the game- tangia are differentiated as lateral appendages, and are multi- cellular, as in the Phseosporese ; in Cutleria it is even possible to distinguish the male from the female gametangium. When the gametangium is unicellular or coenocytic, it usually gives rise to a number of gametes ; but in the Conjugates a single gamete is formed. When the gametangium. is multicellular, each cell usually gives rise to a single gamete ; but in the male gametangium of Cutleria 2-8 gametes are developed in each cell. The female organ, the oogonium, is in all cases unicellular or a coenocyte ; in Sphseroplea it is undifferentiated, retaining the form of a vegetative segment of the incompletely septate plant ; in most cases it is more or less spherical in form, and in some species of Coleochsebe it is prolonged at the apex into a delicate tube, the trichogyne. It opens, in most cases, by the absorption of the wall, at a point opposite the receptive spot of the oosphere when that is present; but in others (e.g. Volvox, Chara) it remains closed. In the former case the spermatozoid enters by the aperture : in the latter, it bores its way through the wall of the oogonium which becomes mucilaginous at its exposed surface. The oogonium of the Fucacese ruptures and sets free the contained female cell or cells. Usually a single female cell (oosphere) is formed in an oogonium, by the rejuvenescence of its protoplasmic contents ; but in various Fucaceae, the protoplasm divides to form two, four, or eight oospheres, and in the coenocytic oogonium of Sphaeroplea there are several oospheres. The female organ of the Rhodophycese, the procarp, is some- times unicellular (e.g. Nemaliese), but more commonly multi- cellular. It is in nearly all cases prolonged into a trichogyne, the basal portion being termed the carpogonium. The trichogyne re- mains closed. The protoplasm of the procarp does not undergo GROUP I. — THALLOPHYTA : ALG^E. 243 differentiation into a female cell comparable with the oosphere of the oogonium. The male organ, the anther idium, is with few exceptions (e.g. Sphaeroplaea) more or less differentiated in form, attaining its highest development in the Characeae. It is usually unicellular ; but in (Edogonium it consists of two cells, and of many cells in the Characese where its structure is highly complex. When the antheridium is unicellular, it usually gives rise to a number of male cells, but in Coleochaete and most Rhodophyceae only to one. When it is multicellular, each fertile cell gives rise to a single spermatozoid. Sexual organs are not known in the following forms : Cyano- phyceae, some Protococcoideae, some Siphonaceae, some Phaeosporese (e.g. Desmarestia ; Laminariaceae, except Chorda). The asexual reproductive cells of the Algae are formed either sexually or asexually : the former are either zygospores, or oospores ; the latter are spores. The zygospores and oospores occur exclusively in the Chloro- phycese and Phaeophyceae : they are spherical nucleated cells with a cell-wall. The differentiation of the cell-wall varies with the nature of the spores and of the conditions to which they are likely to be exposed. In the Phaeophyceae the zygospore or the oospore germinates at once on its formation, and its wall remains thin, and consists only of a single layer. In the Chlorophyeeae, the sexually produced spore usually undergoes a period of quiescence before it germinates, and except in marine forms (e.g. Acetabularia), it is exposed during this period to the danger of desiccation. As a protection, its wall consists of two layers, a delicate endospore, and a hard cuticularised exospore which often grows out into promin- ences, giving to the spore a stellate appearance. The spores produced asexually may be resting-spores with a thick wall, which may consist of two layers as described above (e.g. many Cyanophyceae) : or cells destitute of a cell-wall, either ciliated (e.g. zoospores of (Edogonium, Coleochaete, Sphaeroplaea, Pandorina), or not ciliated (e.g. tetraspores and carpospores of the Rhodophyceae). The asexual reproductive organs are sporangia. In the simple unicellular forms, the whole body may become a sporangium (e.g. Hsematoccoccus) : in some coenobitic multicellular plants there are no definite asexual reproductive organs, but any of the cells of the body may act as such (e.g. Ulothrix, Pandorina, Coleochaete, 244 PART IV.— CLASSIFICATION. Ulva) without any special morphological differentiation ; this is true likewise of the coanocytic Algse Siphonoidese, such as Botry- dium, Vaucheria, Sphseroplea, Cladophora, where the whole or part of the body may act as a sporangium. Specially differen- tiated sporangia occur only in some Phseosporese, and in the Rhodophycese where they usually produce each four spores (tetra- .spores) and are hence termed tetrasporangia : specially differenti- ated sporangia are also developed in the cystocarp of the Rhodo- phycese where they are termed carposporangia : these organs are in all cases unicellular. In the Cyanophycese the formation of spores is effected without any sporangium, for in these plants a cell of the body is converted into a spore by simple encystment. As a rule a sporangium gives rise to a number of spores ; but only one is formed in the sporangium of Vaucheria and of (Edogonium (see p. 85), and in the carposporangium of the Rhcdophycese. Sub-Class I. CYANOPHYCESE (also called Phycochromacese), or blue-green Algse. The body consists of a single, more or less nearly spherical cell, as in most of the Chroococcacese (e.g. Gloeocapsa, Fig. 134) ; or it is a multicellular layer one cell thick (e.g. Meris- mopedia); or it is filamentous, consisting of a row of cells (e.g. Oscillaria, Nostoc, Rivularia, Scytonema). When the body is filamentous, it sometimes presents a distinction of base and apex (e.g. Rivularia) ; and it is frequently branched. In most cases growth and cell-division go on in all the cells of the body, but in the Scytonemacese only at the apex. The plant is usually free, but it grows attached in some species of Rivulariacese and Sytonemaceae. A characteristic feature of the sub-class is the more or less bulky mucilaginous cell-wall which invests the cells and filaments. The filaments of the Oscillariacese exhibit a glid- ing, oscillating movement, but the mechanism of it is not fully understood. Reproduction is mainly effected in a purely vegetative manner. In the unicellular forms (Fig. 134) each cell-division necessarily leads to the formation of new individuals. In the flattened forms (e.g. Merismopedia), when the body reaches a certain limit of size, it simply breaks up into a number of portions each of which becomes a new individual. In the filamentous forms, vegetative propagation is effected by the breaking up of the filament into lengths, each such portion being termed a hormogonium ; in most GROUP I. — THALLOPHYTA : ALGLE. 245- of them (except Oscillariacese) the limits of the hormogonia are indicated by large inert cells, heterocysts (Fig. 135 ^4), which differ both in size and colour from the living cells to the filament. The Pis. 131.— Gkeocapsa (x 300) in various stages : A becomes H CD E by repeated division. (From Sachs.) PIG. 135.— A Filament of Nostoc ; the large unshaded cell is a heterocyst. B Portion of a filament of Oscillaria (+ 300). hormogonia are motile, though the mechanism of their movements: is not understood ; they eventually separate, and escaping from the common mucilaginous cell-wall of the filament, they develope by growth and cell-division into new filaments (Fig. 136 A B}. In many cases special reproductive cells, spores, are produced. Each spore is formed from a single cell of the body, which sur- rounds itself with a thick firm exospore ; the spore germinates s* t~ ** PIG. 136.— (After Thuret : x 330). A and B Development of a filament from a hormo- gonium of Nostoc vesicarium. A Cells of hormogonium dividing at right angles to its long axis; B rows of cells formed as B uniting at alternate ends, so as to constitute a Nostoc- filament ; z heterocysts. C Germinating spores of Anabcena licheniformis. under favourable conditions, the exospore being ruptured (Fig. 136 C). It is possible that zoospores are produced in some forms, but 246 PART IV. — CLASSIFICATION. the evidence is at present inconclusive. No form of sexual repro- duction has been observed in any member of this sub-class. The cells of the Cyanophycese contain nuclear substance, but the nucleus is not well-defined ; and the chlorophyll and the phy- cocyanin appear to be diffused throughout the cytoplasm, and not to be aggregated in special plastids. The Cyanophycese resemble the Schizomycetes, among the Fungi, in many respects ; as, for instance, in their general form and struc- ture, in their vegetative multiplication, in their spore formation, in the absence of sexual reproduction, in the formation of a bulky mucilaginous cell-wall, and in their polymorphism. On these grounds they are frequently placed, along with the Schizomycetes, in a distant class Schizophyta. But this arrangement does not seem to secure any special advantage. It is more natural to regard the Cyanophycese and the Schizomycetes as parallel groups, the one belonging to the Algse, the other to the Fungi. The Cyanophycese are both marine and fresh -water : many grow on damp walls, rocks, etc. Sub-Class IL CHLOROPHYCE.E, or Green Algae. In the simpler forms the plant consists of a single cell (e.g. Protococcoidese, some Desmidiese): or it is coenocytic, as in the Siphonoidese, either unseptate (Siphonacese) or incompletely septate (Cladophoracese, Hydrodictyacese) ; it is, in fact, only in this sub-class that the coanocytic structure occurs among the Algse : or the body is mul- ticellular, with essentially similar cells and therefore coenobitic (e.g. Spirogyra, Pandorina, Ulva), or exhibiting at least a dis- tinction between vegetative and reproductive cells (e.g. Volvox). The only members of the sub-class in which there is any appreci- able differentiation of the vegetative cells are the Characese. The body presents all degrees of morphological differentiation ; it may be a thallus, either spherical (e.g. Hsematococcus, Volvox), or filamentous (e.g. Spirogyra, Ulothrix), or a flattened expansion (e.g. Diva, Coleochsete) ; or a filament with rudimentary differen- tiation into root and shoot (e.g. (Edogonium) ; or it may present differentiation into stem, leaf, and root (e.g. Characese). It may be free or attached. Growth and cell-division commonly go on in all the cells of ,the body, so that the growth is intercalary (e.g. Spirogyra, (Edogonium, Ulva) ; it is but rarely that there is a definite growing-point, and then it is apical (Coleochsete, Characese, -some Siphonoidese) ; and in the cellular plants which have an apical growing-point, there is a single apical celL GROUP I.— THALLOPHYTA : ALG.^. 247 Vegetative multiplication by division occurs in some of the lower forms (e.g. Protococcoidese) of this sub-class. Reproduction by zoospores is general (absent in Pleurococcacese, Conjugate, most Volvocoidese, Characeae) ; they are formed, not in specialised re- productive organs, but in any cell or part of the body. A sexual process has been observed in members of every division of this sub-class : it is either isogamous, consisting in the fusion of piano- gametes or aplanogametes (Conjugatse), with the formation of a zygospore ; or oogamous, consisting in the fertilisation of an oosphere, which is in no case extruded from the female organ, by a spermatozoid, with the formation of an oospore. The sexual organs are either gametangia, or antheridia and oogonia ; they are unicellular in all the cellular forms (except the antheridium of Characese and that of some species of (Edogonium), and present various degrees of specialisation. A gametangium gives rise to many planogametes, but to not more than one aplanogamete ; the oogonium produces but a single oosphere, except in the coenocytic Sphaeroplea ; the unicellular (as also the coenocytic) antheridium gives rise to numerous spermatozoids, except in Coleochsete and in some species of (Edogonium where it forms only one ; in the multi- cellular antheridium of the Characese, numerous spermatozoids are developed singly in distinct mother-cells. There is considerable polymorphism in many members of this sub-class, so that various forms which were considered to be inde- pendent members of the simpler families are now known to be merely phases in the life-history of more complex forms ; for in- stance, various unicellular forms, such as Protococcus, Palmella, Grlceocystis, etc., formerly classed among the Protococcacese, are now known to be stages in the life-history of other Protococcoidese, Confervoidese, Siphonoidese, etc. The Chlorophyceae may be classified as follows :— - Series I. Protococcoidese : plants unicellular, isolated or held together by mucilaginous cell-walls into colonies; non-motile; the body is a thallus, and has no apical growth; reproduction, vegetative by division, asexual by zoospores, rarely sexual and then isogamous with conjugation of planogametes. Series II. Volvocoidese : plants unicellular or multicellular, and when multicellular not filamentous ; not attached, motile by means of cilia ; the body is a thallus, with limited growth ; reproduction, vegetative by division, asexual rarely by zoospores, sexual, either isogamous with conjugation of planogametes, or oogamous. 248 PART IV.— CLASSIFICATION. Series III. Siphonoidese : plants ccenocytic, unseptate or incompletely septate ; non-motile ; the body may be a thalJus or may be differen- tiated into stem, leaf, and root ; with or without apical growth ; reproduction, asexual by zoospores ; sexual, either isogamous (plano- gainetes), or oogamous. Series IV. Confervoideae : plants generally multicellular, filamentous, branched or unbranched ; growth frequently intercalary, rarely apical ; body attached or floating, a thallus, or sometimes with rudi- mentary differentiation into root and shoot ; reproduction, asexual by zoospores, in some cases ; sexual, isogamous (either planogametes or aplanogametes), or oogamous. Series V. Charoidese : plants multicellular ; body attached, differentiated into stem (with apical growth), leaf, and root ; reproduction, vegeta- tive by gemmse, no zoospores ; sexual, oogamous, with multicellular antheridium of complex structure. Series I. PROTOCOCCOIDE^E. The forms included in this series are very various, and, inasmuch as their life-history is for the most part imper- fectly known, it is uncertain to what extent they are independent, or are only phases in the life-history of each other or of higher Chlorophycese. It appears, however, that they may be fairly classified into two orders :— Order 1. Pleurococcaceae : cells isolated, or aggregated into colonies of more or less definite form ; multiply only by "cell-division ; no other mode of reproduction. To this order belong such isolated forms as Pleurococcus and Oocystis ; and such aggregate FIG. l37.-Pl zoospore ; « apex of the green filament ; ic a colourless adventitious root ; F filament with sexual organs; og oogonium ; h autheridium after rupture. (After Sachs.) GROUP I.— THALLOPHYTA : 251 tubular and much, branched (usually in Vaucheria, Fig. 140) ; or it may be differentiated into root and shoot, the shoot assuming various forms, such as a rounded cushion ((Jodium Bursa), or a simple vesicle (Botrydium, Fig. 142) ; or the shoot may be differentiated into stem and leaf (Acetabu- laria, Fig. 141); it has sometimes continuous apical growth (e.g. Vaucheria) ; the wall is sometimes impregnated with chalk (e.g. Acetabularia). Asexual reproduction is known to take place in only a few forms ; it is effected by zoospores, which are uniciliate in Botrydium, or multiciliate, as in Vaucheria, where they are sometimes non-motile. In Vaucheria the spores are formed singly in simple sporangia formed by septation of branches of the body ; in Botrydium they are formed in large numbers from the protoplasm of the tubular body. Sexual reproduction is generally isogamous by planogametes : Vau- FIG. 141. -Acetabularia crenulata (after Kutz- ing ; nat. size) : the terminal circular disc con- sists of a whorl of coherent leaves ; in these the gametangia are formed. FIG. 142.— Botrydium granulatum ( x 6) : s the green shoot ; w the colourless root. cheria is the only known oogamous form. Isogamy is known in Botry- dium, Acetabularia, and others. In Botrydium and Acetabularia the gametangia are formed by the aggregation of the protoplasm (in the coherent leaves of the latter) into rounded masses which become sur- rounded by a wall, and are set free by the rupture of the parent organism; their contents undergo frequent division to produce the planogametes which are eventually set free. In Botrydium and Acetabularia the con- jugating gametes are quite similar. In Vaucheria, the sexual organs are unilocular antheridia and oogonia, and are developed as lateral branches which become shut off by a septum (Fig. 140); each antheridium gives rise to a number of biciliate sperma- tozoids ; each oogonium gives rise to a single oosphere which is not 252 PART IV. — CLASSIFICATION. extruded, and has a hyaline receptive spot directed towards the opening of the oogonium. Vaucheria forms the green felt which covers the soil in damp flower- pots or other moist situations. Aquatic species occur in ditches, etc., often forming a thick scum on the surface. Order 2. Cladophoraceae. The body is incompletely septate and the segments are coenocytic ; it is filamentous, branched or unbranched, some- times differentiated into root and thalloid shoot, attached or free-floating, the shoot with or without apical growth : reproduction, asexual by zoospores ; sex- ual, isogamous, or oogamous. Fam. 1. SphceropJece : the body consists of floating unbranched filaments, without distinction of base and apex, and with intercalary growth. Each segment con- tains numerous small chloroplastids. Any segment of the body may become a sexual reproductive organ, either an oogonium or an antheridium, without any change of form; in the oogonium several oospheres are formed by free cell- formation (see p. 87), and likewise in the antheridium, after repeated nuclear divi- sion, a great number of sperm atozoids ; the oospheres are not extruded, but are fertilised in the oogonium by spermato- zoids which enter through an opening formed in the wall ; the oospore, at first green, assumes a bright red colour, and on germination sets free 2-8 zoospores, each of which gives rise to a new fila- ment. Sphseroplea is the only genus, and it comprises but one species — S. annutina : it inhabits fresh water. Fam. 2. Cladophorece : body filamentous, generally attached by a basal root-seg- ment, branched (e.g. Cladophora) or un- branehed (e.f the foot. The sexual organs (Fig. 150) are in all cases borne on the leaves ; the antheridium is developed from the terminal cell of a leaf or of a leaflet ; the oogonium replaces a leaflet. The plant may be either monoecious or dioecious. The antheridium is a spherical body, of a green colour when young, but orange when mature, borne on a stalk. Its wall consists of eight cells, each of which is termed a shield, presenting marginal infoldings of the wall ; the wall of the upper half of the antheridium consists of four tri- angular shields ; that of the lower half consists likewise of four triangular shields, each of which has its lower angle truncated to admit of the passage of the stalk-cell. On the inner surface of each shield, at its centre, is attached a cylindrical cell, the manubrium, which extends to near the centre of the antheridium. Each manubrium bears at its inner end a somewhat spherical cell, the capitulum. To each capitulum are attached usually six rounded cells, the secondary capitula. Connected with each secondary capitulum are two cells, each of which bears a pair of long filaments, each filament consisting of about two hundred cells. The cells of the filaments are the mother-cells of the spermatozoids, each cell giving rise to a single spermatozoid. The male cell or spermatozoid consists of a club-shaped spirally-wound mass of protoplasm bearing two long cilia at its pointed anterior end. When the antheridium is mature the shields separate, the spermatozoids are set free from their mother-cells and escape into the water. The oogonium is the enlarged terminal cell of the leaflet which it repre- 262 PART IV. — CLASSIFICATION. sents. Beneath, the oogonium proper is a node, the central cell of which constitutes the stalk-cell of the oogonium, whilst the five peripheral cells of the node grow out into filaments which gradually become spirally twisted and enclose the oogonium ; the tips of these filaments project at the free end of the oogonium, constituting the crown or corona, and are cut off from the rest of the filaments either by one transverse wall, so that the FIG. 150.— CTiarafragilis, reproductive organs (after Strasburger). A Median longitu- dinal section through a leaf (gametophyll) r, and the sexual organs which it bears; a an- theridium, borne on a nodal cell na by the stalk-cell p ; m the manubria ; ob an oogonium, borne on a nodal cell no and an internodal stalk-cell po ; c corona (all x 90). B spermato- zoids(x540). GROUP I.— THALLOPHYTA : ALG.E. crown consists of five cells as in the Chareae, or by two transverse walls, so that the crown consists of ten cells as in the Nitelleae. Each oogonium contains a single oosphere, a nucleated mass of protoplasm containing starch-granules, with a well-marked clear area, the" receptive spot, at the apical end. At the time of fertilisation, the cells of the crown separate so as to form a channel leading to the apex of the oogonium. The wall of the oogonium is not ruptured, but it becomes mucilaginous. The spermatozoids enter the channel and reach the apex of the oogonium; one of them makes its way through the mucilaginous cell-wall, and, entering the oosphere at the receptive spot, fertilises it. After fertilisation, the oosphere becomes an oospore, surrounding itself with a proper wall. The more internal walls of the investing filaments become thickened, and assume a dark brown colour. The whole organ falls off and undergoes a period of quiescence. On germination, the oospore does not at once give rise to an ordinary Chara plant. It produces, in the first instance, an em- bryo, consisting of a filamentous root and a shoot of limited growth. The adult form is developed upon the embryo by the development of a lateral growing-point at the node of the embryonic shoot (see Fig. 151). Fresh water. Sub- Class III. PILEOPHYCE.E, or Brown Algae. The body may consist of a single cell (e.g. Diatomaceae), but is generally multicellular. When multicellular, it presents various de- grees of morphological differentiation, being usually differentiated into shoot and root, and in some cases (e.g. Cladostephus, Sargassum) into stem, root, and leaf. Vegetative multiplication is com- mon in the unicellular forms, in which it is effected by division ; in a few forms (e.g. species of Sphace- laria) it is effected by means of gemmae. In all but the lowest forms there is a distinction between re- productive and vegetative cells, the former developing into more or less highly differentiated reproductive organs. I'iG. 161.— Chara frag His (after Pringsheim: x 4). Embryogeny: ap apical portion of shoot of the embryo ; r primary root of embryo, springing from the oospore ; tr adventitious roote,; I leaves, amongst which lies the growing- point of the adult shoot; i inter- mediate cell. 264 PART IV. — CLASSIFICATION. Asexual reproduction is effected by means of spores, either zoo- spores (as in the Phseosporese), or non-motile spores (as in some Phseogamte). The spores are developed either singly, or more commonly several together, in unicellular (and also necessarily unilocular) sporangia. Sexual reproduction is either isogamous or oogamous : when isogamous, it may be effected by aplanogametes (Diatomacese), but more commonly by planogametes (Phseosporese) which usually resemble each other ; but in some cases (e.g. species of Ectocarpus, Cntleriacese) they are of two kinds, differing in size and in the duration of their movement, the one which is smaller and more active being the male ; when oogamous, it is effected by means of spermatozoids and oospheres, and is peculiar in that the oospheres, though not ciliated, are extruded from the female organ before fertilisation takes place. The sexual plants may be monoecious or dioecious. The sexual organs, in the isogamous forms, are gametangia, sometimes unicellular (Diatomacese) but more com- monly multicellular (Phseosporese) : in the latter case each cell of the gametangium gives rise either to a single planogamete or to several : they are in most cases all alike, though some (e.g. in species of Ectocarpus, Cutleriacese) consist of smaller and more numerous cells than the others and give rise to the smaller planogametes. In the oogamous forms, the oogonium is unicel- lular, giving rise to one or more (2-8) oospheres : the antheridium is sometimes multicellular, but it is unicellular in the Fucacese ; in the former case each cell gives rise to a single spermatozoid, in the latter several spermatozoids are developed in the one cell. Of the motile reproductive cells of this sub-class, the zoospores and the planogametes contain chromatophores, and have two cilia inserted laterally ; the spermatozoids, however, have no chromato- phores, nor have the smaller planogametes in those cases in which the conjugating planogametes differ in size ; the oosphere has no receptive spot. The following groups of the Phseophycese will be considered :— Unicellular Forms : Order Diatomaceae: sexual reproduction isogamous by aplanoga- metes. Multicellular Forms : Series (a) PHJEOSPORE^E : sexual reproduction isogamous by planoga- metes ; asexual by zoospores. Order Ectocarpaceae (Ectocarpus, Sphacelaria, Cladostephus, etc.). GROUP I.— THALLOPHYTA : ALG.E. 265 Order Laminariacese (Laminaria, Alaria, Chorda, etc.). Order Cutleriacese (Cutleria Zanardinia). Series (l>) PH^OGAM^E : sexual reproduction oogamous; asexual, want- ing, or by non-motile spores. Order Fucacese. Order Diatomaceae. Unicellular plants, either free, or connected into filaments or masses by mucilage; sometimes attached. Reproduction, vegetative by division; or by means of asexual ly produced spores; or sexual isogamous by the conjugation of aplanogametes. The cell-wall is impregnated with silica. Both fresh-water and marine. The Diatomaceae resemble the Desmidiese in many respects; the two orders are, in fact, corresponding forms in the Phseophycese and the Chlorophycese respectively ; but, besides their colour, the Diatoms differ from the Desmids in the presence of silica in their cell-wall. The cell, or frustule, as it is called, is enclosed by a rigid wall. The wall, like that of the Desmids, consists of two halves, called valves, of different ages? whiph are not directly continuous, but are related to each other as the two parts of a pill-box, the one overlapping the other (Fig. 152). The cell-contents consist of a more or less vacuo- lated mass of protoplasm, which forms a laj-er in close contact with the inner surface of the cell-wall ; in this there is a nucleus, sometimes parietal sometimes central, and chromato- phores which may be very numerous and small, or few in number (sometimes only one) in the form of relatively large plates. Vegetative multiplication by division takes place by the division of the protoplasm into two cells; each of these cells has one of the two valves of the parent frustule on one side of it ; it then secretes a new valve on its naked side, which is smaller than the older valve and fits inside its rim; thus two new individuals are formed. It will be noted that this process of multiplication is accompanied by a decrease in size ; and, were it repeated indefinitely, the cells would become very small. This process of diminution is interrupted by the formation of auxospores, either asexually or sexually. In the former case the pro- toplasmic contents of a cell escape from the separated valves, as an auxo- spore, which, after growing considerably, secretes two new valves forming a n.-w and larger frustule. In the latter case, two naked cells which have thus escaped, conjugate to form an auxospore which becomes a new frustule. This process of conjugation differs, however, from that of the Desmidiese, in that no resting zygospore is formed, but simply a new individual. Series PH^EOSPOEE^. The multicellular body is attached; it some- FiG.162.— Pinnularia, a Dia- tom (mug. and diag.); « lateral view, showing the mode of connection of the two halves of the frnstule; s surface view. PART IV.— CLASSIFICATION. times consists of a flattened dorsiventral branched filament, the branches of which are often coherent into a disc which adheres to the substratum by the ventral surface and bears vertical shoots on its dorsal surface (e.g. Ectocarpus, Myrionema, Pylaiella) ; the body is frequently more or less clearly differentiated into root and shoot, and in some cases (e.g. Clado- stephus, Cheetopteris) the shoot is differentiated into stem and leaf ; adven- titious roots are very generally developed. The body presents a considerable variety of structure. In the simplest forms (e.g. Ectocarpus, etc.) it is filamentous and branched, the filament con- sisting of a single row of cells (monosiphonous) ; in others it is filamentous, consisting of several coherent longitudinal rows of cells (poly$iplionous}\ in the most highly developed forms it consists of parenchymatous tissue frequently differentiated into a small-celled cortex and a medulla of large cells elongated parallel to the long axis of the plant (e.g. Laminariacese). Growth in length may be effected without a de- finite growing-point, all the cells being merismatic (e.g. generally in Ectocar- pacese) ; or there may be a definite growing-point, which may be apical, with an apical cell (e.g. Sphace- lariese) ; or the growing- point may be intercalary, either sub - apical (e.g. Chordaria), or more or less basal (e.g. Laminari- acese). The division of the apical cell, or of the initial cells, of the grow- ing-point takes place only Fia. 153. - Longitudinal section through three inter- nodes of a sexual plant of Cladostephus verticillatus (Ectocarpaceae) : a gametophyll ; the larger appendages are foliage-leaves. (x60: after Pringsheim.) in one plane, the transverse. The segments thus formed undergo division either only transversely (monosiphonous forms), or longitudinally (poly- siphonous), or in several planes. The sporangia are in all cases unicellular. In the simple filamentous forms they are somewhat enlarged and rounded cells, either intercalary in position (e.g. Pylaiella), or terminal, occupying the place of a lateral branch, and generally sessile (e.g. Ectocarpus, etc.). In the more bulky thalloid forms, the sporangia may be merely developments of single superficial cells (e.g. Laminariacese) scattered singly or in groups (sori) over the whole surface. In others again they are borne as lateral branches on hair-like outgrowths from the superficial cells. In certain cases, where the shoot presents differentiation into stem and leaf (e.g. Cladostephus), the sporangia are borne on specialised leaves, sporophylls (Fig. 154). The gametangia are in all cases multicellular. each cell constituting a GROUP 1.— THALLOPHYTA : ALG.E. 267 loculus which gives rise to one or more planogametes. In their distribu- tion and general morphology they resemble the sporangia. The game- tangia of any one species are, as a rule, all exactly alike, but in some few cases they present two forms which differ in the size, "and consequently in the number, of their constituent cells (e.g. Ectocarpus fenestratus and E. secundus, Cutleriacese) ; the small-celled gametangia are considered to be the male, and the large-celled the female organs. The plants may be monoecious or dioecious (Cutleria). The zoospores and the planogametes are generally all very much alike : in Cutleria, however, and in those species of Ectocarpus which have two kinds of gametangia, the one kind of planogamete (female) is considerably PIG. 151.— Fertile leaves of Cladostephus vertivillatus: A sporopbyll ; one of the uni- cellular sporangia has discharged its zoospores with a mass of mucilage ; B gametophyll, bearing the multicellular gametangia. (x280: after Pringsheim.) larger than the other (male), and has a shorter period of motility ; the smaller planogametes are developed in the small-celled gametangia. A sexual process has been observed in but few cases (Ectocarpus silicuiosus, Scytosiphon lamentariiis, Cutleria). In the two former the planogametes are externally similar, but they behave differently in the process of con- jugation, some coming to rest earlier than others, thus indicating that they are female. When the female plauogamete is at rest, it is approached 208 PART IV. — CLASSIFICATION. by a number of the still motile male planogametes (Fig. 155), one of which fuses with it. In Cutleria the larger planogamete soon comes to rest, and then one of the smaller planogametes fuses with it. In Ectocarpus silicu- losus the zygospore gives rise to a plant which resembles its parents : it has been observed that, if the planogametes fail to conjugate, they are capable of germinating independently. The Phseosporese are almost exclusively marine, the only fresh-water forms being the genus Pleurooladia (Ectocarpacese) and two species of the genus Lithoderma. The size of the plants included in this series varies widely, from microscopic Ectocarpacese to gigantic tree-like Laminariacese, such as Macrocystis, Nereocystis, etc., which may attain a length of geveral hundred feet. In some of the Laminarias, which have cylindrical stalk-like region in their thalloid shoot, secondary growth in thickness takes place by means of a merismatic layer. In these large forms too, the conducting tissue is sometimes so far developed as to form sieve-tubes; though no woody tissue is developed, nor is it required in view of the fact that these plants live submerged. Series PH^EOGAM^E. The orders comprised in this group are character- ised by the oogamous sexual process. Order Fucaceae. Body differ- entiated into root and shoot; shoot usually thalloid, either cylindrical or flattened ; differentiated into stem and leaves in Sargassum ; growth in length by a single apical cell; branching generally dichotomous. No asexual production of spores. Sexual organs, unicellular antheridia and oogonia; spermatozoids, ciliated, formed several together in the antheridium : odspheres, set free but not ciliated; one (Py- cnophycus, Himanthalia), two (Pel- vetia), four (Ascophyllum), or eight (Fucus) formed in each oogonium. Marine. The body consists of what may be termed cortical and medullary tissue. The cortical tissue consists of closely-packed parenchymatous cells, the external layer of which, the limiting layer, is for a time merismatic, and plays an important part in the growth of the body. The medullary tissue consists of filamentous rows of cells the walls of which are mucilaginous and much swollen. The cortex is essentially the assimilatory tissue and the medulla the conducting tissue. Fio. 155. — Sexual process in Ecto- carpus siliculosus : I a-/, female piano- gamete coming to rest: II resting female planogamete suspended from the surface of the water, with numerous motile male planogametes : III con- jugation of a male and a female planogamete. (x790: after Berthold.) GROUP I.— THALLOPHYTA : ALG.E. 269 In some of these plants (e.g. Fucus vesiculosus, Ascophyllum, Halidrys, Cystoseira, Sargassum) there are large intercellular spaces, filled with air, which project on the surface, and are known as air-bladders ; they serve as floats. In Halidrys and Sargassum the air-bladders -are borne on special branches. The sexual organs are in all cases borne in depressions of the surface known as concepfades (Fig. 157). The conceptacles are commonly confined to special portions of the thallus ; either to the tips of the branches (e.g. FIG. 166.— Pucus vetieulosMS, about half nat. size : b air-bladders j / fertile branch. Fucus, Cystoseira) or to special branches, the gametophores (e.g. Himau- thalia, Ascophyllum). From the inner surface of the conceptacle there arise a number of hairs (pqraphyses) among which the sexual organs are borne. The oogonia (Fig. IBS) are nearly spherical, and are borne on a short stalk consisting of a single cell ; the antheridia (Fig. 158) are the lateral branches of some of the hairs. The plants may be monoecious (e.g. Fucus piatgcarptu, Halidrys, Pelvetia, Cystoseira), or dioecious (e.g. Himanthalia, Ascophyllum, Fucus vesiculosus and serratus); in the former case each conceptacle contains both antheridia and oogonia. 270 PART IV. — CLASSIFICATION. The oospore, which is the product of the fertilisation of an oosphere, germinates without any period of quiescence. It first becomes somewhat pear-shaped ; it is then divided into two by a transverse wall ; the more pointed of the two cells forms the primary root, whilst the other gives rise to the shoot (Fig. 158 d). FIG. 157.— Section of a female conceptacle, with surrounding tissue, of Fucus tjesicutosus. (x50: after Thuret.) FIG. 163. — Fucus vesiculosus. a Paraphysis, from male conceptacle, bearing antheridia ; b an oogonium (with paraphyses), showing division of its contents to form eight oospheres ; c process of fertilisation, an extruded oosphere surrounded by spermatozoids; d develop- ing embryo. (x!60: after Thuret.) GROUP I.— THALLOPHYTA : ALG.E. 271 Sub-Class IV. RHODOPHYCE.E (FLORIDELE) or Red Algae. Multi- cellular plants ; body, generally differentiated into shoot and root ; shoot, sometimes differentiated into stem and leaf ; flattened or filamentous ; when filamentous, consisting of a single longitu- dinal row of cells (monosiphonous} or of several rows (polysi- phonous") with or without a small-celled cortex , the filamentous forms grow by means of a single apical cell from which segments are cut off either by transverse walls, or by oblique walls alter- nately right and left ; the flattened forms grow by means of a marginal series of initial cells ; but in the Bangiacese there is no growing-point, all the cells being merismatic : branching, generally monopodial, but sometimes sympodial (eg. Plocamium, Dasya) ; adventitious roots common. Vegetative reproduction by gemmae (e.g. Monospora, Melobesia) is rare. The plant, as a rule, produces tetrasporcs asexually, but they are usually not borne on individuals which produce sexual organs, but on distinct individuals, though there are exceptions to this rule (e.g. Lomentaria kaliformis, Callithamnion corymbosum, Polysiphonia variegata, etc.). The spores are produced in unilocular sporangia, either singly, or two together, or sometimes as many as eight, but most commonly in fours; hence they are generally termed tctraspores. They may be formed t etrahedrally ; or by transverse divisions, when they are said to be zonate ; or by two divisions at right angles to each other, when they are said to be cruciate. The arrangement of the sporangia on the shoot is various. In simple monosiphonous forms (e.g. Callithamnion) the terminal cells of short lateral branches become sporangia. In forms of more complex structure the sporangia are developed internally, within the superficial layer of tissue. The sporangia may be scattered over the surface of the shoot, or collected into special receptacles of various forms. In some cases (e.g. some Rhodome- lacese, such as Polysiphonia) the sporangia are confined to certain specially modified branches which are termed sticliidia. The tetra- sporas are set free as spherical unciliated cells without a cell- wall. The sexual organs are antheridia and procarps ; they are usually borne by distinct individuals, but in some cases on the same (e.g. Grateloupia, Halymenia, Halarachnion, Xemastoma, Dudresnaya ciccinea &n& purpurifcra, Glceosiplionia capillaris, ITelminthora dicaricata}. 272 PART IV. — CLASSIFICATION. The antheridia are small and unicellular ; in the simple fila- mentous forms they occur singly or in groups at the ends of the branches ; in others of more complex structure, they are produced in special receptacles (e.g. Corallinacese) ; in the flattened paren- chymatous forms they occur in groups on the surface ; in those forms in which the shoot is differentiated into stem and leaf (e.g. some Rhodomelacese such as Polysiplionia fastigiata and nigre- scens, Chondriopsis tcnuissima) the antheridia are confined to the leaves, the whole or part of the leaf being specially modified for this purpose. The male cells (spermatia) are formed singly in the antheridia, and are set free as small, spherical or oval, unciliated cells destitute of a cell-wall ; they acquire a cell-wall at the time of fertilisation ; they contain no chromatophores, except in Bangiacese. The procarp presents considerable variety of form and structure. It may be unicellular (e.g. Bangiacese, Chan- transia, Batrachospermum, Lemanea, Nemalion), or multicellular, as is more commonly the case. The unicellular procarp consists simply of a carpo- gonium prolonged (except perhaps in Bangia) into a filament termed the tri- chogync. Various descriptions are given of the structure of the multicellular pro- carp ; however, it appears to consist essentially of a unicellular carpogonium (with a trichogyne) together with one or more specially differentiated auxiliary cells. In some cases (e.g. Dudresnaya coccinea, Squamariacese), the carpogonium and the auxiliary cells are not developed in the same procarp, but in distinct organs. Whether the procarps be unicellular or multicellular, the carpo- gonia agree in that the trichogyne remains closed, and further, in that the protoplasm of the carpogonium does not undergo re- juvenescence to form a distinct female cell (oosphere) as is the case in the oogamous Algae (see p. 241). The carpogonium is (except in the Bangiacese) developed from the terminal cell of a lateral appendage; in some cases (e.g. FIG. 159. — Portion of a branch of Dosya elegans, bearing a slichidium (»), with tetrahedral tetrasporangia (t); V empty tetrasporangium. (x 25 ; after Kutzing.) GROUP I. — THALLOPHTTA : ALG^E. 273 Polysiplionia fastigiata and nigrcscens) the lateral appendage is a leaf, the whole or part of which goes to form the procarp ; in the Corallinacese the procarps are aggregated in receptacles. The sexual process consists in the fusion of the protoplasmic con- tents of a spermatium with those of a trichogyne. The sperina- tium is brought by the water into contact with the projecting trichogyne to which it adheres, the spermatium being at this time covered with a cell-wall; the intervening cell- walls are absorbed at the point of contact, and the protoplasm of the sper- matium enters the trichogyne. The product of fertilisation is a fructification termed a cysto- carp, consisting of a number of carposporangia. The cystocarp is developed either directly or indirectly from the carpogonium : directly, when the procarp is uni- cellular ; indirectly, when it is de- veloped from both carpogonial and auxiliary cells : the trichogyne takes no part in the development of the cystocarp, being shut off by a septum. The simplest mode of direct formation of the cystocarp occurs in the Bangiacese ; the cavity of the carpogonium becomes chambered, by the formation of cell- walls, into usually eight cells, each of which is a sporangium, giving rise to a carpospore : only a single spore is formed in the genus Erythrotrichia. In other cases of direct formation (e.g. Nemalion, Batrachospermum), the carpogonium gives rise to a number of filaments, termed oo- blastema-filaments, which bear a cluster of sporangia (Fig. 160). In the indirect formation of the cystocarp, the carpogonium fuses with one or more of the auxiliary cells. In the simplest case (e.g. G-igartinacese, Rhodymeniaceae), the carpogonium fuses directly with the auxiliary cell (or cells), and from the latter the M.B. T FIG. 160.— Sexual organs of Nemalion ( x 300). A. lends of branches bearing a unicellular procavp t-o, and a group of antheridia s ; the trichogyne (t) of the procarp has two spermatia («) adhering to it. B early stage in the development of the cystocarp ; the fertilised carpo- gonium is undergoing growth and division. C nearly mature cystocarp, consisting of a number of short fila- ments each terminating in a carpospo- rangium. The development of the cystocarp is direct. 274 PART IV.— CLASSIFICATION. sporangia, or filaments bearing sporangia, are formed. In other cases the carpogonium gives rise to one or more elongated, branched, ooblastema-filaments which fuse with one or more auxiliary cells, and the sporangia are produced either from the ooblastema- filaments (e.g. Gelidiacese) or from the auxiliary cells (e.g. Squama- riacese and other Cryptoneminse). In the Corallinacese, where the procarps are aggregated in re- ceptacles, only a single cystocarp is formed from the whole group of procarps. Some of the procarps appear to be altogether abor- tive, and only those toward the centre of the group have tricho- gynes, whilst others seem to have only auxiliary cells : after Fio. 161.— Sexual organs of Spermothamnion Hennaphro&itwn. A Male and female organs ; c multicellular procarp ; t trichogyne ; V trichophore ; on terminal cluster of antheridia. B cystocarp developing from the fertilised procarp ; a cluster of carposporangia is springing from each of the two opposite lateral auxiliary cells. The development of the cystocarp is indirect ( x 300 ; after Naegeli). fertilisation, the carpogonia of the central procarps fuse with each other, and with the auxiliary cells of the other procarps, forming a large cell from the periphery of which the corpo- sporangia, constituting the cystocarp, are developed. In many cases the cystocarp consists merely of the cluster of sporangia (e.g. Bangia, Chantransia, Callithamnion, Dudresnaya) ; in other cases the cluster of sporangia is surrounded by a cellular investment, termed the pericarp, formed by the growth of adjacent sterile cells. Each sporangium always gives rise to a single carpospore, which GROUP I.— THALLOPHYTA : FUXGI. 275 is set free as a somewhat spherical unciliated cell destitute of a cell- wall, and germinates without any quiescent period. The germination of the tetraspores and of- the carpospores has only been followed in a few cases. Generally speaking the spore becomes elongated in form, and is attached by the more pointed end which is almost colourless ; division by a transverse wall then takes place ; the elongated attached cell developes into the root, the other into the shoot. The Rhodophyceae are almost exclusively marine ; the only fresh-water forms are Batrachospermum, Lemanea, and species of Chantransia, Bangia, and Hildenbrandtia. CLASS II.-FUNGI. This class, like the preceding, includes many very simple organisms, as well as others of tolerably high development. None of them contain chlorophyll ; hence they cannot assimilate so simple a carbon-compound as carbon dioxide, but must take up their carbonaceous food in the form of rather complex compounds, and their structure and mode of life are correlated with this peculiarity (see p. 189). Some are parasites, such as the Rusts and Smuts, and absorb these complex carbon-compounds from other liv- ing organisms, whether plants or animals. Others are saprophytes, absorbing these compounds from the remains of dead organisms, or from organic substance formed by living organisms ; the numer- ous and often large Fungi which grow on humus or leaf-soil in forests, or on the bark of trees, are examples of the former case ; the Yeasts and Moulds which make their appearance on juicy fruits, saccharine liquids, etc., are examples of the latter. Some Fungi are symbiotic ; that is, they live in intimate relation (symbiosis) with plants which possess chlorophyll, and obtain from them the necessary carbonaceous food, but without destroy- ing, or apparently injuring them. They commonly live with Algae, forming Lichens ; or in connexion with the roots of trees (esp. Cupuliferae) and of Orchids, Leguminosae, and other plants, or with prothallia (e.g. Lycopodium), forming what is known as Mycorhiza. The vegetative body may be unicellular, or coenocytic. In the former case it is small and rounded or rod-shaped in form. In the latter case the body is always a mycelium, consisting of more or less branched filaments, termed hyphce. The mycelium may be 276 PART IV. — CLASSIFICATION. unseptate, as in the Phycomycetes, in which case the body re- sembles in structure that of the Siphonaceae among the Green Algae (see p. 250 1. Or the mycelium may be septate, as in the higher Fungi, in which case it appears to be always incompletely septate ; that is to say, the segments of the hyphae which are marked out by the transverse septa, are not cells, each with a nucleus, but contain several nuclei, and are coenocytes (as in the Cladophoraceae among the Chlorophyceae). The hyphae grow in length at the apex in the manner of such Algae as Vaucheria and Cladophora (see p. 239). In some of the more complex forms, the hyphae of the repro- ductive organs form compact masses of tissue of a somewhat parenchymatous appearance, in which there is no differentiation of tissue-systems, but the superficial layers of hyphae form a kind of tegumentary tissue, termed generally cortex. Considerable differences in the nature of the cell-wall may obtain in different parts of such organs, some walls being soft and mucilaginous, whilst others are relatively hard without, however, ever being lig- nified. In a few Mushrooms (e.g, Lactarius) some of the hyphae form a system of laticiferous tissue, and in others glandular struc- tures occur. Except in the simplest forms, the body is generally more or less clearly differentiated into root and shoot. These members can be distinguished partly by their relative position, the root-hyphae growing into the substratum, and the shoot-hyphae into the air ; and partly by the fact that the shoot-hyphse bear the reproductive organs. Some parasitic forms have root-like organs, termed haustoria, which penetrate into the cells of the host ; similar organs occur in some saprophytes, and in others (e.g. crustaceous Lichens) the roots (sometimes called rhizines) consist of bundles of hyphae. There is in no case any differentiation of the shoot into stem and leaf. The foregoing account does not apply to the body of the Myxomycetes, which consists of a multinucleate mass of protoplasm, termed a plas- modium, without any cell-wall. It is formed by the cohesion of a number of small, originally independent amoeboid cells, like that of the Hydrodictyacese among the Algae (see p. 253). Vegetative propagation is common among the Fungi. The simplest form of it is simple cell-division (e.g. Schizomycetes), or that form of cell-division known as budding or sprouting (gemm- GROUP I. — THALLOPHYTA : FUNGI. 277 at ion) (e.g. the Yeast-forms of various Fungi). It is effected in some cases (e.g. in some Zygomycetes, Ascomycetds, and Basi- diomycetes) by unicellular gcmmce of various .sizes (termed chlamy- dospores when they are relatively large and thick -walled; and are adapted for a period of quiescence ; oidium-cells, when they are small and thin-walled and capable of immediate germination) which are formed by the segmentation of a hypha by transverse septa into short cells which become somewhat rounded and separate from each other ; on germination, each may give rise to a mycelium. In other cases (e.g. many Ascomycetes, such as the Sclerotiniese, Pezizese, Claviceps, etc. ; some Basidiomycetes, such as Coprinus stcrcorarius, species of Typhula and Agaricus), it is effected by bodies termed sclcrotia ; each sclerotium consists of a compact mass of hyphge, filled with reserve materials, covered by a cortex of one or more layers of tissue, which are thick- walled and of a dark colour. They become detached from the mycelium on which they are formed, and are capable of retaining their vitality during a long dormant period ; on germination they give rise to shoots bearing reproductive organs. A form of sclerotium is found also in the Myxomycetes. Here it consists of a plasmodium, or a part of a plasmodium, which has surrounded itself with a membrane, and remains for a longer or shorter time in a dormant condition : the individual amoeboid cells may also surround themselves with a membrane and remain dormant, in the form of microcysts. Reproduction is effected sexually or asexually. A sexual process takes place in the Zygomycetes, in the Peronosporacese, and in some Ascomycetes. The modes of the sexual process are the following : — I. Isogamy : sexual cells, similar aplanogametes which are not set free ; process, conjugation ; product, a zygospore ; Zygomy- cetes. II. Heterogamy: — a. Oogamy : sexual cells, oospheres and undifferentiated male cells ; process, fertilisation ; product, an o'ospore ; Peron'o- sporacese. b. Carpogamy : no differentiated female cell ; female orga'ri fertilised by the undifferentiated contents of the male organ or by differentiated male cells, spermatia : product, a fructifi- cation termed an ascocarp : all the forms in which this mode occurs belong to the Ascomycetes. 278 PART IV.— CLASSIFICATION. There is no sexual process in the Schizomycetes, the Myxomy- cetes, in some of the Phycomycetes (Saprolegniacese), the great majority of the Ascomycetes, the vEcidiomycetes, and the Basidi- omycetes. In the Schizomycetes and Myxomycetes, the absence of a sexual process may be attributed to their rudimentary charac- ter ; in the higher groups it is due to sexual degeneration. In the Saprolegniacese, female and, generally, male organs are deve- loped, but the male organs are functionless ; still the female organs produce oospores. In the majority of the apparently sexual Ascomycetes, even when both kinds of sexual organs are present (e.g. Erysiphese, Penicillium, Sordaria) it is a question if any sexual process takes place : yet in all these cases an ascocarp is pro- duced, either from the female organ or from the mycelium. The sexual organs, with the exception of those of some Ascomy- cetes, are unicellular. They are either quite similar to each other, as in the Zygomycetes and some Ascomycetes (e.g. Eremascus), when they may be termed gainetangia ; or they may be more or less differentiated, as in the Oomycetes, and in some Ascomycetes (e.g. Erysiphese, etc.), as male and female. The male organ is a pollinodium in the Oomycetes and in some Ascomycetes (e.g. Pyronema, Erysiphese, Ascobolus) ; it is generally unicellular but sometimes multicellular (e.g. Ascobolus). As it is developed in close proximity to the female organ, fertilisation is effected, in these forms, by absorption of the cell- walls at the point of contact of the two organs, or the development of a tube placing their cavities in communication. In some Ascomycetes (the Laboulbeniacese) non-motile male cells (spermatia} are formed in unicellular antheridia. Spermatia occur in other Ascomycetes, as also in the ^Ecidiomycetes, but their sexuality has not been established in these cases. The female organ is either a unicellular closed oogonium (Oomy- cetes), or a unicellular or multicellular archicarp (Ascomycetes). The archicarp may consist (like the procarp of the Rhodophycese) of two parts : a receptive portion, the trichogyne, which is a more or less elongated multicellular filament, and a sporogenous portion, the ascogoniuntj from which, after fertilisation has taken place, the one or more sporangia (asci) of the ascocarp are developed. Sexual cells are only clearly differentiated in the case of the female cells of the Oomycetes and of the spermatia of some Ascomycetes. The female cells of the Oomycetes are oospJieres, GROUP I. — THALLOPHYTA : FUNGI. 279 spherical cells destitute of a proper wall : the spermatia generally have a cell- wall. In all other cases the protoplasmic contents jof the sexual organs are not differentiated into cells of definite form ; but the fusing masses of protoplasm of the Zygomycetes may be regarded as aplanogametes ; and that portion of the protoplasmic contents of the pollinodium of the Peronosporacese which enters the oogonium and fertilises the oosphere, may be regarded as a male cell. An asexual formation of spores is of general occurrence. In the Schizomycetes there are no special spore-bearing organs, but the protoplasm of the cells surrounds itself with a proper cell-wall, and becomes a spore. In the Myxomycetes sporangia are formed, attaining, in some forms, a high degree of complexity of structure. In the higher Fungi, the spores are formed, speaking generally, either in the interior of unilocular sporangia (e.g. Phycomycetes), or by abstriction, either singly or a number in succession, from certain special hyphse (as in the Ascomycetes, JScidiomycetes, and Basidiomycetes) ; in the latter case the spores are often distin- guished as conidia. In either case, the spores are borne upon an organ, a special branch of the mycelium, termed a sporophore or conidiophore. This may consist of a single hypha (e.g. Mucor, Peronospora, Penicillium, Puccinia), when it is said to be simple ; or of a number of coherent hyphse (e.g. the Mushroom, and the fructifica- tions of other Basidiomycetes ; the ascocarp of the Ascomycetes ; the secidium of the ^Ecidiomycetes) when it is said to be com- pound. The conidiophores may be scattered over the mycelium, or they may be collected into receptacles termed pycnidia. The asexually-formed spores are but rarely motile (e.g. ciliated zoospores of Myxomycetes and Oomycetes) ; in all other Fungi they are non-motile and have a cell-wall. There is considerable variety in their form, colour, etc. In some cases the spores are compound ; that is, they appear to consist of two or more cells (e.g. teleuto- spores of Puccinia Graminis and other vEcidiomycetes ; ascospores of some Ascomycetes such as Pleospora, Hysterium, Cordyceps, etc.) ; each cell, however, germinates independently and is there- fore itself a spore. These compound spores are formed by the division of a primary mother-cell. The Life-History of the Fungi is generally very much compli- cated by . polymorphism. In most of the Schizomycetes there is 280 PART IV. — CLASSIFICATION. remarkable polymorphism especially in the higher forms which pass through several distinct phases in the course of their life. Again, in some Ascomycetes and ^Ecidiomycetes there may be two or three forms bearing different kinds of reproductive organs, the different forms being parasitic on different hosts (hctercecisiri). The Fungi may be classified as follows : — Sub-Class I.— SCHIZOMYCETES : Body unicellular, or multi- cellular and filamentous ; no special spore- bearing organs ; no sexual reproduction. Sub-Class II. — MYXOMYCETES : Body a plasmodium ; spores formed in more or less well-developed spor- angia ; zoospores ; no sexual reproduction. Sub-Class III. — PHYCOMYCETES : Body generally a coenocytic unseptate mycelium ; sexual reproduction general ; zoospores present in most orders. Section A. — Zygomycetes: sexual process isogamous ; product, a zygospore. Section B.— Oomycetes : sexual process oogamous ; product, an oospore. Sub-Class IV. — ASCOMYCETES : Body usually an incompletely septate mycelium; sexual process carpoga- mous ; the product is an ascocarp. Sub-Class V.— JEciDiOMYCETES : Body an incompletely septate mycelium ; no sexual process. Sub-Class VI.— BASIDIOMYCETES : Body an incompletely septate mycelium; no sexual process; compound sporophores are always formed. Sub-Class I. — SCHIZOMYCETES. These organisms are either uni- cellular or multicellular ; most of the unicellular forms are very minute. The cell consists of a mass of protoplasm, with a rudimen- tary nucleus, surrounded by a cell- wall which consists in some cases of cellulose, and in others of a proteid substance. In some cases the cells are coloured red, green, blue, etc. ' a starchy substance, turning blue with iodine, is found in the cells of some forms (e.g. Bacillus Amylobacter). The forms presented are extremely various. The individuals may be spherical, the Coccus-form (Fig. 162, a) ; or rod-shaped, the Bacterium-form (Fig. 162, &) ; or spirally-wound, the Spirillum- and Spirochsete-forms (Fig. 162, d) ; or straight free filaments, the Bacillus-form ; or straight filaments attached by one end, the Cre- GROUP i.— THALLOPHYTA: FUNGI. 281 0 nothrix-form ; or the individuals may form cubical masses, as in Sarcina Ventriculi. Some forms (e.g. Bacillus, Spirillum, and some Coccus-forms) are capable of locomotion";- they are provided with one (Coccus-form) or more (one or more at each end in Bac- illus and Spirillum-forms) vibratile cilia, by means of which movement is effected. A remarkable phase, common to the life-history of nearly all forms, more especially the unicellular, is the zoogloea-stage. It consists of great numbers of cells held toge'ther by bulky mucilage, to form either a membrane (e.g. the scum on putrefying liquids) or masses of the most various form. A striking zoogloea-stage is that known as Leucoriostoc mesenterioideSj which consists of wavy chains of cocci imbedded in a mass of mucilage, the whole resembling the structure of Nostoc ini the Cyanophycese (Fig. 136, A, B). Although a special name has been given to each of the multi- farious forms assumed by the Schizomycetes, it must not be assumed that each form to which a name has been given con- FIG. 162.— pifferent forms of Schizo- mycetes : a Micrococcus ; b Bacterium ; c Bacillus with spores; d Spirillum (diag.: x600). FIG. 163.— Bacillus subtilts: A zooglcea-stage; B motile stage; C zooglcea-stage, with spore-formation. After Strasburger : x 800.) stitutes a distinct species. On the contrary, the Schizomycetes are highly polymorphic, and the various simpldr forms are, for 282 PART IV. — CLASSIFICATION. the most part, merely phases in the life-history of the more com- plex forms. The Schizomycetes multiply mainly by cell-division (whence their name), and they do so with great rapidity under favourable conditions. In many forms reproduction is also effected by means of spores (e.g. Bacillus subtilis, Bacillus Anthracis, Clostridium butyricum). Each spore is formed from a cell, the protoplasmic contents contracting from the cell-wall and surrounding them- selves with a thick proper wall ; the spore is set free by the decay of the old cell-wall. Spore-formation generally takes place in the zooglo3a-stage, and is promoted by conditions which are un- favourable to growth and multiplication by division. The vitality of the spores is remarkable, being retained under conditions, such as extremes of temperature, desiccation, etc., which would prove fatal to the organisms themselves. A comparatively simple life-history is that of Bacillus subtilis, which makes its appearance in infusions of hay when allowed to stand. The in- fusion gradually becomes turbid, owing to the rapid multiplication of the Bacillus. At first the organisms move actively by means of cilia (Fig. 163, B), the motile form: after this the cells cease to be motile, and remain connected into filaments, the bacillus-form: then a membranous scum forms on the surface of the liquid, the zoogloea-form : at this stage spore-formation begins (Fig. 163, C) and the life-cycle is at an end. The spores give rise, on germination, to the motile form, and so the cycle is repeated. The most conspicuous feature in the physiolog}- of the Schizomy- cetes is their capacity for decomposing organic compounds, indu- cing various fermentative processes, such as the lactic and the butyric fermentation of various kinds of sugars, etc. (but never the alcoholic fermentation), and the putrefactive fermentation of complex nitrogenous organic substances, such as proteids, etc. Some are parasitic in the bodies of animals, such as Sarcina Ven- triculi, Leptothrix buccalis which causes decay of the teeth, and the various forms of Bacteria which cause Phthisis, Cholera, An- thrax, and other diseases. The particular form presented, and the degree of the physiolo- gical activity manifested, at any given time, is determined by the external conditions, such as the nature of the obtainable food, the temperature, the presence or absence of oxygen, etc. ; important variations in any of these conditions may induce change from one GROUP I. — THALLOPHYTA : FUNGI. 283 form of the organism to another, and may modify its physiological activity. There is a general resemblance in organisation and reproduction between the Schizomycetes and the Cyanophycese, as well as a remarkable correspondence between individual forms belonging to the two groups. On this ground they are sometimes placed to- gether in a distinct group, the Schizophyta. It is, however, pre- ferable to place them respectively in the classes Fungi and Algae as corresponding sub-cltiesee. Sub-Class II. MYXOMYCETES. These organisms are character- istically saprophytic, living on decaying organic substances, such as spent tan, decaying leaves, tree-stumps, etc. Their life-history is, in most cases (Endosporese), as follows : — On the germination of the spores, the contents of each spore escape as a zoospore, a naked mass of protoplasm, enclosing a nucleus and a contractile vacuole, provided with a single cilium ; this con- stitutes the mastigopod stage, and in this stage the cells multiply by division. After a period of active swimming, the zoospore draws in its cilium, and now creeps about by means of temporary protrusions of its protoplasm termed pseudopodia ; this is the amceboid or myxopod stage, and in this stage also multiplication by division takes place. The amoebae then collect together, cohering into a plasmodium ; the protoplasm of the amoebse in some cases fuses completely so that the plasmodium presents no cellular structure, whereas in others (pscudoplasmodium} the outlines of the coherent amoebse persist ; but, in any case, there is no fusion of the nuclei of the constituent amoebae,, so that the plasmodium is multinucleate and syncytic. The plasmodium creeps about, like a gigantic amoeba, by means of pseudopodia, until spore-formation begins. At this time the plasmodium comes to rest ; and it either forms a single sporangium, or divides into several portions each of which forms a sporangium. The mass of protoplasm then assumes the form of the future sporangium ; the external portion of it hardens to form the wall ; while the internal portion, after rapid nuclear division, separates into cells each of which secretes a proper wall and becomes a spore. In most forms a portion of the internal protoplasm goes to form a number of filaments, generally tubular, either free or connected into a net-work, which constitute the capillitium. The wall dries, and eventually ruptures, and' the spores are scattered by the expansion and hygroscopic movements of the 284 PART IV. — CLASSIFICATION. elastic capillitium. In many cases the sporangium has a stalk, (sporophore) which is sometimes continued into the cavity of the sporangium as a columella. In the Exosporese the spores are not formed in the interior of a sporangium, but by abstriction from the ends of filaments de- veloped from the surface of the sporophore. In some forms (e.g. Fuligo vdridns) a compound sporangium is II Fie. 164J —A Part of a plasmodium of ZKdt/mium leucopu* (x 300). J? A closed sporangium of Arcyria incarmito. C The same after the rupture of its wall (p) and expansion of the capillitium cp ( x 20) . (After SachsO formed, termed ^Ethalium, by the combination of a number of plasmodia. The sporangium:wall and capillitium give the reactions of cuticularised cell-wall. The life-history, as sketched above, varies somewhat in different forms'. In some (e.g. Dictyosteliacese, Gruttulinese) the mastigopod stage is wanting; the spores" giving rise directly to amoebae. Again, the mastigo pods' or the amoebae may surround themselves GROUP I.— THALLOPHYTA : FUNGI. 285 with a membrane and go through a resting-stage as microcysts ; or the whole or part of a plasmodium may do the same as a sclerotium. Sub-Class III. PHYCOMYCETES. Section A : 'Zygomycetes. The section Zygomycetes includes several orders of which, how- ever, only the characteristic order Mucorinse will be considered. Order Mucorinae. Body an unseptate mycelium, septa being only developed in connection with the formation of reproductive organs ; repro- duction by spores, and by zygospores formed by conjugation; mostly saprophytic, but some are parasitic on other Fungi. The mycelium ramifies in the substratum (Fig. 165). The asexual repro- ductive organs are developed as simple sporophores which grow erect into the air. In the Mucoracese the simple sporophores are unbranched, and each FIG. 165.— Mucor Mucedo : m a mycelium bearing a simple sporophore with a terminal sporangium s ; S a sporangium much magnified; t the end of the sporophore dilated into the columella c ; ic the wall of the sporangium ; gp the spores ; z zygospore formed by the fusion of the contents of two gametangia. bears at its apex a single sporangium ; the sporophore projects into the cavity of the sporangium as a columella (Fig. 165). In the Chaetocladieae and the Piptocephalideae the sporophore is branched and more or less septate ; it produces a number of spores by abstriction from the tips of its branches. On germination, the spore gives rise to a mycelium similar to that from which it was derived. The gametophores are short swollen hyphse; by the formation of a septum near the tip of the gametophore, a terminal cell is produced, which is the sexual organ or gametangium; the protoplasmic contents of the gametangium constitute the gamete. Two gametophores from adjacent vegetative hyphse come into contact at their tips ; the walls of the two 286 PART IV.— CLASSIFICATION. gametangia are absorbed at the point of contact ; the protoplasmic con- tents (gametes) of the gametangia fuse to form the cell which surrounds itself with a coat of two layers and becomes a zygospore (Fig. 166). In many cases the zygospore, on germination, gives rise to a small branched or unbranched mycelium, which bears a single simple sporo- phore. The spores derived from this sporophore give rise, on germina- tion, to the ordin- ary mycelium. In other cases, how- ever, the zygo- spore gives rise directly to a my- celium bearing sexual organs. The mycelium, when under un- favourable con- ditions, gives rise to unicellular gemmae, either c h 1 a m y d ospores or oidium - cells : the latter multi- ply by gemmation in a yeast -like manner (e.g. Mu- cor racemosus) and, like Yeast, have the power of caus- ing alcoholic fer- mentation ; this takes -place espe- cially when the hyphse are im- mersed in liquid. The hyphse be- come segmented into a row of cells by the formation of transverse septa, and the cells then sepa- rate and become free. The chla- mydospores are FIG. 166 — Mucor Mucedo: A diagram of sexual process: two gametophores in contact ; at the end of each gametophore a cell, the gametangium, has been cut off by a septum ; B commencing development of the zygospore from the fused gametangia; C ripe eygospore, still connected with the gametophores; D free zygo- spore, showing one point of attachment ; E germinating zygo- spore, bearing a small ,'promycelium with a single sporangium (after Brefeld). GROUP I. — THALLOPHYTA : FUNGI. 287 thick- walled and large; the oidium-cells are smaller and thin-walled (see p. 277). Mucor Mucedo may be obtained by keeping fresh horse-dung under a bell-jar in a warm room: it may be further cultivated by sowing the spores on slices of bread moistened and kept under a bell-jar. Section B. Oomycetes. This section of the Phycomycetes includes the following orders : Order 1. Peronosporaceae: body branched; oogonia terminal or intercalary ; pollinodium functional. Order 2. Saprolegniacese : body branched ; oogonia generally terminal, rarely intercalary ; pollinodium absent, or, if present, functionless. Order 1. — Peronosporaceae. The forms comprised in this order are mostly parasitic, chiefly on Phanerogams, but some species of Pythium inhabit the dead bodies of plants and animals. The asexual reproduction of the plant is effected, in most forms, by sporangia developed at the ends of the branches of the simple sporophores (Fig. 168 A): no such organs have, however, been observed as yet in Pythium vexans or P. Artotrogus. In some forms the sporangium gives rise to zoo- spores either before or after, it has fallen off the sporophore (Fig. 168 B, C) ; whilst in other forms it falls off and germinates as if it were itself a spore, growing out into a hypha, and so into a pIG- 167- _ mycelium. phthora omnivora. An The oogonium is spherical, and remains closed oogonium (Og), contain- (Fig. 167). The protoplasmic contents undergo *** an °°spore ('f ' a \" . . . ' . . ° a pollinodium which has differentiation into a single oosphere which is fertiii8ed the oosphere. surrounded by the remainder of the protoplasm, (x400.) the peripi-asm. The pollinodium is developed terminally, either on a hypha springing from beneath the 'oogonium, or on an adjacent hypha, and is club-shaped- Its protoplasmic contents undergo differentiation into a male cell (aplano- gamete) and into periplasm. At the time of fertilisation, the pollinodium is closely applied to the oogonium and sends out a delicate tube which penetrates through the wall of the oogonium and reaches the oosphere. The tube then opens, and the male cell passes out of the pollinodium into the oosphere and fertilises it. The oosphere then surrounds itself with a proper wall and becomes the oospore. In some genera (Peronospora, Cystopus) an external coat, the episporium or perinium, is formed round the oospore from the peri- plasm. The germination of the oospore takes place in different ways in different species. In Phytopltthora omnieora and Pythium proliferum it gives rise to a small mycelium (promycelium) which produces a few spores, from 288 PART IV. — CLASSIFICATION. which, sexual plants are developed. In other species (e.g. Cystopus candidus) the contents of the oospore are set free as a number of zoospores. In yet other species (e.g. Pythium de-Baryanum, Pythium Artotrogus, Peronospora Valerianellce), the oospore directly gives rise to a sexual plant. Pythium can be obtained by sowing cress in a pan of earth. When the seedlings spring up, they should be well watered and be kept covered with a bell-jar so as to keep them moist. Some of them will be seen to fall owing to the failure of the stem just above the surface of the soil ; these are infected with Pythium. If an in- fected stem be kept in a drop of water on a slide for a day or so, and be examined from time to time, hyphse bearing the reproductive organs of the Fungus will be seen to be de- veloped at the surface. In the genus Peronospora, which is represented by many species (P. parasitica on Capsella, P. calotheca on Rubiacese, etc.), only one sporangium is borne by each branch of the sporophore which protrudes through a stoma. In Phytophthora the sporangia are displaced laterally by branches which arise from the hyphte bearing the sporangia, at their points of origin. To this genus belongs P. infestans, which produces the potato-disease. The tissues of the host un- dergo decomposition in the infected parts and turn black : the mycelium of the Fungus extends from the circumference of these spots, and throws up sporophores through the stomata (Fig. FIG. 168.— .4 Surface-view of the epidermis of a Potato-leaf with the sporophores of Phytophthora infestant projecting out of the stomata ( x 90). B A ripe sporangium. C Another undergoing division. D A zoospore. ( x 540 : after Strasburger.) 168). The sporangia of the parasite are carried by the wind to healthy plants and infect them : the zoospores also penetrate through the soil to the tubers, and the mycelium which is developed from them extends into the young Potato-plant which grows from the tuber. No sexual reproductive organs have been observed in this Fungus as yet. GROUP I. — THALLOPHYTA : FUNGI. PhytopJtthora omnivora infects and destroys the seedlings of the Beech and other plants. In Cystopus (C. Candidas on Capsella and other Crucifers, C. cubicus on Compositse) sporophores bearing numerous sporangia are formed in great numbers close together under the epidermis, and cause its rupture. Order 2. — Saprolegniaceae. The Saprolegniacese all live in water, and are mostly saprophytic, though some are parasitic ; one species causes the Salmon-disease. Asexual reproduction is effected entirely by zoospores ; they are formed in terminal but not otherwise especially differentiated sporangia (Fig. 169). On coming to rest they germinate to form a mycelium. They are, in some forms, surrounded by a thin cell-wall at their first formation. The oogonia and pollinodia (when pre- sent) resemble those of the Peronosporaceae. The number of oospheres in the oogonium varies widely in different individuals ; sometimes there is only one (Leptolegnia, Aphanomyces) ; but as a rule there are many, as many as 30-40 ; in either case they are developed from the whole of the protoplasm of the oogonium. The male and female sexual organs are commonly borne on the same hypha, but in some cases (e.g. Saproleynia dioica and anisospora) this is not the case ; however, it is not clear that these species are actually dioecious. In some species (Saprolegnia Thureti, (orulosa, monilifera, and Achlya stellata) no male organs are developed as a rule; in others (Saprolegnia mixta, Achlya spinosa) they are as often absent as present ; in others they are frequently absent (Aphanomyces stellatus, Saprolegnia hypogyna, Aplanes BrauniY); in others, finally, they are always present (Achlya racemosa and polyandra, Saprolegnia monoica). When pollinodia are present, they are closely applied to the oogonium ; sometimes several are applied to one oogonium. In some forms (e.g. Saprolegnia asterophora) the pollinodium undergoes no change, or it sends out a short tube which enters the oogonium but does not touch the oospheres. In most others the pollinodium sends out one or more tubes which enter the oogonium and come into close contact with the oospheres. But in all cases the tubes remain closed, and no act of fertilisation has M.B. U FIG. 169.— Zooeporangmm of an Achlya : A closed ; B the zoo- spores are escaping ; c a lateral branch ; a zoospores just escaped ; b empty membranes ; e swarming zoospores. ( x 550 : after Sachs. ) 290 PART IV. — CLASSIFICATION. been observed: the oospheres, however, all become oospores. The ger- mination of the oospores presents the same variations as in the Perono- sporacese. Saprolegnia can be obtained by placing dead flies in water from a ditch or pond: in a day or two the flies will be found covered with mycelium, if the temperature has been sufficiently maintained. Sub-Class IV.— ASCOMYCETES. This sub-class includes a vast number of forms, both saprophytes and parasites. Some of them (e.g. Penicillium glauctim, Eurotium Aspergillus) are fami- liar as the blue or green moulds appearing on jam, damp boots, etc. ; others (Erysiphese) as mildew on roses, etc. : Cordyceps infests the larvse of insects. In some cases the life-history is complicated by polymorphism, including one or more entirely asexual conidia-bearing forms. These various life-histories are briefly illustrated by the following examples. In some cases (e.g. Eremascus albus, Gymnoascus, most Asco- mycetous Lichen-fungi, Ascobolus furfuraceus, Pyronema) the life-history is perfectly simple, presenting only the plant bearing sexual organs, and, subsequently, the ascocarp. On germination the spores (ascospores) produced in the ascocarp give rise to the plant. In other cases (e.g. Erysiphese, Eurotium, Penicillium) the plant reproduces itself by means of conidia ; in the Erysiphese and Euro- tium it generally produces sexual organs eventually ; but in Penicillium the formation of sexual organs takes place only excep- tionally under special conditions, so that many successive genera- tions may be produced by means of conidia before a sexual plant makes its appearance. This may occur also in the Erysiphese. A more complicated life-history can be clearly traced in Clavi- ceps purpurea, the Ergot of Rye. The mycelium is developed in the ovary of the Rye-flower, and forms a continuous layer of hyphse, a compound conidiophore, at the surface, from which immense numbers of conidia are formed by abstriction, imbedded in a mucilaginous substance known as Honey-dew. This substance is eaten by insects, and thus the conidia are carried to other flowers and there reproduce the fungus. This is the Sphacelia- form. When the rye is ripening, the mycelium forms a dense sclerotium (see p. 277), fusiform, about an inch long, of a dark purple colour at the surface. This is the Ergot, and it remains dormant until the following spring. On germination the sclerotium gives GROUP I.— THALLOPHYTA : FUNGI. 291 rise to several filaments termed stromata, about an inch long, each composed of a strand of hyphse, which bear a swollen knob at their apices (Fig. 176). All over the surface of the 'knob are a number of depressions, in each of which there is an ascocarp (perithecium) containing a number of asci, and in each ascus there are eight filiform ascospores. The ascospores are carried by the wind to the Rye-fbwers and there give rise to the Sphacelia-fonn. In some cases only conidia-bearing forms are known (e.g. Asper- gillus clavatus, Botrytis Bassii, species of Isaria, Cladosporium Iferbarum, etc.). The Reproductive Organs are asexual and sexual. The asexual organs are conidiophores, either simple or compound (see Figs. 170, 175), branched or un- branched ; the conidia are formed by abstriction from short tubular outgrowths of the unbranched, or of the terminal cells of branches of the branched, conidiophore, termed sterigmata. In many cases the conidiophores are collected into special receptacles termed pycni- dia. The sexual organs are modified hyphse. They may be unseptate (e.g. Eremascus, Eurotium Asper- gillus, Pyronema), or septate (e.g. Ascobolus, Laboulbeniacese) ; they may be quite similar (e.g. Eremas- cus) or more or less differentiated ; they may come into close contact (e.g. Eremascus, Eurotium, Pyronema). When, as in Eremascus, the sexual organs are undifferentiated, no special names are given to them ; but when they are differen- tiated the female organ is termed an archicarp, and the male organ a pollinodium, when its contents do not undergo differen- tiation into cells, or an antheridium when (as in the Laboul- beniaceee) male cells are formed in it. In some forms (e.g. Laboulbeniacese, Pyronema) the archicarp consists of two parts ; a receptive portion, filamentous in form, the trichogyne; a fertile portion, the ascogonium (compare Rhodo- phycese, p. -272). In the simpler forms, the trichogyne is absent FIG. 170.— Conidiophore of Penieil- lium j/lnucnm : s a row of conidia on a sterigma ; m hypha of the mycelium (x!50.) 292 PART IV. — CLASSIFICATION. (e.g. Eurotitim, E^sipheae, Ascobolus), the archicarp consisting solely of the ascogonium. The form of the ascogonium is either filamentous, sometimes spirally coiled (e.g. Eurotium, Fig. 175) ; or, it is dilated, and spherical or oval (e.g. Pyronema, Fig. 172, Erysiphese). The pollinodium may be filamentous (e.g. Eurotium), or dilated and club-shaped (e.g. Pyronema, Erysiphese). The antheridium of the Laboulbeniacese is unilocular, and produces several non- motile male cells (spermatia) within it. Some of these plants bear receptacles termed spermogonia. The spermogonium consists of a wall formed of coherent hyphse from which a number of free hyphse, the sterigmata, grow into the interior and produce, by repeated abstriction at their apices, a number of small, rod-shaped cells, the spermatia, with a cell- wall, as to the nature of which there is some doubt. These cells reach the surface through the small opening of the spermogonium. A process of fertilisation has not been observed in all forms in which sexual organs are present j but it has been observed in the c. FIG. 171.— Sexual reproduction of Eremascus albus. A Sexual organs in contact. B Fusion of the organs at the apex, with developing ascocarp. C Mature ascocarp, consist- ing of a single ascus containing eight ascospores. ( x 1000 : after Eidam.) following cases which are representative of the various modes in which it may take place. In Eremascus (Fig. 171) the apices of the undifferentiated sexual organs come into contact, and, the cell-walls being absorbed at the point of contact, the protoplasmic contents fuse. In Pyronema the trichogyne comes into close contact with an adjacent pollinodium ; the cell-walls become absorbed at the point where the apex of the trichogyne presses against the pollinodium, and the contents of the two organs fuse (Fig. 172). In the Laboulbeniacese it appears that the male cells spermatia GROUP I.— THALLOPHYTA : FUNGI. 293 are brought by means of water into contact with the projecting trichogyne. One of them adheres to the trichogyne ; the cell-walls are absorbed at the point of contact, and the protoplasm of the spermatium enters the trichogyne, with the result that the ascogonium developes into an ascocarp. It is probable that, in consequence of sexual degeneration, the sexual organs are f unctionless in the majority of those Ascomycetes in which both kinds of them are present. The Ascocarp. In those Ascomycetes in which there is an archicarp, the ascocarp is developed directly or indirectly from that organ : when no archicarp is present, the ascocarp is developed directly from the mycelium. The simplest form of ascocarp is found in Ereinascus (Fig. 171). After the sexual process has taken place, a large spherical cell is formed at the point of junction of the two sexual organs. This cell is an ascus, and produces within it eight ascospores. Here the whole ascocarp consists of a single naked ascus. The ascocarp of some of the Ery- sipheae (e.g. Sphserotheca) is but little more complex than that of Eremascus. Here likewise the archicarp gives rise directly to a single ascus ; but an in- vestment is formed round the develop- ing ascus by the growth round it of hyphae from the adjacent mycelium, which cohere to form a layer of paren- chymatous tissue. In the majority of forms the development of the ascocarp is indirect. The archicarp gives rise to a greater or smaller number of filaments, branched or unbranched, the ascogenous hyphce (which closely correspond to the ooblastema-filaments of the Rhodophycese, see p. 273), from which the asci are formed as branches, and which together form a compound sporophore. The asci are developed close together, forming a hymenial layer or group, and may or may not be enclosed, either completely or partially, by an investment formed from the surrounding myce- lium. In the latter case, vegetative hyphae grow in among the ascogenous hyphae and terminate in a number of sterile filaments, PIG. 172. — Sexual reproduc- tion in Pyronema confluent : e archicarp with trichogyne («) which has fused with the pollinodium a. ( x 300 : after Kihlman.) 294 PART IV.— CLASSIFICATION. the paraplujses, which are situate in the hymenial layer between the asci. The following forms of ascocarp may be distinguished amongst those which have a cellular investment : — the cleistothecium ; the investment remains closed until it decays and ruptures to permit of the escape of the ascospores (see Figs. 173, 175): ihe> pcrithe- cium; a narrow aperture is developed opposite to the hymenial layer (see Fig. 176) : the apothecium ; the investment is somewhat saucer-shaped, so that the hymenial layer is fully exposed (see Fig. 177). The ascus is in all cases unicellular. It may be either spherical (e.g. Eremascus, Eurotium), or oval, or club-shaped (e.g. Peziza) in form. In some cases the ascospores are ejected with considerable force ; in others they are set free on the mucilaginous degeneration of the wall of the ascus. The ascopores are formed by free cell-formation (see Fig. 64, p. 86) from a portion only of the proto- plasmic contents of the ascus, pre- ceded by nuclear division. The unused portion of the protoplasm is termed the cpiplasm, and is rich in a carbohydrate called glycogen. In nearly all cases eight ascospores Fro. 173. — A Ascocarp of UnetrmZa It- , , «oroi. (Erysipheie), slightly magnified: are formed ; m some cases each of m mycelium; / cleistothecium; ft, in- the eight Spore-rudiments under- vesting filaments. B An ascus from tho -,? . . , , cleistothecium. containing eight asco- &oes dlvision to form a compound snores (more highly magnified). Spore (e.g. Hysterium, Pleospora, etc.), the cells of which may either separate or remain coherent. The form of the ascospore is spherical, or oval, or rarely filamentous (e.g. Claviceps, Fig. 176). The wall generally consists of exospore and endospore: the protoplasm generally contains oil-drops. The germinating ascospore usually gives rise directly to the ordinary mycelium. The Ascomycetes may be classified as follows:— Order L— Gymnoasceae : asci withqut any investment, or with only a rudimentary investment, either solitary, or forming a hymenial layer. The typical members of this group are Eremascus (Fig. 171), Gymnoas- cus, and Exoascus parasitic on various trees. It is now customary to place in this order the family of the SACCHA- GROUP I. — THALLOPHYTA : FUNGI. 295 ROMYCETES, or Yeast-Fungi, which is familiar on account of the alcoholic fermentation of saccharine solutions which some of its members excite (e.g. Saccharomyces Cerevisice used in brewing, and S. eHipsoideun which causes the fermentation of the grape-juice in the manufacture of wine: see p. 198). The plant is usually a single small spherical or oval nucleate cell, and multiplies rapidly by gemmation (Fig. 174: see p. 149). When budding is proceeding very rapidly, the successive cells may remain coherent for a time; but a true mycelium is only rarely found, as in S. Mycoderma, which forms a scum on decomposing wine and beer. Under certain conditions, particularly the absence of a sufficient supply of food, the plant forms spores. Usually four spores are formed in a cell, by free cell-formation, from a portion of the protoplasm, the rest remain- ing as a parietal layer of epiplasm. The spores surround themselves with a membrane, and are set free by the disorganisation of the wall of the cell. The spores retain their vitality under conditions, such as desicca- tion, absence of food, extremes of temperature, etc., which would prove fatal to the Yeast-plants. The spores germinate, on attaining appropriate conditions of moisture and temperature, and give rise to Yeast-cells by budding. Inasmuch as the formation of the spores in a Yeast-cell takes place in the same manner as the formation of spores in an ascus, the Yeast-cell may be regarded as an ascus. It is on this account that the Saccharo- mycetes are included in the Ascomycetes, and in the Gymnoascese on account of their naked asci. They are, however, reduced and sexually de- generate forms. It must be borne in mind that cells very Fio. 174,-BuddinR cells similar to those of the true Saccharomycetes, of Yea8t J*^***"**" ..-.., if Cerevisue) ; the clear spaces multiplying in the same manner, and often in thfl ce]lg are vacuo]es> capable of exciting the alcoholic fermentation (x300.) of sugar, may be formed by gemmation from the conidia of various kinds of higher Fungi (e.g. Mucor racemosus, Peni- cillium glaucum, some Ustilaginese and Basidiomycetes) under special conditions. These Yeast-like cells, however, grow into mycelia -under appropriate treatment. However, it is still a question whether all the forms of Saccharomycetes may not be merely secondary cpnidial forms of gernmse of mycelial Fungi. Order II. — Pyrenomycetes : asci forming a hymenial layer, with an investment; the ascocarp is either a cleistothecium or a perithecium; a stroma is present in some families. The ascocarp is a cleistothecium in the sub-order Perisporiacese, includ- ing the families Erysipheae (the Mildews) and Perisporieae (e.g. Eurotium and Penicillium) ; in these families there is no stroma. Eurotium Aspergilhis is the greenish Mould which so often attacks jam. It can be well cultivated for study on stewed prunes, by sowing conidia obtained from infected jam. The prunes soon become covered with a white, dowjiy substance, which is the vegetative mycelium ; this grad- 296 PART IV.— CLASSIFICATION. ually assumes an olive-green colour as the conidiophores are developed ; and finally bright yellow patches appear, indicating the formation of the ascocarps. Specimens of Mildews can generally be obtained, in a wet summer, from the leaves of the Rose or of the Hop. The ascocarp is a perithecium in the sub-order Hypocreacese, Sphseria- cese, and Dothideacese (e.g. Claviceps, Cordyceps) : a stroma, which varies much in form, is frequently present. FIG. 176.— Eurolium repens. A A portion of the mycelium with a simple conidiophore (c) bearing conidia; the conidia have already fallen off from the sterigmata (st) ; at, a young ascogonium. B Ascos;onium (as) with a pollmodium(p). C Another, with hyphse growing up round it. L> A cleistothecium seen on the exterior. E F Sections of unripe cleistothecia ; 10 the investment; / ascogenous hyphffl arising from the ascogonium, which subsequently bear the asci. 0? An ascus. H A ripe ascospore. (Magnified : after Sachs.) Order III.— Discomycetes : the ascoearp is an apothecium of various form ; a stroma sometimes present; The order may be divided, according to the form of the apothecium, into the two sub-orders Pezizaceae and Helvellacese. In the former the apothecium is cup-shaped, the hymenium covering the concave surface GROUP I. — THALLOPHYTA : FUNGI. 297 and is closed in the early stages of its development ; in the latter the apothecium is borne on the convex, smooth, or reticulate surface of an erect strorna. A FIG. 176.— Clainceps purpurea. A A sclerotium (c) bearing stromata (x2). B Section of a stroma ; cp the perithecia. C A perithecium more highly magnified. D An ascus ruptured ; the elongated spores (sp) are escaping. (After Sachs.) The sub-order Pezizacese includes several families, the Phacidiese, Pezizese, Bulgarieee, etc. As representative may be mentioned Rhytisma Acerinum, the mycelium of which infests the leaves of the Maple, but the development of the apothecium does not take place until after the leaves have fallen ; and other similar forms which inhabit the leaves of the Silver Fir, Spruce, and other trees : Ascobolus, which grows on dung : the various species of Peziza, with brightly- coloured apothecia, growing on rotting wood, etc. : Bulgaria, with Fl°- !?7. — Longitudinal section of the a gelatinous apothecium, growing aP°thecimn of Peziza convexula -. h the hy- menium. (After Sachs.) on dead branches of the Oak. The sub-order Helvellaceae includes the genera Morchella (the Morell, esculent), Gyromitra, Helvella, etc. 298 PART IV. — CLASSIFICATION. Sub-Class V.— ^ECIDIOMYCETES. This sub-class includes a con- siderable number of parasitic plants known as Rusts and Smuts. They are characterised by their remarkably complex life-history, due to polymorphism, presenting two or more spore-bearing forms : and by the fact that the spores are not developed in the interior of a sporangium, but are formed by abstriction. FIG. 178. — Puccmta Graminis. I Transverse section of a leaf of Barberry, with secidia (a) ; p the wall of the aeculiuin ; u lower, o upper surface of the leaf, which has become thickened at u, y, in consequence of the presence of the parasite ; on the upper surface are spermogonia (sp). A A young secidium which has not yet opened. II Sorus of teleuto- spores (t) on the leaf of Triticum repens ; e its epidermis. HI Part of a sorus of uredo- spores on the same plant ; ur the uredospores; t a teleutospore. (After Sachs.) The sub-class is divisible into two orders : — Order 1. Uredineae : have an secidium-form, as a rule. Order 2. Ustilaginese : never have an secidium-form. GROUP I. — THALLOPHYTA : FUNGI. 299 Order I. — Uredineae. This order comprises those parasites which are generally known as Busts, on account of the rusty appearance which they give to their host-plants at a certain stage of their life-history, when they bear at the surface a great number of orange-coloured spores. Puccinia G-raminis affords an example of the most complex life-history with heteroecism : that is, that the different forms inhabit different hosts. It infests Wheat, Rye, and other Grasses, and developes its mycelium in the tissues of the young plants. During the summer it produces groups of simple sporophores, at the apex of each of which a single oval spore, termed a uredospore, of en orange colour, is formed by abstriction (Fig. 178, III) • in consequence of the grea,t development of cells at these points the epidermis of the host is ruptured, and the groups of uredospores are visible on the surface as rusty spots. These uredospores are scattered by the wind, and infect other Grass- plants ; on reaching a leaf, the uredospore germinates at once, forming a hypha which enters through a stoma into the in- terior of the leaf, where it de- velopes into a mycelium bearing uredospores. This stage in the life-history is termed the Uredo- form. Later in the season, when the tissues of the hosts are becoming hard and dry, the Uredo-form no longer produces uredospores, but dark-coloured often com- pound spores, known as teleuto- spores (Fig. 178, 77), developed in the same way as the uredospores. The teleutospores remain qui- escent during the winter. When they germinate in the following spring, one or both of the cells gives rise to a small, free, non- parasitic mycelium (promycel- ium), from each of the cells of which a delicate conidiophore is produced, which developes a small conidium (termed a sporidium) by abstriction at its apex (Fig. 179). The sporidia are scattered by the wind, and if they fall on the leaves of the Barberry they germinate, giving rise to a hypha which pierces the epidermis of the leaf, and then forms a dense mycelium in the inter- cellular spaces of the mesophyll. At certain points the tissue of the leaf is hypertrophied, forming cushions which project on the under surface. c. A. FIG. 179. — Germination of teleutospores of various Uredineae : A of Puccinia Graminis ( x 40C) ; B of Melampsora ( x 300) ; C of Coleo- sporium ( x 230) ; t teleutospore ; pm promy- celium ; sp sporidia. 300 PART IV. — CLASSIFICATION. Towards the upper surface of the cushion there are formed on the my- celium small receptacles, the spermogonia (Fig. 178 sp\ each of which con- tains a number of uuseptate hyphse, radiating from the wall towards the centre, which are termed sterigmata ; each of these produces at its apex by abstriction a small cell, the spermatium, which escapes from the spermo- gonium ; spermogonia are formed, though less frequently, on the under surface : the significance of the spermogonia and spermatia is not known. Large spherical structures are formed on the under surface of the cushion (Fig. 178) ; these are the aicidia. This form of the fungus is known as sEcidium Berberidis. Each secidium consists of a hymenial layer of simple unseptate sporophores at its base, from the apices of which a number of spores (cKcidiospores) are formed by successive abstriction; the secidium has a definite wall which ruptures at the surface to set free the spores. JJ. FIG. 180.— Transverse section of a Willow-leaf FIG. 181. — Germinating resting. infested by Melampsora salicina -. par mesophyll of leaf: eo upper, eu lower epidermis. On the under side a gorus of uredospores (st) with para- physes (p) has broken through the epidermis ; beneath the upper epidermis is a sorus of young teleutospores (t). (x 260.) spores: A of Vstilago receptaculorum ; B of Tilletia Caries ( x 460) : sp the spore ; pm the promycelium ; d the sporidia : in B the sporidia have coalesced in pairs at e. The secidiospores are conveyed by the wind to Grass-plants, on the leaves of which they germinate, putting out hyphse which penetrate into the interior through the stomata, giving rise to the mycelium which bears the uredospores, and subsequently the teleutospores. Order 2. — Ustllagineae. This order comprises those parasites which are known as Smuts. The life-history of most of the members of this order is briefly as follows. The plant produces numerous thick-walled, often black (Smut) resting-spores, the development of which is usually inter- calary (resembling that of chlamydospores) on more or less specialised mycelial branches (conidiophores). On germination, the resting-spore GROUP i. — THALLOPHYTA: FUNGI. 301 forms a number of reproductive cells, sporidia, of various form ; the sporidia are usually developed on a small promycelium, which may be either multicellular (Fig. 181 A), or unicellular (Fig. 181 B). In most forms these sporidia then coalesce in pairs ; but in any case they germin- ate, either producing at once the mycelium which will bear the resting- spores (e.fj. Protomyces), or a second promycelium bearing secondary sporidia. from which the mycelium bearing resting-spores is developed (e.g. Tilletia Caries). In some species (e.g. Entyloma Ranunculi, Tuburc.inia Trientalis) the my- celium, before it produces the resting-spores, developes conidia ; these are small, thin-walled, somewhat spindle-shaped cells, developed by abstric- tion from the ends of unbranched simple conidiophores. The sporidia, when cultivated in nutrient solutions, may be made to multiply actively by gemmation, producing a number of yeast-like cells. The most important and the inost common species are Ustilago Carbo, which especially attacks Oats, but other Cereals and Grasses as well : U. Maidis, which produces large tumours in the Maize, filled with resting- spores : Urocystis occulta, which fructifies in the leaves and haulms of the Eye : Tilletia Caries, the Smut of Wheat ; this is dangerous because the grains filled with resting-spores remain closed, and are therefore harvested with the sound ones. Many other species and genera infest wild plants. Sub-Class VI.— BASIDIOMYCETES. This sub-class includes a large number of plants, both, saprophytes and parasites, the fructifications of which are well known as Mush- rooms, Toad -stools, and Puff-Balls; they are the most highly organised of the Fungi. The body is a branched septate mycelium, growing in the substratum, and bearing the re- productive organs which come to the surface. That of the common edible Mushroom is gener- ally termed " mush- room-spa wn." FIG. 182.— A Section of young compound sporopbore of Agarieus (Amanita) vaginatui: v the velum universale; st the stipe; h the pileus ; I the lamella: B the same Romewhat older; the velum v is ruptured. C Agarieus nwlleiis: m the mycelium (Rhizomorpha) ; in the smaller specimen to the right the hymenophore is still covered by the velum partiale o ; in the larger specimen the velum is almost completely ruptured, and remains attached to the stipe as the ring, a. (i nat. size.) 302 PART IV.— CLASSIFICATION. The reproductive organs are of two kinds, compound and simple. Of these the compound sporophore is universal, and is character- istic of the sub-class ; it constitutes the fructification commonly known as a Mushroom, a Toadstool, etc. The structure of this fructification may be illustrated by reference to the common Mush- room (Agaricus campestris). It consists of a stalk, termed the stipe, bearing at its apex a large circu- lar, somewhat um- brella-shaped expan- sion, the pileus. On the underside of the pileus are a number of radiating plates of tissue, the lamella* (Fig. 183), covered with the spore-bearing layer of cells, called the hymenial layer or hymenium. The lamellae collectively constitute the hy- menophore. To- wards the upper end of the stipe is a ring of tissue, the annulus, the torn remains of a mem- brane (the velum) which extended from the stipe to the margin of the pileus, FIG. 183. — Agaticus campestris. A Tangential section of the pileus, showing the lamellae (1) of the hymenophore. B A similar section of a lamella more highly mag nifled hy the hymenium; t the central tissue called the trarta. C A portion of the same section more highly magnified (x 650): q young basidia and paraphyses; «' the first formation of spores on a basidium ; s" more advanced stages ; at »"" the spores have fallen off. (Alter Sachs.) enclosing the hy- menial cavity (Fig. 182). The stipe consists of a number of close- ly-packed branching hyphse, which, at its apex, spreads out to form the tissue of the pileus. In the pileus the hyphse branch repeatedly, the hyphae GROUP I.— THALLOPHYTA : FUNGI. 303 towards the lower surface forming the lamellae. Each lamella (Fig. 183 B) consists of a mass of hyphae, constitxiting the trama ; as the hyphae approach the surface of the lamella, the cells become shorter. The last cells, before reaching the hymenial laj'er, are very short, and constitute a definite layer, known as the sub- hy menial layer (Fig. 183 B, C, sh}. The terminal cells of the hyphae constitute the hymenial layer (Fig. 183 B hy). This con- sists of somewhat elongated club-shaped cells, some of which bear spores, and are termed basidia, whilst the others are sterile, and are termed paraphyses (Fig 183 C q}. Each basidium developes at its apex 2-4 delicate outgrowths, the sterigmata, and at the apex of each sterigma a single small spore (C s' s") is formed. These spores are termed basidiospores, with reference to their mode of origin. The form of the compound sporophore, as also the relation of its different parts, varies widely in the orders and families of the sub- class. In some families (e.g., Auricularieae, Tremellineae, Clavariese Hydneae, most Polyporeee, and some Agaricinae), the hymen- ium is exposed from its first development, and the sporophore is consequently said to be gymnocarpous. In Polyporus volvatus, species of Boletus, and in some Agaricinae (e.g. sub-genera Armillaria, Psalliota, of the genus Agaricus, etc.) the hymenium is covered FIQ ]gl _Multicellular for some time by a membrane, termed a basidium of Tremeiia : velum partiale. as described above (see Fig. 8 sterigma ; «p basidio- spores. (x 350.) 182); the sporophore is then termed hemi- angiocarpous. Finally, the whole sporophore may be surrounded by a membrane, which is dehiscent or indehiscent, and is then said to be angiocarpous. This is due to the fact that the sporo- phore is developed from the internal portion of the primitive mass of hyphal tissue, the external portion constituting the enveloping membrane. This arrangement obtains in various genera of Agari- cinae, such as Agaricus (sub-genera Amanita, Fig. 182, Lepiota) and Coprmus, and generally in the order Gasteromycetes. This membrane is termed, in the case of the Agaricinae, a velum uni- versale ; in that of the Grasteromycetes, a peridium. When it is dehiscent, and the sporophore is stipitate, a portion of it remains surrounding the base of the stipe as a volva. 304 PART IV. — CLASSIFICATION. In the higher Basidiomycetes (Autobasidiomycetes) the basidia are unicellular, but in the lower forms (Protobasidiomycetes) they are multicellular, either with transverse septa (Pilacrese, Auricu- lariese), or with longitudinal septa (Tremellinese, Fig. 184). The number of spores borne by a unicellular basidium is usually four ; but it may be one (species of Hymenogaster), or two (Calo- cera, Dacryomyces, species of Octaviana and Hymenogaster), or 4-8 (Phalloidese). In the case of the multicellular basidium, each cell bears one basidiospore. Simple conidiophores have been discovered in several forms (e.g. Pilacre Petersii, Auricularia sambucina, Exidia, Trcmella mesenterica and lutcscens. In these forms the basidiospore gives rise, on germination, to a mycelium, sometimes small and un- branched, which is either itself the simple conidiophore, or bears simple conidiophores, on which conidia are formed by abstriction. The same mycelium may subsequently bear the compound sporo- phores ; or the conidia-bearing form may reproduce itself through successive generations until at length, under appropriate con- ditions, the form bearing the compound sporophores occurs. The conidia of Tremella, cultivated in nutrient solution, mul- tiply rapidly by budding, producing yeast-like cells which have not, however, the power of exciting alcoholic fermentation. The formation of unicellular gemmae (see p. 277) is of common occurrence in the Basidiomycetes ; either in the form of chlamydo- spores (e.g. Nyctalis, Oligoporus, Fistulina) ; or, more commonly (e.g. species of Coprinus, Clavariese, Polyporus, Cyathus, etc.) in the form of oidium-cells. The chlamydospores are especially de- veloped in the basidial fructifications of the plants in which they occur : the oidium-cells are generally developed from the vegeta- tive mycelium, either the whole of it or individual hyphse, forming sometimes a more or less definite fructification. In some Agaricinse (e.g. Coprinus, Clavariese) the oidium-cells appear to be incapable of germinating. Sclerotia (see p. 277) are known in some cases. The mycelium (e.g. species of Typhula, Coprinus stercorariuSj Tulostoma) pro- duces sclerotia as an antecedent to the formation of the compound sporophores; the sclerotia become quite free from the mycelium, and may be kept for months without losing their vitality. On germination each sclerotium gives rise to one or more compound sporophores. The most remai'kable sclerotia are those of Agaricus mclleus, a Fungus which is very destructive to timber. The GROUP I.— THALLOPHYTA : LICHENTS. 305 mycelium gives rise to dark-coloured compact strands of hyphse, of the pseudo-parenchymatous structure characteristic of sclerotia ; but they are peculiar in possessing continued apical growth, and by this means they soon become long filaments, known as Rhizo- morpha. It is in this way that the Fungus spreads from tree to tree : the Rhizomorpha-filaments grow underground from the roots of an infected tree to those of a healthy tree (usually a Conifer) ; it penetrates into them and spreads in the tissues external to the wood in the form of a white fan-shaped mycelium. The compound sporophores (Agaricus melleus) are borne either on the subter- ranean Rhizomorpha-filaments, or on the parasitic mycelium ; in either case they come to the surface. The Basidiomycetes are classified as follows : — Series I. PROTOBASIDIOMYCETES : basidia multicellular. four-celled, each cell bearing a spore ; simple conidiophores generally present. Principal genera 5 Pilacre, Auricularia, Tremella, Exidia. Series II. Autobasidiomycetes : basidia unicellular ; simple conidio- phores in some forms. This series consists of the two orders Hymenomycetes and Gasteromy- cetes, which are distinguished by the facb that in the former the hymenium is exposed before the maturity of the lasidiospores, whereas in the latter, the hymenium either remains altogether enclosed in the tissue, or is exposed only after the spores are ripe. The principal genera of Hymenomycetes are, Clavaria, Hydnum, Poly- porus, Agaricus (Mushrooms) 5 and of Gasteromycetes, Lycoperdon (Puff- ball), Ehizopogon, Cyathus, Geaster, Phallus. Subsidiary Group. LICHENES. A Lichen consists of a Fungus and an Alga, living in intimate connexion^ and both contributing to their mutual welfare — that is,symbiotically (see p. 275). The Lichen-Fungus has al- ways a mycelioid body, and is the constituent of the Lichen which bears the re- productive organs. From the nature of these organs the Lichen-Fungi have been found to belong chiefly to the discomycetous and py- renomycetous Ascomycetes, but a few are basidiomy- , , . FIG. 185. — Section of a spermogonium of Ana- cetous, belonging to the ptMcWa Cl,^8: sp the apePrtnre Kat the 8urface ; orders Hymenomycetes and c cortex> and TO medullary portion, of the thallus ; Gasteromycetas. g layer of algal cells. (After Strasburger.) M.B. X 306 PART IV. — CLASSIFICATION. The reproductive organs of the Ascolichenes are sterigmata, producing spermatia, contained in spermogonia (Fig. 185) ; archicarps (in the order Collemacese), differentiated into a coiled ascogonium and a multicellular projecting trichogyne ; and ascocarps, which are either apothecia (discomycetous) or perithecia (pyrenomycetous) ; the archicarp, apparently after fertilisation by means of spermatia, gives rise to filaments which form the hymenial layer (consisting of asci and paraphyses) of the apothecium, and out- growths from the adjacent FIG. 186.— A-D Soredia of Usnea barbata. A A. simple soredium , consisting of an algal cell covered vegetative hyph •with a web of hyphse. B A soredium, in which the algal cell has multiplied by division. C A group of simple soredia, resulting from the penetration of the hyphffi between the algal cells. D K Germin- ating soredia : the hyphae are forming a growing- point, and the algal cells are multiplying. (Alter Bachs.) wall of the apothecium. In the fructification of the Basidiolichenes there is a hymenial layer consisting of paraphyses and basidia, the latter bearing apical sterig- mata, on each of which a basidiospore is produced by terminal abstriction. Lichens are also reproduced by gemmse, termed soredia, which consist of one or more algal cells invested by hyphse ; they are budded off from the surface of the thallus, and grow into new plants (Fig. 186). The Lichen- Algse belong either to the Cyanophycese or to the Chloro- phycese. The algal cells or filaments may be distributed throughout the thallus, when it is said to be homoiomerous ; this is usually the case in gelatinous Liphens (such as the Collemacese), in which the Alga belongs to the Cyanophyceae, but also in some non-gelatinous forms in which the Alga belongs to the Chlorophycese (such as Coenogonium, Eacodium, and others, in which the Alga is Trentepohlia) : or they may be arranged in a definite layer near the surface of the thallus, when it is said to be heleromerous (Fig. 188), as in the case of nearly all those Lichens of which the Algae belong to the Chlorophycese, and some in which the Algse belong to the Cyanophyceee (e.g. Peltigera, Pannaria). In some heteromerous forms (e.g. Theli- dium) the Algse are quite on the surface. Occasionally (e.g. Endocarpon) algal cells are present in the hymenium. It may be generally stated that the form of the thallus is determined in the homoio- merous Lichens by the Alga, in the hetero- merous Lichens by the Fungus. In the latter case three main forms are distinguished: — FIG. 18". — A gelatinous Lichen, Cnllema jiulposum, slightly magni- fied. It is homoiomerous, and the Alga is Xostoc. (After Sachs.) GROUP I. — THALLOPHYTA : LICHENES. 307 (a) frulicose Lichen*, in which the thallus grows erect, branching in a shrub-like manner. Of this form are the various species of Usnea (Fig. 189 .4), and allied genera with a cylindrical thallus, which grow on trees : Roccella tinctoria grows on rocks in regions bordering on the Mediterranean ; from it and other allied Lichens litmus is prepared : Ramalina and Evernia, with a ribbon-shaped flattened thallus, occur on trees and wooden fences: Cetraria islandica is the Iceland Moss, which forms a mucilaginous fluid when boiled with water : Cladonia, the Cup Moss, has a scaly decumbent thallus, from which erect branches spring bearing the apothecia ; Cladonia fimbriata is common ; Cladonia rangiferina, the Reindeer Moss, occurs on moors. (i) foliaceous Lichens, in which the thallus is flat- tened and adheres to the substratum: the green (rarely bluish-green) algal cells form a single layer beneath the upper surface (Fig. 188). The margin of the thallus is usually lobed. Parmeiia (or Physcia) pa- rietina occurs, with its bright yellow thallus bear- ing apothecia, on tree- 'trunks and walls, together with other species of a grey colour ; Sticta pulmonacea (Fig. 189 B) has a reticu- lated yellowish thallus, and grows on tree-trunks : Pel- tigera is represented by several species which grow on mossy banks in woods ; the apothecia are borne on the margin of the lobes of the thallus. (c) crustaceous Lichens, in which the thallus is usually indefinite in outline, and can often be scarcely dis- tinguished from the substratum, the fructification alone being con- spicuous. The Lichens of this form are extremely numerous. Among them may be mentioned the Lecanorese. of which Lecanora subfusca occurs on the trunks of trees : the Lecideaceae, which occur mainly on earth and rocks, Lecidea geographica forming bright yellow incrustations of considerable extent on silicious rocks : the Graphideae, of which Graphis scripta is common on the trunks of Beeches and other trees. FIG. 188.— Transverse section of the heteromerous thallus of Sticta fuliginnsa (x 500). o Cortex of the npper surface ; tt under surface ; m network of hyphse forming the medullary layer ; g algal cells ; r root-like outgrowths (rhizines) of the under sur- face. (After Sachs.) 308 PART IV. — CLASSIFICATION. FIG. 189.— jl A fruticosa Lichen, TJsnea larbata, with apothecia. a. B A foliaceous Lichen, Sticta pulmonacea, with apothecia, a (nat. size). (After Sachs.) Many species of crustaceous Lichens inhabit the highest peaks of the Alps, and other lofty mountains, on which there is no other vegetation, and they contribute materially to the weathering of the rocks and to the formation of a vegetable soil. When they grow on the trunks of trees, they occur more especially upon those which have a smooth surface ; the formation of a rough bark seems to interfere with their growth. Lichens may become completely dried up without losing their vitality. FIG. 190.— Crustaceous Lichens. A and B Qraphit elegans : B slightly magnified. C Per- tusaria, Wulfeni, slightly magnified. (After Sachs.) GROUP II.— BRYOPHYTA. 309 GROUP II. BRYOPHYTA (Muscinese). The plants forming this group, that is the Liverworts (Hepa- ticae) and the Mosses (Musci), are characterised by the following distinctive features. Their life-history presents a regular and well- marked alternation of generations : the gametophyte is the more conspicuous form, constituting " the plant " : the sporophyte is a sporogontum, presenting indications of differentiation into root and shoot, but not of the shoot into stem and leaves ; it never becomes an independent individual, but remains attached to the game- tophyte, from which it derives all or much of its nutriment. In some of the Mosses there is an indication, in both the sporophyte and the gametophyte, of a differentiation of vascular tissue. The GAMETOPHYTE. The germinating spore does not at once give rise to what is known as the " Moss-plant," but produces an embryonic body, the protonema, which consists generally of a branched filament, but occasionally of a flat layer, of cells which contain numerous chloroplastids. The protonema is generally in- conspicuous and short-lived in the Hepaticse, whilst in the Musci it is more amply developed and may, either wholly or in part, persist from year to year. The "Moss-plant " is the adult sexual form. It does not possess any true roots, but is attached to the soil either by unicellular root-hairs (Hepaticse), or by multicellular protonematoid filaments termed rhizoids (Musci). The body of the " Moss-plant " is essen- tially a shoot, which is highly developed and specialised in con- nexion with the functions which it performs — the development of the sexual reproductive organs and, in the case of the shoots bearing female reproductive organs, the nourishment of the at- tached sporophyte developed in consequence of fertilisation. The adult shoot arises as a lateral (rarely terminal) bud on the proto- nema : the protonema may give rise to a single shoot (Hepaticae) or to several (generally in Musci). In the latter cases, the adult shoots may become distinct " plants " by the complete or partial dying away of the protonema. The symmetry of the shoot is, almost uniformly, dorsiventral in the Hepaticse and radial in the Musci. It is either thalloid, as in most Hepaticae ; or it is differentiated into stem and leaf, as in the higher Hepaticae (foliose Jungermanniaceae) and in the Musci. 310 PART IV. — CLASSIFICATION. The sexual organs are borne on the adult shoot, and are an- theridia and archegonia. They are rarely borne singly or scat- tered, but more commonly in groups (sori) surrounded by some kind of protective investment to which the general term involucre may be applied. In some cases the portion of the shoot which immediately bears the sexual organs is more or less specialised as a receptacle, and in others special reproductive branches, gameto- phores, are differentiated, and may be either antheridiophores or archegoniophores. In the lower Hepaticse the sexual organs are generally borne on the upper (dorsal) surface of the shoot, whilst in the higher Hepaticse (Jungermanniacese acrogynse) and in the Mosses they are borne at the apex. PIG. 191.— Funaria hygrometrica (Moss). A Germinating spores : u rhizoid; s exospore. H Part of a protonema, about three weeks after the germination of the spore : 7i a pro- cumbent primary shoot with brown wall and oblique septa, out of which arise the ascending branches with limited growth : K rudiment of a leaf-bearing axis with rhizoid (tc). (A x 550 : B about 93.) The distribution of the sexual organs is various : the male and female organs may be borne on distinct shoots, when they are dio3Cious ; or on different branches of the same shoot, when they are moncc.cious but diclinous ; or together in the same sorus, when they are monoclinous. In Mosses it appears to be the rule, in dicecious forms, that a protonema always bears both male and female shoots. The sexual organs are always multicellular. The antheridium (Figs. 192, 193) is a capsule of various shape, having a longer or GROUP II.— BRYOPHYTA. 311 shorter stalk ; its wall consists of a single layer of cells which contain chloroplastids when young ; internally it consists of very numerous small cells, each of which eventually gives rise to a single spermatozoid. The spermatozoid is a cell, consisting of a naked filament of protoplasm, spir- ally twisted, thickened at the posterior end where lies the nucleus, tapering at the anterior end where it terminates in two long cilia by means of which it swims (Figs. 192, 193) ; the spermatozoids are set free by the rupture of the an- theridial wall, which usually takes place at the apex of the antheridiuni. The archegonium is flask-shaped and shortly stalked (Figs. 194, 195) ; it con- sists of a slightly dilated basal portion, the venter, and of a long slender neck. The axis of the archegonium, when young, is occupied by a central row of cells ; the basal cell of this row, lying in the venter, is the central cell of the archegonium ; it grows considerably, and eventually divides into two unequal parts, an upper and smaller, the ventral canal- cell, and a lower and larger which is the female re- productive cell or oospJiere : the upper cells of the central row constitute the neck - canal - cells. At maturity the terminal cells, lid- cells, of the neck separate ; the neck- canal -cells and the FlG- 193--^ Antheridium of if arclwntia polymarplia ( Liver. , , wort) iu optical longitudinal section: p parapbyses (x 90). Ventral Canal-cell B Sparmatozoids ( x 600) : (after Strasburger). FIG. 192.— Fitnaria hygromet- rica (Moss). A Anantberidium bursting -. a the spermatozoids (x 350). B Spermatozoids (x 800); bin the mother-cell: c free spermatozoid of Poly- trichutn. 312 PART IV.— CLASSIFICATION. become mucilaginous and disorganised, so that the oosphere is placed in communication with the exterior by the canal of the neck. Fertilisation takes place when the plants are more or less covered with water from rain or dew. Then the antheridia dehisce, the spermatozoids are set free, and, since the male and female organs are at no great distance, they, swimming by means of their cilia, come into the neighbourhood of the archegonia ; they are attracted to enter the necks of archegonia by the escaping mucilage formed by the disorganisation of the canal - cells, which contains cane-sugar which substance has been shown to be especially attrac- tive to them (see p. 220). One of the entering sper- matozoids travels down the canal to the oosphere, which it pene- trates, the nu- cleus of the sper- matozoid fusing with that of the oosphere. Fer- tilisation is now complete ; the fertilised oosphere surrounds itself with a cell- wall and becomes the oospore, which begins to divide and to develope into the sporophyte. The effect of fertilisation is not confined to the oosphere. The adjacent tissue of the shoot is stimulated to growth, and in some forms (Sphagnaceae) it grows out into a long leafless stalk, the pseudopodium, which carries up the fertilised archegonium on its apex. The venter of the archegonium also grows, forming, either FIG. 194.— Marchantia ^olymorpha. A young ; -B mature, but unfertilised, archegonium. C Fertilised archegonium, with dividing ooepore. fc' Neck-canal-cells ; fc" ventral canal-cell ; o oosphere ; pr perigynium. (x 640 : after Strasburger.) GROUP II.— BRYOPHYTA. 313 by itself or together with the adjacent tissue of the shoot (as commonly in the Hepaticse), an investment, termed the calyptra, which surrounds the developing embryo within and, for a longer or shorter time, keeps pace with its growth. The gametophyte has a remarkable power, especially in the Musci, of reproducing itself vegetatively. This is effected fre- quently by the gemmce, formed from various parts of the body : the leaves, for instance, in the foliose Hepaticse ; or in distinct receptacles termed cupules, as in the Marchantiese and some Musci. n The gemmae are either unicellular or multicellular, and, in the latter case, may be either spherical or flattened in form. In the branched forms vegetative propagation is effected by the dying away of the main shoot or of the larger branches, the smaller branches be- coming isolated and constituting independent plants. In the Musci almost any part is capable, under favourable conditions, of growing out into protonemal filaments on which new adult shoots are de- veloped. With regard to the histology of the adult shoot, it need only be pointed out that rudimentary vas- cular tissue, absent in the Hepa- ticae, is to be found in the stems and the midribs of the leaves of many Musci ; and, further, that the epidermis is not clearly differ- entiated as a tissue- system, and is destitute of stomata. It is true that in some Hepaticse (e.g. Antho- ceros, Marchantia, etc., Fig. 199) there are structures in the super- ficial layer which are erroneously called stomata ; these are merely pores, and differ altogether in FIG. 195.— Futiaria hygrometriea. A Longitudinal section of the summit of a weak female plant (x 100) raarchegonia ; b leaves. B An archegoninm ( x 550) : b ventral portion with the oosphere ; neck ; m mouth still closed ; the cells of the axile row are beginning to be con- verted into mucilage. C The part near the mouth of the neck of a fertilised archegonium with dark red cell-walls. (After Sachs.) 314 PART IV. — CLASSIFICATION. structure and development from the true stomata which are to be found on the sporophyte of Anthoceros and of most Musci, as well as on the sporophyte of the higher plants. The SPOROPHYTE, the asexual spore-producing form, is developed from the oospore within the venter of the archegonium (Fig. 196). The oospore divides first into two cells by a transverse wall, the basal u-allj at right angles or obliquely to the long axis of the archegonium ; the upper cell, the one next the neck, is termed the epibasal cell, the lower the hypobasal cell. This is followed in some Hepaticse (Marchan- tiaceae, Anthocerotacese) by the formation of two walls, at right angles to the basal wall and to each other, which are known as the quadrant and octant walls, since they respectively seg- ment the oospore into quad- rants and octants of a sphere. In other Hepaticse, and generally in the Musci, the segmentation into oc- tants is confined to the epibasal cell, the hypobasal cell either remaining un- divided, or dividing irregu- larly. With the exception of some of the lower Hepa- ticae (Riccieae), where epi- basal and hypobasal cells alike contribute to the formation of the capsule in which the spores are de- veloped, the epibasal cells alone give rise to the cap- sule. The hypobasal cell gives rise to the foot, which is well- developed in the lower forms of both Hepaticae and Musci, but is rudimentary in the higher. The foot is essentially an em- FIG. 196. — Funaria hygrometrica. A Develop- ment of the sporogonium (/ /) in the ventral portion (6 I) of the archegonium (longitudinal section x 600). B C Different farther stiiges of development of the sporogonium (/) and of the calyptra (c) ; 7i neck of the archegonium. ( x about 40.) GROUP II.— BRYOPHYTA. 315 bryonic organ ; but it persists, acting, when sufficiently developed, as the organ of absorption and attachment, throughout the life of the Moss-sporophyte, because the sporophyte, since it does not become free and independent, does not altogether develope beyond the embryonic stage. In most forms the epibasal half of the oospore also gives rise to a longer or shorter stalk, the seta, by the elongation of which the capsule is raised up out of the calyptra. In those Hepaticse which have a seta, its elongation, and the consequent rupture of the calyptra, takes place suddenly when the capsule is already mature and the spores fully developed ; in the Musci its elongation is gradual, whilst the capsule is still rudi- mentary, and the rupture of the calyptra takes place relatively early. In the Hepaticse and some Musci (Sphagnaceae, Phascum, Ephemerum) the whole of the ruptured calyptra remains as a sheath, the vaginula, round the base of the seta : but in the higher Musci (most Bryineae) the calyptra is ruptured trans- versely into an upper and a lower half ; the latter constitutes the vaginula, whereas the former is carried up as a cap on the top of the capsule. In some forms, where the true hypobasal foot is rudimentary (some Jungermanniaceae and Bryinese) and is function- less, the base of the seta becomes dilated to form a false foot (epibasal), which performs the functions of attachment and ab- sorption. The body developed from the oospore, which constitutes the asexual generation or sporophyte of the Bryophyta, is termed the sporogonium. With regard to its general morphology it may be considered (except in Ricciese) to present differentiation into root and shoot ; the foot, however rudimentary, developed from the hypobasal half of the oospore, represents the root ; the capsule and the seta (when present), developed from the epibasal half of the oospore, represent the shoot. The shoot is in no case differentiated into stem and leaf. In the Riccieae the products of the hypobasal and epibasal cells are quite similar, so that the whole thalloid sporogonium consists only of a capsule. Hence, whilst it is the rule in the Bryophyta that sporogenous cells are only developed in the shoot-portion of the sporophyte, that is, are derived only from the epibasal cell, in the Ricciese the derivatives of the hypobasal cells are also sporogenous. The sporogonium is not an independent sporophyte, but remains attached to the gametophyte, obtaining from it either the whole or a portion of its food. It must, however, be clearly understood 316 PART IV. —CLASSIFICATION. that there is no continuity of tissue between the two generations ; the sporophyte is simply inserted into the tissue of the gameto- phyte. In the Hepaticse (except Anthoceros) the sporophyte is short-lived, and is entirely dependent upon the gametophyte for its nutrition. In Anthoceros, and in most of the Musci, the capsule possesses more or less well-developed assimilatory tissue, and its epidermis is provided with stomata, so that the sporophyte is capable of using the carbon dioxide of the air as its carbonaceous food, and is dependent upon the gametophyte only for its supply of water and salts. In many of these forms the seta has a central strand of rudimentary vascular tissue through which the water and salts, absorbed from the gametophyte, can travel to the region of the capsule where assimilation and transpiration are carried on. FIG. 197.— Comparative morphology of the sperogonium in the Bryophyta : diagram- matic transverse sections of the young capsule. A Sphserocarpus (typical Liverwort) ; B Ceratodon (typical Moss) ; C Anthoceros (aberrant Liverwort). E Endothecium : 9-9 primary divisions (quadrant and octant walls) ; s (shaded) archesporium ; C columel'.a. (A and C after Leitgeb ; B after Kienitz-Gerioff.) The tissue of the developing capsule becomes differentiated into an external layer (or layers) of cells, termed the amphithecium, which, in nearly all cases (except Anthocerotacese and Sphagnacese) forms only the wall of the capsule ; and an internal solid mass of cells, the cndothecium. The spores are developed from a mass or a layer of cells termed the archesporium. In the Hepaticae the archesporium (Fig. 197 .4) includes the whole of the endothecium (except in Anthocerotacese, Fig. 197 (7), and the archesporial cells are either all sporogenous (Ricciese) or, as is more frequently the case, some of them are sterile and generally become spirally thickened and elongated in form when they are termed elaters. In the Antho- GROUP II. — BRYOPHYTA. 317 cerotaceae and in nearly all Musci the archesporium is a layer of cells : it is generally the external layer of the endothecium, but in most of the Anthocerotaceae and in the Sphagnacese it is the innermost layer of the amphithecium. In those forms where the archesporium is a layer of cells, the internal sterile tissue of the endothecium constitutes what is termed the columella. The arche- sporial cells are either themselves the mother-cells of the spores, or they undergo division to form these cells. Each mother-cell gives rise to four spores ; the nucleus divides into two, and each of these divides again ; the protoplasm aggregates round the four nuclei, constituting four cells which surround themselves with a proper wall and which are the spores. They do not usually all lie in one plane, but are placed tetrahedrally. The mature spore is a cell, consisting of a mass of protoplasm, with a nucleus, and containing chloroplastids, starch-grains and oil-drops ; the wall consists of two layers of the usual structure (see p. 50). During the formation of the spores the mother-cells become isolated from each other, floating freely in a mucilaginous liquid in the interior of the capsule. The escape of the spores from the capsule is effected in various ways. In some cases the wall of the capsule simply decays (e.g. Riccieae, Phascum) ; or it splits into valves (e.g. Jungermanniaceae) ; or the upper part is thrown off as a lid or operculum (e.g. some Marchantieae, Sphagnaceae, most Bryineae). On being set free, the spores germinate, when the conditions are favourable, giving rise to the protonema. The brittle exo- spore being ruptured, the contents, covered by the endospore, then generally grow out in the form of a filament, which is the beginning of the protonema. In some rare cases (e.g. Pellia) cell- divisions take place within the spore before the exospore is ruptured, so that the protonema is from the first a mass or a layer of cells. The Bryophyta (Muscineae) are divided into two classes, the distinctive characters of which are as follows : — Class III. — HEPATICJE (Liverworts). Gametophytic Characters. Protonema, generally short-lived, inconspicuous, a flattened expansion. Adult shoot, generally dorsiventral ; thalloid in many forms ; has unicellular root-hairs ; no trace of vascular tissue : leaves (when present) destitute of a midrib. Sporophytic Characters. The sporogonium remains within the 318 PART IV.— CLASSIFICATION. calyptra until the spores are ripe ; the ruptured calyptra remains as a vaginula, no portion of it being raised as a cap on the sporo- gonium ; the elongation of the seta (when present) is sudden ; the growth of the sporogonium is not effected by a two-sided apical cell. The archesporium (except in the Anthocerotacese) is a mass of cells co-extensive with the endothecium ; in all cases (except Ricciere) some of the archesporial cells are sterile, being frequently developed into elaters ; a columella is present only in the Antho- cerotacese. There is no trace of vascular tissue in the sporophyte, nor are there any stomata in its epidermis (except Anthocerotacese). Class IV.— Musci (Mosses). Gamctophytic Characters. Protonema frequently persistent, well-developed, generally filamentous. Adult shoot, radial or isobilateral ; always differentiated into stem and leaf ; no root- hairs, but branched multicellular rhizoids ; stem frequently with a central strand of rudimentary vascular tissue ; leaves generally with a midrib. Sporophytie Characters. The sporogonium escapes from the calyptra at an early stage ; a portion of the calyptra (with certain exceptions) is carried up as a cap on the sporogonium ; the elonga- tion of the seta is gradual ; the growth of the sporogonium is (except Sphagnacese) effected by a two-sided apical cell. The archesporium is not co-extensive with the endothecium, and is generally a layer of cells ; the archesporial cells are all sporo- genous, none being sterile or forming elaters ; there is usually a well-developed columella in the capsule. The seta frequently has a central strand of rudimentary vascular tissue ; the epidermis of the capsule is generally provided with stomata. Class III. — HEPATIOE (Liverworts). A. The GAMETOPHYTE. The spore gives rise, on germination, to a small protonema which is sometimes filamentous, but more generally a flattened cellular expansion. The Adult Shoot springs from the protonema. Its symmetry is generally dorsiventral. It is commonly thalloid, but is differ- entiated into stem and leaves in some forms (e.g. foliose Junger- manniacese). Its growth is effected by an apical growing-point in GROUP II. — BRYOPHTTA : HEPATIC^. 319 which there is either a group of initial cells (Marchantiacese, Anthocerotacese), or a single apical cell (Jungermanniacese). The branching is commonly dichotomous, taking -place in the plane of expansion ; but the development of branches from the ventral surface is constant in several genera. The dorsiventral shoot bears numerous unicellular root-hairs on its ventral (lower) surface ; when thalloid it also bears multi- cellular scales (ventral scales') on the same surface ; when foliose, it bears on this surface a row of small rudimentary leaves, termed amphigastria, the fully developed foliage-leaves being borne in two lateral rows, one on each flank of the shoot. In the great majority of Hepaticae, the sexual organs are borne on the dorsal (upper) surface, either scattered or in groups ; and sometimes upon a specially modified portion of the shoot, termed the receptacle, either sessile or stalked ; in the latter case the shoot (e.g. higher Marchantiese) may be more or less clearly differentiated into a vegetative and a reproductive part (gametophore). It is only in some of the Jungermanniacese (Jungermanniacese acrogynse) that the sexual organs are developed at the apex of the branches of the shoot, a feature in which they approach the Musci. The protonema bears but a single adult shoot ; and this, owing to the transitory nature of the protonema, soon becomes an inde- pendent plant. The plant is generally monoecious, but sometimes dioecious. B. The SPOROPHYTE is developed from the fertilized oosphere (oospore) in the archegonium (see p. 312). It is a sporogonium, which may consist merely of a capsule (Ricciese) ; or it may be differentiated into a capsule and a foot (e.g. Anthoceros) ; or into a capsule, a longer or shorter seta, and a foot (e.g. Marchantiese) ; or into a capsule, a seta, and a rudimentary (hypobasal) foot (some Jungermanniacese), a false foot (epibasal) being in some cases de- veloped from the lower part of the seta. It never grows by means of a two-sided apical cell as it does in the Mosses. The internal differentiation of the capsule presents the following varieties : — It is in all cases differentiated into amphithecium and endothecium ; in all, except most Anthocerotaceae, the archesporium is co-extensive with the endothecium ; in the Anthocerotacese, the whole or part of the endothecium constitutes a columella, a feature in which the Anthocerotacese resemble the Musci. In the Riccieae, as a rule, the whole archesporium is sporogenous ;. whereas in all other forms some of the archesporial cells are 320 PART IV.— CLASSIFICATION. sterile, and in many they are developed into elatcrs, elongated cells with spirally thickened walls, generally becoming free from each other. The sporogonium remains enclosed in the calyptra until the spores are mature when, if a seta be present, it suddenly elongates and ruptures the calyptra, which persists as a vaginula at its base. The capsule opens either by the decay of its wall, or more gener- ally by the splitting of the wall from the apex downwards into valves ; in some Marchantiese a lid, the operculum, is formed and the capsule is opened by the throwing off of the lid. The Hepaticse are classified as follows : — Order I. Marchantiacese. Order II. Jungermanniacese. Order III. Anthocerotacese. Order I. Marchantiaceae. A. The GAMETOPHYTE. The spore gives rise on germination to a short unbranched filamentous protonema which developes at its apex into a flattened cellular expansion, from the margin of which the adult shoot (commonly known as the plant) springs as a lateral branch. The Morphology of the Adult Shoot. The adult shoot is undiiferentiated into stem and leaf. Its symmetry is dorsiventral ; on the lower (ventral) surface it bears numerous root-hairs, and also scales which are arranged in one or two rows, or irregularly. Growth is effected by an apical growing-point, situated in a depression, possessing a transverse row of initial cells from which segments are cut off dorsally and ventrally; the initial cells also undergo longitudinal division, and thus increase in number. The normal mode of branching is that which takes place in the plane of expansion; it is dichotomous, and is effected in the manner described on p. 132. The sexual organs are in all cases developed on the dorsal surface, each antheridium or archegonium arising from a single superficial cell. In the simpler forms they are arranged in a continuous median row, developed in acropetal succession ; in the higher forms they are borne on a special structure termed a receptacle. The receptac'e. In the higher Marchantiese the adult shoot is frequently differentiated into a vegetative and a reproductive portion, the gameto- phore; the gametophore is a branch (or a branch-system) bearing a terminal receptacle, in which either the male (antheridiophore) or the female (archegoniophore) organs are developed. In the simpler forms the archegoniophore is simple, that is unbranched ; the stalk presents a single furrow which represents the ventral surface of the shoot. In Marchantia the stalk has two ventral furrows, showing that it consists of the two coherent branches of the first dichotomy. The receptacle itself is repeatedly branched: thus in Marchantia there are eight groups of archegonia, corresponding to eight branches. The receptacle is more or less distinctly lobed, thus showing its compound GROUP II.— BRTOPHYTA : HEPATIC-E. 321 nature : each group of archegonia is situated between the bases of two adjacent lobes. The complete elongation of the stalk does not take place until the archegonia are fully developed, or even until one of them has been fertilised. It is only in a few of the higher Marchantieae that there is a highly de- veloped antheridiophore. In Marchantia a definite terminal receptacle is formed; it is discoid inform, and it is elevated on an erect stalk (see Fig. 198 A) : it is compound, having several growing-points, each of which gives rise to antheridia in acropetal succession, and then ceases to grow ; the stalk has two ventral furrows, showing that it consists of two coherent branches. In Marchantia the venter of each archegonium becomes surrounded by a sac-like membrane, developed from the stalk-cell of the archegonium, which is termed the perigynium (Fig. 194). The development of the perigynium begins when the archegonium is nearly mature. FIG. 1S>8.— J 1'ortion of a plant of 3Iarchantia polymorp'ia (t), with atitheridiophores. B Portion of a plant with a cupule containing gemmae; v v apices of the two branches. (After Sachs.) C An archegoniophore with a doubly furrowed (r) stalk t, bearing a terminal branched receptacle of which s is one of the rays ; h perichaetium ; fc sporogonia. The distribution of the sexual organs is various : the plants may be monoecious or dioecious (Marchantia). The Structure of the Adult Shoot. The dorsal portion of the shoot consists in all the Marchantiacese, of parenchymatous tissue, made up of cells con- taining chloroplastids, which includes a number of air-chambers, giving it an areolated appearance, whence it is termed the air-chamber-layer. The chambers are formed by the unequal growth of the cells near the growing-point, in consequence of which the surface presents alternating elevations and depressions. In Marchantia the primary air-chambers be- come completely closed in ; at the central meeting-point the growth of the superficial cells take place vertically, leading to the formation of vertical rows of cells which subsequently separate, leaving a canal between them. This structure is distinguished as a compound pore. Compound pores of M.B. Y 322 PART IV.— CLASSIFICATION. this sort are found in the receptacles of other Marchantiese, the vegetative parts of which have simple pores. In many forms, the cells containing chloroplastids (assimilatory tissue) are simply those which form the walls of the air-chambers ; in Marchantia (Fig. 199) the cells forming the floor of the air-chamber, or the sides, or even the roof, grow out into branched or unbranched filaments which fill most of the air-cavity, thus largely increasing the assimilatory tissue. Beneath the air-chamber-layer is a compact tissue, consisting of several layers of cells elongated in the direction of the long axis of the branch, which is without intercellular spaces, and contains but few chloroplastids. In the Marchantieae the walls of these cells are generally thickened and pitted ; some of the cells contain mucilage, and in Fegatella the mucilage- cells form continuous rows ; other cells contain a dark-coloured oil-drop, though such cells also occur in the air-chamber layer. The ventral surface is formed by a layer of cells which, in the simpler FIG. 199. — Marchantia polymorplia. A A pore seen in surface view. B Section of a portion of the dorsal region of the thallus, showing the air-chamber containing assimilatory tissue, and the compound pore. ( x 210 : after Strasburger.) forms, is not specially differentiated, but in some the cells of this layer are remarkable for their small size; in Marchantia and Preissia there are several layers of these small cells, forming a sort of ventral cortex. The ventral scales consist of a single layer of cells, the walls of which generally assume a violet colour ; each scale is developed from a single superficial cell, or, as generally in the Bicciese, from a transverse row of cells. In Marchantia polymorpha, in addition to the scales which arise from the midrib, there are others which spring from the surface of the lamina. Unicellular root-hairs are produced in all Marchantiacese ; the com- monest form has thin walls ; in the Marchantieae a second form occurs, in which peg-like thickenings of the wall project into the cavity of the cell : the simple root-hairs are developed mainly on the midrib, the thickened hairs mainly on the lamina. GROUP II.— BRYOPHYTA : HEPATKLE. 323 Ge.nmce are produced in Lunularia and Marchantia in special receptacles, termed cupules, borne on the dorsal surf ac3 of the shoot ; in Lunularia the cupule is crescsnt-shaped, in Marchantia it is circular (Fig. 198J5). The cupule is formed by an outgrowth of the air-chamber layer, and in Marchantia its margin is prolonged into laciniae. The gemmae spring from single cells of the floor of the cupule, which elongate upwards and divide transversely into a stalk call and a terminal cell, which, by repeated growth and division, forms a flattened plate of tissue, several layers of cells thick at the middle, thinning out to a single layer at the margin, with a growing-point in a depression on each lateral margin. The symmetry of the gemmae is isobilateral ; but when they fall on to the soil and begin to grow, the undermost surface becomes the ventral, and the uppermost the dorsal. Some of the superficial cells have no chloro- plastids ; those of the surface next the soil grow out into root-hairs. B. The SroROPHYTK. The degree of morphological and histological differentiation of the sporophyte presents wide divergences in the different groups. In the Ricciese, the whole embryo simply forms a spherical capsule : in the Marchantiese, the capsule is developed entirely from the epibasal cells, whilst the hypobasal cells give rise to a bulbous foot, which attaches the embryo to the parent, and to a short stalk which bears the capsule, and is formed at a relatively late stage by intercalary growth. The differentiation of the tissue of the capsule into amphithecium and endothecium is well-marked, except in the Riccieae. The archesporiuni is coextensive with the endothecium. In the Riccia the whole of the archesporial cells are sporogenous ; in Corsinia, some of the archesporial cells are sterile, but these undergo no special differentiation; in the Marchantiese the sterile cells assume an elongated form, and their walls undergo spiral or annular thickening: these specially modified sterile cells are the elaters, and, being very hj'gro- scopic, they assist in the scattering of the spores. Each sporogenous cell gives rise to four spores. The wall of the capsule, which consists generally of a single layer of cells, is but slightly developed in the Riccieae, and becomes entirely dis- organised during the development of the spores. In the Marchantiese the cells of the walls are generally thickened and the thickenings fibrous, in which case the capsule opens by the splitting of the wall longitudinally into a number of teeth. The spores are generally tetrahedral, with two coats, the outer of which (exospore) is tuberculate or reticulate on the surface. On germination the exospore of the tetrahedral spore ruptures at the point of junction of the three projecting angles. The spores of Lunularia and Marchantia are small and spherical ; the exospore is feebly developed, and presents a granular thickening. In consequence of the thinness of the exospore, the whole spore is enabled to enlarge considerably on germination, and it does not rupture at any special point. In Fegatella, cell-divisions take place in the spores before they are scattered. The venter of the archegonium keeps pace with the growth of the developing embryo, forming the calyptra, and encloses it until the spores 324 PART IV. — CLASSIFICATION. are ripe. In the Riccieae the spores are set free by the gradual disorgan- isation of the calyptra and of the tissue of the thallus in which the cah-ptra is embedded ; in the other Marchantiacese the capsule is forced out of the catyptra by the elongation of the short stalk. The order Marchantiacese includes the families Kicciese (Riccia, Oxy- mitraX Corsinieae (Corsinia, Boschia), and Marchantiese (Marchantia, Lunularia, Fegatella, etc.). Order II. Jungermanniaceae. A. The GAMETOFHTTE. On germination the spore gives rise to a proto- nema which may be a solid ellipsoidal mass of cells (as in Pellia) with a root-hair at one end ; or a flattened plate of cells (Eadula, Frullania) ; or a filament, some- •f " times branched (Lophocolea, C h i 1 oscy phus) ; however, the differences in form of the protonema are not of great morphological importance since, in many cases, flattened and filamentous forms have been found to be pro- duced from spores of the same plant. The protone- ma gives rise to the adult shoot by the forma- tion, either from a marginal cell, if it is flat, or from the terminal cell, if it is filamentous, of a growing-point with a s ingle apical cell. The Morphology of the Adult Shoot. The adult shoot may be differenti- ated into stem and leaf, as in ihefoliose forms ; or undifferentiated, as in the thalloid forms. Its symmetry is generally dorsiventral ; the only radially symmetrical, erect-growing forms being Haplomitrium and some species of Riella (e.g. R. hellcophijlla and Parian). Most of the thalloid forms have a distinct midrib. The shoot bears numerous unicellular root-hairs, as also club-shaped glandular hairs which secrete mucilage, on its ventral (under) surface. In the dorsi- ventral foliose forms, the stem bears a row of leaves on each flank, and FIG. 200.— Growing-point of thallus of Metzgeria furcata : t apical cell ; sf etc., successive segments ; m' m" marginal cells ; p' super- ficial cell ; i t cells of the midrib ; c clavate hairs. ( x 540 : after Strasburger.) GROUP II.— BRYOPHYTA : HEPATIC^. 325 generally a row of amphigastria on its ventral surface. In the radial foliose forms, the leaves are borne in three rows in Haplomitrium, and in two rows in the radial species of Riella; here there is no distinction of amphigastria. The growth of the shoot is effected by an apical growing-point which possesses a single apical cell. The apical cell of the thalloid forms is most commonly two-sided (Fig. 200) ; the base is directed outwards, the apex inwards, and from the two sides segments are cut off alternately right and left. But in Pellia the cell is bounded by four surfaces — an external free surface, an internal, and two lateral ; segments are successively cut off along the internal and the two lateral surfaces. The apical cell of the foliose forms, with the exception of Fossombronia and Riella which have a two-sided apical cell, is a three-sided pyramid; its base is directed out- wai-ds, its apex inwards, one side is ventral and the other two are dorso- lateral ; this latter statement does not, of course, apply to Haplomitrium, which is radial. The normal mode of branching in the dorsiventral forms is that which takes place at the growing-point in the plane of expansion. In the thal- loid forms, as also in the foliose Fossombronia and Blasia, it may be described as dichotomous (see p. 132) although the apical cell does not undergo division so as to form the apical cells of two branches ; the apical cell of the parent shoot persists, and that of the branch is developed from an adjacent segment, either before or after further division. When the two shoots develope with equal vigour, the resulting branch-system re- sembles a dichotomy ; but when the parent shoot grows the more vigor- ously throughout, the branches are lateral upon it and the branch-system is a monopodium (see p. 19). In the foliose forms the mode of normal branching is generally monopodial. The apical cell of a lateral branch is developed from the lower (ventral) half of a dorso-lateral segment cut off from the apical cell ; either from the whole of the segment, or from the posterior (basiscopic) portion of it. In some of these plants there is a formation of gemmx. In Aneura certain cells of the margin and of the dorsal surface of the shoot each become divided into two, anl the two cells thus formed are set free as a bicsllular gemma, with probably a proper wall of its own, by the rupture of the enclosing wall. In Blasia, the gemmae, which are solid multicellular nearly spherical bodies, are developed in special receptacles (cupules) situated on the dorsal surface of the apex ot the shoots ; their mode of origin resembles that of the gemmae of Marchantia. In most foliose forms the gemmae are developed from marginal cells of the leaves (e.g. Jumjer- mannia ventricosa) or from cells near the growing-point of the stem (e.g. .Junyermannia blcuspidala). In these forms the gemmae are usually uni- or bi-cellular, but in Rtdula complanata (where they are formed on the leaf- margin) they are flat multicellular plates of tissue. The leaves are developed, generally speaking, one from each segment formed from the apical call. In the typical Acrogynae each dorso-lateral segment gives rise to a lateral leaf, and each ventral segment to a ventral leaf (amphigastrium) ; though, as alread3r mentioned, the amphigastria 326 PART IV.— CLASSIFICATION". are wanting in many species. A characteristic feature of the leaves of this group is that they are distinctly bilobed, at least when young ; this is due to the fact that the mother-cell of the leaf is divided into two which give rise to the two lobes. The leaves are sessile, and their insertion is at first transverse to the long axis of the stem, so that one lobe is superior or dorsal, the other inferior or ventral ; but by subsequent displacement it becomes oblique. Since the leaves are situated close together, they thus come to overlap each other, and this overlapping presents two forms: either the posterior edges of the leaves overlap the anterior edges of those next behind them (Fig. 201), when the leaves are said to be succubous ; or the anterior edges of the leaves overlap the posterior edges of those next in front of them (Fig. 202), when the leaves are said to be incubous. The growth of the leaf is generally apical at first, and subsequently basal. Fie. 201. — Brooches of one of the aero- rrrnous Junxermanniacese, P'a/iiochila as- flenioides, seen from above: the leaves are succubous ; at the apex, two of the shoots benr sporopon'a, the one (b) having de- l.isced, the oiher(n) being still closed; p the involucre. Fm. 202 —Part of a shoot of Frul- lania dilatata seen from below (x 20): ul auricnlate lower leaf-lobes ; ol upper leaf-lobe; the leaves are incubous; u amphigastrium. The sexual organs are generally borne on the main axis and its normal branches, but in many cases they are confined to more or less specialised ventral branches (gametophores). The place of development of the arche- gonia affords the basis for the classification of the Jungermanniacese into the two main groups, Acrogynee and Anacrogynse. In the former, which includes nearly all the foliose forms, the archegonia are produced from the apical cell and its youngest segments at the growing-point ; hence when the formation of the archegonia takes place on a shoot its further elongation is arrested. In the latter group, which includes all the thalloid forms and some exceptional foliose forms, the archegonia are GROUP II. — BHYOPHTTA : HEPATIC^E. 327 produced laterally, on the dorsal surface in the dorsiventral forms, on all sides in the radial forms (species of Biella, Haplomitrium) ; hence the growth in length of the shoot is not necessarily arrested. The archegonia of the Acrogynae are borne eithe'r singly or in groups of two or more. Each archegonium is developed from a single cell; when the archegonium is single it is developed from the apical cell ; when there are several archegonia, the development of them begins in the youngest segment-cells of the growing-point. The archegonia are surrounded by the leaves of the apex ; and in most cases the leaves of the last whorl are coherent, forming an involucre, surrounding the single archegonium or the group of archegonia. The archegonia of the thai lo id Anacrogynae are borne in median dorsal groups : in the radial Anacrogynae (Riella helicophyUa, Haplomitrium) they are borne singly, scattered over the whole length of the stem as in the former, or confined to the apical region as in the latter. They are in all cases provided with some sort of protective organ. Among the thalloid Anacrogynae the group of archegonia is surrounded, in Metzgeria, Aneura, and Pseudoneura, by an involucre consisting of the short modified game- tophore (ventral in Metzgeria) ; in Pellia, Symphyogyna, and Sphaero- carpus a group (or each archegonium as generally in Sphaerocarpus) is surrounded by an involucre developed as an outgrowth of the tissue of the fertile branch. The antheridia are borne, in all Jungermanniacese (except Haplomitrium) on the dorsal surface of the shoot ; in Haplomitrium they are borne in three rows on the sides of the apical region. The antheridia are shortly stalked and are in all cases provided with a protection. In Metzgeria the group of antheridia is invested by an involucre which consists of the short modified ventral gametophore : in the other thalloid Anacrogynae (e.g. Pellia) each antheridium is invested by an involucre which grows up around it, so that it appears to be sunk in the tissue of the shoot. In the Acrogynae the antheridia are borne, singly or several together, in the axils of leaves; and in some forms (e.g. Scapania, Lejeunia, Frullania) the upper lobe of the protecting leaf is modified in form. The distribution of the sexual organs varies even in the species of some of the genera. Some of the Anacrogynse (e.g. Metzgeria, Pseudoneura, Sphaerocarpus, Haplomitrium, etc.) are dioecious ; whereas others (e.g. Pellia, some species of Aneura, Fossombronia, Symphyogyna) ar« gener- ally monoecious. In the monoecious forms the antheridia and archegonia are generally borne on distinct branches (diclinous), but sometimes on the same branch (monoclinous). When a dorsiventral shoot bears only antheridia or archegonia, they are developed in the median line; but when it bears both organs, the archegonia are median and the antheridia lateral. The Acrogynae are generally monoecious and diclinous. The structure of the adult shoot of the Jungermanniaceae is very simple. In thalloid forms which have no distinct midrib, the shoot consists of parenchymatous cells forming a single layer at the margin and several 328 PART IV.— CLASSIFICATION. layers (e.g. Pellia, Aneura) in the middle line of the shoot ; in those which have a well-defined midrib (e.g. Metzgeria), the midrib consists of several layers of cells, whereas the lamina consists of only a* single layer. In Symphyogyna and Blyttia the midrib is traversed by a strand of elon- gated prosenchymatous cells having thickened and more or less pitted walls : a similar tissue occurs in the thick central portion of the shoot of Pellia. In the Acrogynse, the stem generally consists of an axial strand of relatively thin-walled cells, surrounded by a cortex of narrow thick-walled cells : the leaves are simply single layers of similar cells, and have no midrib. The root-hairs are, in all cases, destitute of the peculiar thickenings so characteristic of the Marchantiacese. B. The SPOKOPHYTE. The course of the development of the sporophyte is, in its main features, essentially the same throughout the Jungerman- niacese. The oospore is divided by a transverse (basal) wall into two halves, epibasal and hypobasal. The epibasal cell gives rise to the capsule and its stalk (seta). It divides transversely, and the longitudinal divisions follow in both cells so that the epibasal half of the embryo consists of two tiers of each consisting of four cells. Further growth in length is effected by the cutting off, by transverse walls, of segments from the cells forming the apical tier ; but this apical growth is arrested, sooner or later, by the formation of walls parallel to the free surface (periclinal) in the apical cells, and also frequently in some of those below them, which indicate the differentiation of the capsule-wall (amphithecium) from the internal mass of cells (endothecium) which give rise to the spores and elaters. The cells balow the capsule may, however, continue to grow and divide transversely, and by means of this intercalary growth the full length of the seta is attained; In many of the Jungermanniacese (e.g. Pellia, Jungermannia, Frullania) the lower end of the seta developes into a bulbous mass of cells forming a false foot, the upper margin of which grows up so as to form a sheath round the lower part of the seta in some cases. The development of the hypobasal portion of the embryo is compara- tively insignificant •, in most cases it is merely a small appendage to the lower end of the seta. The hypobasal cell enlarges somewhat, without undergoing any division (e.g. Radula, often in Pellia); or it undergoes transverse division to form a filament of two or three cells, the lowest of which becomes elongated and grows down among the cells at the base of the archegonium (e.g. Metzgeria, Aneura). In some forms, however (e.g. Fossombronia), the hypobasal cell appears to give rise to a true foot, bulbous in form, C9mparable to that of the Marchantieae (see p. 323). In the further differentiation of the capsule, the cells of the amphithe- cium undergo periclinal division so that the wall eventually consists of two or more (up to six) layers of cells. In the wall-cells transverse annular thickenings are usually developed. The planes of dehiscence of the capsule, except in those forms which dehisce irregularly (Riella, GROUP II.— BRYOPHYTA : HEPATIC JE. 329 Sphserocarpus), are marked out by four longitudinal rows of small-celled tissue which correspond in position with the walls between the four apical cells of the growing embryo. The archesporium, which is co-extensive with the endothecium, presents various degrees and forms of differentiation. In the Riellese it comes to consist of a number of cubical cells, some of which become the mother- cells of the spores, whereas the others persist as unaltered sterile cells. In all the other Jungermanniacese &ome of the cells of the endothecium are sterile, but they develope into elaters, becoming elongated in form and spirally thickened, having sometimes two spirals, or only one. The relative arrangement of the sterile and fertile cells, dependent upon the growth of the capsule along different diameters, varies somewhat. In the lower forms, the elongated archesphorial cells are arranged more or less longitudinally, either quite straight (e.g. Frullania, Lejeunia), or radia- ting from the apex of the capsule (Metzgeria, Aneura), or radiating from the base of the capsule (Pellia, Badula): whereas in the higher forms (Lepidozia, Calypogeia, Jungermannia), these cells are placed horizontally round a central longitudinal axis, except at the apex where they radiate. In most cases the sterile and fertile archesporial cells are mingled to- gether, but in some cases certain parts of the archesporium give rise especially to spores and others to elaters. Thus, in Pellia, the cells to- wards the base and those in the longitudinal axis of the capsule form only elaters, whereas in Jungermannia the formation of elaters is confined to the cells near to the wall. Whilst the development of the embryo is taking place, growth is also proceeding in the archegonium and the adjacent tissue, giving rise even- tually to the calyptra. Several of the archegonia of a group may be fertilised, but generally only one gives rise to a fully developed sporo- gonium, and itself takes part in the formation of the calyptra. The calyptra is sometimes developed from the venter of the archegonium alone (e.g. generally in the Acrogynae) ; in the Anacrogynae the adjacent tissue of the shoot frequently takes part in its formation, as is shown by the fact that the unfertilised archegonia of the original group are found on the sides, or even on the top of the calyptra (e.g. Aneura, Pellia). The wall of the calyptra consists of one or more layers of cells, and keeps pace with the growth of the embryo which it encloses until the spores are mature. The cells of the seta then rapidly elongate, causing the rupture of the calyptra, and the capsule is exposed. The capsule then dehisces, generally into four valves, sometimes irregularly, and the spore* are set free. The Jungermanniaceae may be classified as follows : — Series I. ANACROGYX.E : growth in length not necessarily arrested by the development of archegonia. Section A. Anelatereae : the sterile cells in the capsule do not develope into elaters. This section consists of the family Riellece: including the two genera Kiella (foliose) and Sphaerocarpus (thalloid). odO PART IV. — CLASSIFICATION. Section B. Elatereae : the sterile cells in the capsule develope into elaters. o. Thalloid Forms : Metzgeria, Aneura, Pellia, etc. ft. Foliose Forms : Fossombronia, Blasia, Haplomitrium (with radial symmetry), etc. Series II. ACBOGYN^: : growth in length arrested by the development of archegonia ; all foliose. This series includes a number of families of which the more familiar genera are Plagiochila, Jungermannia, Scapania, Lepidozia, Badula, Lejeunia, Frullania; etc. Order III. Anthocerotaceae. A. The GAMETOPHYTE. The protonema developed from the germinating spore is a flattened plate of cells ; in Anthoceros the formation of the flattened plate is sometimes preceded by the outgrowth of the contents of the spore, covered by the endospore. into a filament at the apex of which the plate of cells is developed. The adult shoot is developed as a lateral out-growth from the flattened protonema. The Morphology of the adult shoot. The adult shoot is thalloid, and its symmetry is dorsiventral. It is semi-circular, or nearly circular, in out- line in Anthoceros. There are no ventral scales on the under surface, but numerous unicellular root-hairs. The growth of the shoot is effected, in Antho- ceros, by a series of marginal growing-points. In the growing-point there is a row of initial cells, each of which acts as an apical cell ; their form is wedge-shaped in Anthoceros, dorsal and ventral segments being alternately cut off by the formation of oblique walls. (nat size) K the *ca "sules* Branching, or at least the formation of new some as yet unopened. growing- points, takes place in the manner de- scribed for the Marchantiacese (p. 132). The sexual organs are developed from the dorsal segments formed in the growing-point, and are situated in the middle line behind each growing- point in Anthoceros. The antheridia are developed endogenously, and remain enclosed in the tissue until maturity ; they are developed either singly (some species of Anthoceros) or in groups. The archegonia are sunk in the tissue, the apex of the neck reaching to the dorsal surface of the shoot. The shoots are monoecious ; the sexual organs are sometimes intermingled in the same group (frequently in Anthoceros). The structure of the adult shoot. The adult shoot of Anthoceros consists of several lajrers of cells in the middle line, thinning out to a single layer of cells at the margins. The tissue in the middle line consists of longi- tudinally elongated cells, the walls of which, especially in the older parts of the shoot, frequently present reticulate or even spiral thickening. The chloroplastids of the Anthocerotaceae are peculiar, on account of their relatively large size, and of the fact that they occur singly in the cells and contain a pyrenoid (see p. 71). GROUP II.— BRYOPHYTA : HEPATIC^. 331 B. The SPOROFHYTE. The early stages in the development of the sporophj'te of the Anthocerotaceae appear to be much the same as in the case of other Liverworts. The oospore divides transversely into an epibasal and a hypobasal half : and each of these divides by two perpendi- cular walls so that the embryo consists at this stage of eight cells. The cells of the epibasal half divide transverse^ several times, and then further apical growth in length is arrested by the formation of periclinal walls, marking the differentiation of amphithecium and endothecium, first in the four apical cells, and subsequently in those below them. By the repeated formation of periclinal walls, the amphithecium comes to consist of several layers of cells. The hypobasal cells undergo but few divisions, giving rise to a bulbous foot, the superficial cells of which grow out into papillae and penetrate between the cells of the adjacent tissue of the gametophyte. As regards the differentiation of the epibasal portion of the embryo, in Anthoceros (Fig. 197) the archesporium is developed from the innermost layer of cells of the amphithecium, a peculiarity, the only other instance of which, in the Muscinese, is to be found in the Sphagnaceae: the endothecium gives rise to an axial strand of sterile tissue, termed the columtlla, which is completely invested (except at the base, where it is continuous with the tissue of the foot) by the archesporium. In all the genera some of the cells derived from the archesporium are sterile. In some species of Anthoceros (e.g. vicentianus, giganteus, etc., constituting the subgenus Anthocerites) these cells develope into elaters with spiral thickening, each elater consisting of a row of cells with an apparently continuous spiral band : in other species (e.g. tuberculatus, glandulosus) the elaters have the same form, but they have no spiral band; in others (Icevis, punctatus) the sterile cells do not form distinct elaters, but a network of short cells, with spiral thickening, in the meshes of which lie the mother-cells of the spores. The sporogonium of Anthoceros has no seta ; when the apical growth has ceased, the capsule continues to elongate by basal growth, and hence does not ever become fully mature throughout. The pod-shaped capsule splits from the apex into two valves (Fig. 203). Stomata occur in the epidermis of the capsule in most species of Anthoceros, but they appear to be wanting in the other genera. Since the archegonia are sunk in the tissue of the shoot, the calyptra, which invests the developing embryo, is developed mainly from the surrounding tissue, and only to a small extent from the wall of the arche- gonium. 332 PART IV. — CLASSIFICATION. CLASS IV. MUSCI (Mosses). A. The GAMETOPHYTE. The protoncma is more conspicuous in the Musci than in the Hepaticse : it sometimes persists until the sporogonia are developed and the spores are ripe (e.g. Ephemerum), and in many cases the subterranean portion persists from year to year. It is generally filamentous and much branched ; but in some forms it is a flattened expansion (e.g. Sphagnum), or cylindri- cal branched and shrubby, or it bears lateral flattened expansions which are assimilatory organs (e.g. Tetraphis, etc.). The filamen- tous protouema consists of a subaerial and of a subterranean portion, which differ in that the cells of the former contain chloro- plastids, their walls are colourless, and the septa are transverse ; whereas those of the latter do not contain chloroplastids, and their walls are brown and their septa oblique. The protonema presents, in fact, a certain differentiation into shoot and root, the term rhizoids being applied to the root-like filaments. This differ- entiation is, however, of little morphological value, since the differences between the shoot- and root-filaments depend entirely on external conditions : thus, if the rhizoids be exposed to light they assume the characters of the subaerial filaments. The flat- tened protonema of Sphagnum bears rhizoids on its margins and under surface. The growth in length of the protonemal filaments is apical : the terminal cell behaves as an apical cell from which segments are successively cut off by transverse or oblique walls. The Adult Shoot arises as a lateral bud on the subaerial portion of the protonema. In some cases (Bryinese) the subterranean portion gives rise to lateral buds : these are small, spherical or lenticular, multicellular bodies of a brown colour, filled with re- serve materials, and are termed bulbils ; when they are brought to the surface they give rise to adult shoots, either directly or with the intervention of protonema. The adult shoot is in all cases differentiated into stem and leaves, and may be branched or unbranched. Its symmetry is commonly radial or isobilateral, less commonly dorsiventral. In the former case it is attached to the soil by rhizoids springing from its basal portion ; in the latter, by rhizoids developed on its under surface. In Sphagnum, rhizoids occur only on young shoots. GROUP II. — BRTOPHYTA : MUSCI. 333 The growth of the adult shoot and its branches is effected by means of an apical growing-point with a single apical cell which is generally a three-sided pyramid. Each segment cut off from the apical cell gives rise to a leaf : hence the arrangement of the leaves, and the symmetry of the shoot, is generally determined by the form of the apical cell. Thus in Fissidens, the leaves are arranged in two rows and the symmetry of the shoot is isobilateral : in other cases (e.g. Fontin- alis) the leaves are in three rows, and the symmetry of the shoot is radial. Branching is confined to perennial shoots, and is lateral, never dichotomous. When the growth of the main shoot is arrested by the formation of sexual organs at the apex (acrocar- pous), one (or more) of the lateral branches (termed innovations) close behind the apex assumes the cha- racters of the main shoot and carries on the growth : the resulting branch- sj'stem is cymose, either sympodial or dichasial according to the number (one or more) of the in- novations at each branching. When the growth of the main stem is not thus arrested, the sexual organs be- ing borne on lateral branches (pleurocarpous), the branch-system is monopodial and racemose. The branches (except the innovations) frequently differ in various ways from the primary shoot. Thus, in Sphagnum and other pleurocarpous Mosses, the leaves of the branches differ in FIG. 204. — Longitudinal section through the apical region of a stem of Fnntinalis antijtyretica, a Moss growing in water (after Leitgeb) : v the apical cell of the shoot, producing three rows of segments which are at first oblique and afterwards placed transversely (distinguished by a stronger outline). Each segment is first of all divided by the wall a into an inner and an outer cell; the former produces a part of the inner tissue of the stem, the latter the cortex of the stem and a leaf. Leaf-forming shoots ari^e beneath certain leaves, a triangular apical cell (z) being formed from an outer cell of Hie segment, which then, like v, produces three rows of seg- ments ; and each segment here also forms a leaf. 334 PART IV.— CLASSIFICATION. size and shape from those of the primary shoot : in other forms (e.g. Thuidium) the lateral branches have limited growth. The development of the branches, though never axillary, is in- timately connected with the arrangement of the leaves, since the apical cell of a branch is developed from the same segment as the corresponding leaf. Each branch is developed beneath the corres- ponding leaf, either in the median line (e.g. Fontinalis), or on one side of it (e.g. Sphagnum). However, a branch is not developed in connexion with each leaf. In most cases the adult shoot does not present any differentia- tion into a vegetative and a reproductive portion (gametophorej, but such a differentiation is to be found in certain forms. Thus, in Splachnum, the male organs are borne upon a leafless prolongation of the shoot. In Sphagnum the apex of the female shoot grows out, after fer- tilisation of the -archegonium, into a long leafless shoot termed a pseudopodium, which bears the sporogonium (here destitute of a seta) at its apex. In Aulacomnium and Tetraphis there is a some- what similar terminal shoot, likewise termed a pseudopodium, which bears at its apex a cluster of gemmae. The Leaves present considerable variety in size and form : they may be divided, in the first instance, into foliage-leaves and in- volucral leaves. The foliage-leaves are simple and sessile ; they are usually inserted transversely on the stem, and are closely packed. They are generally larger towards the upper than towards the lower part of the shoot. In most pleurocarpous Mosses the leaves of the lateral branches differ more or less from those of the main stem. In some forms (e.g. Bryum roseum, Climacium, etc.), where the branches take the form of creeping runners or stolons, the leaves of these branches are reduced to scales (cataphyllary leaves). The involucral leaves are arranged in one or more whorls, form- ing an involucre round the sexual organs. Those surrounding a group of male organs are commonly larger than the foliage-leaves, and in some cases (e.g. Polytrichacese) are coloured red or yellow. Those surrounding a group of female organs differ but little from the foliage-leaves: the more internal leaves are smaller than the external : the innermost leaves, distinguished as perichcetial leaves, are quite rudimentary when the archegonia are mature, but after fertilisation has taken place they grow up round the base of the seta of the sporogonium. GROUP ii. — BRYOPHYTA: MUSCI. 335 The Sexual Organs are borne in groups (rarely singly) at the apex either of the main shoots (acrocarpous), or of lateral branches (pleurocarpous), surrounded by involucral leaves, each group with its involucre constituting a receptacle. Generally speaking the growth of the shoot or branch ceases with the development of the sexual organs, the apical cell itself giving rise to an antheridium or an archegonium ; but in some male receptacles (e.g. Polytrichacese and some other Bryinese, also Sphagnum) the apical cell persists as such ; consequently the elongation of the shoot or branch is not necessarily arrested by the development of the antheridia, and appears to grow through the receptacle. Among the sexual organs there are usually present multicellular hairs, termed paraphyses: they are often filamentous, but in some cases (e.g. male receptacle of Funaria) the terminal cells are large and rounded ; they are hyaline, or coloured red or yellow, some- times brownish, and the cells frequently contain chloroplastids. They are more numerous and more highly developed in the male than in the female receptacles ; they are rarely absent in plants which grow in dry situations, but frequently in those forms which grow in water or in damp places. Their function seems to be that of sesreting water to prevent the drying-up of the sexual organs. The antheridia are generally club-shaped (spherical in Sphag- num), and are shortly stalked (see Fig. 192). The antheridia generally open at the apex to allow the spermatozoids to escape. The archegonia are stalked ; the neck is long, and the venter is but slightly dilated (Fig. 195). The distribution of the sexual organs is various. The plant may be monoecious (i.e. may bear both kinds of sexual organs), or it may be dioecious : amongst the monoecious forms may be distin- guished those which are monoclinous, that is, which have both male and female organs in the same receptacle (e.g. Bryum la- custre, cuspidatum, etc.), the archegonia being in the middle ; and those which are diclinous, that is, which bear the male and female organs in distinct receptacles : sometimes (e.g. Bryum pendulum, arctic um, etc.) the plants are heteroclinous or polygamous, that is, some receptacles are hermaphrodite, whilst others are unisexual. In the dioecious species the male plant is usually the smaller, and is short-lived. In some species the plants are sometimes monoecious (either monoclinous or diclinous) and sometimes dioecious. The Structure of the Adult Shoot. The stem presents more or 336 PART IV.— CLASSIFICATION. less well-marked histological differentiation. The outer portion consists of an epidermal layer, followed by one or more layers of elongated prosenchymatous cells, with thickened walls which are yellow or brown, forming the cortex which passes by gradual transition into thin-walled parenchymatous ground-tissue ; in the subterranean shoots of the Polytrichacese, however, the cortex is parenchymatous and thin-walled, whilst the ground-tissiie is thick- walled. In species which live under such conditions that both transpira- tion and the absorption of water may be actively carried on, a central strand is differentiated in the longitudinal axis of the stem, the structure of which presents two principal varieties ; it may be simple, consisting of a group of thin-walled tracheides, destitute of protoplasmic cell-contents (e.g. Funaria, etc.) ; or it is compound, consisting of a group of thick-walled tracheides, or of several groups of thin- walled tracheides with intervening paren- chymatous or prosenchymatous cells, surrounded by several layers of thin-walled elongated cells with oblique ends, containing abun- dant protoplasm and starch-grains (e.g. Polytrichum). This cen- tral strand is, in fact, a rudimentary vascular stele : the tracheides, though unlignified, represent the wood or xylem : in the simple form, the phloem is unrepresented; in the compound form it is represented by the elongated cells which surround the xylem. The structure of the leaves shows considerable variety. Most commonly the leaf-blade consists of a single layer of cells, con- taining chloroplastids, with or without a midrib. In the midrib of those forms which have a central strand in their stems, there are one or more rudimentary vascular bundles of a structure corres- ponding to those in the stein. These bundles enter the stem as leaf-traces, and either end blindly, or join the central strand of the stem. The rest of the midrib is made up chiefly of thick-walled prosenchymatous cells. It must not be overlooked that the absorp- tion of water is effected, in the Mosses, mainly by the leaves. The most remarkable deviations from the usual structure of the lamina are those offered by the Sphagnacese and the Polytrich- acese. In the Sphagnacese the constituent cells are of two kinds : large empty cells with perforated walls (see p. 94, Fig. 73 A}, and small cells containing chloroplastids. In the Polytrichacese, the assimilatory tissue is borne on the surface of the broad midrib in the form of numerous longitudinal plates, one cell thick. The rhizoids which spring from the shoot are essentially similar GROUP ii. — BRYOPHYTA: MUSCI, 337 to those of the protonema: in the Polytrichaceae they become wound together into strands. The gametophyte of the Musci possesses a remarkable capacity for vegetative propagation. Thus the perennial protonema of many species serves year by year to produce new adult shoots which, not un frequently, become distinct plants. In the pleurocarpous forms (e.g. Sphagnum, Hypnum) the main axes gradually die away from behind, the lateral branches becoming isolated, and constituting the main axes of new plants. In probably the majority of the Musci almost any portion of the body, a piece of stem or a leaf, will, under proper conditions, grow out into protonemal filaments, which give rise to adult shoots in the usual manner. In certain species, belonging to the Bryinese (Aulacomnium palustre, A. androgynum, Tetraphis pellucida\ multicellular gemmae are pro- duced at the apex of the stem, either free or enclosed in a cupule (Tetraphis). In Aulacomnium palustre the gemmae appear to be modified leaves ; in A. androgynum and in Tetraphis the gemmae are smaller, and consist of but few cells ; in Tetraphis they are borne on long stalks. On being placed under favourable conditions, the cells of the gemma grow out into protonema. B. The SPOROPHYTE. The oospore divides into two by a wall (basal wall) transverse to the long axis of the archegonium : from the epibasal half is developed the capsule (theca] and its long or short stalk (seta\ whilst the hypobasal half gives rise to a more or less well -developed foot : the whole being termed the sporogonium. The segmentation of the oospore into octants (see p. 314) is confined to the epibasal cell, and even this can only be traced in Sphagnum, which in this respect resembles the Liverworts. In the other Mosses, the epibasal cell undergoes one or more trans- verse divisions, after which two oblique walls, cutting each other at an acute angle, are formed in the terminal cell ; the cell marked out by these two oblique walls is a two-sided apical cell by the growth and segmentation of which the further development of the embryo is effected. At an early stage in the differentiation of the capsule (see Fig. 197) the amphithecium, consisting of one or more layers of cells, can be distinguished from the endothecium. The amphithecium constitutes eventually the wall of the capsule ; the internal tissues being formed for the most part from the endothecium. The archesporium becomes differentiated in various positions within the' young capsule. It is differentiated, in the Sphagnaceae, od8 PART IV. — CLASSIFICATION. from the innermost layer of the amphithecium ; in the Bryinese, from the external layer of the endothecium. The cells which bound the archesporium on each side constitute the spore-sac. The endothecial tissue which lies internally to the archesporium con- stitutes the columella. In the Sphagnacese the archesporium is a hollow hemisphere covering the top of the columella like a cap ; in the Bryinese the archesporium is a hollow cylinder surrounding the columella which extends to the apex of the capsule. In the Bryinese a large intercellular space is developed in the amphi- thecium, between its outer and its two inner layers ; in most Polytrichacese a similar intercellular space is developed in the endothecium internally to the spore-sac, between it and the central portion of the columella. At maturity the internal cells of the capsule become dry and disorganised, so that it simply contains the spores which now lie loose in its cavity. It dehisces by the throwing off of its apical portion as a lid or operculum in Sphaguacese and the higher Bryinese (Stegocarpse) ; or it ruptures irregularly or simply decays, as in the lower Bryinese (Cleistocarpse). In the higher Bryinese the mouth of the dehisced capsule bears a fringe, the peristome, the development and structure of which will", be described sub- sequently (p. 344). The basal portion of the capsule, where it joins the seta, is termed the neck. In the Polytrichacese the neck is considerably dilated, as also in various species of Splachnum ; whilst in other species of Splachnum it grows out into an umbrella-shaped struc- ture. When the neck is thus markedly developed it is termed the apophysis. The histological differentiation of the sporogonium is well- marked. There is a well-defined epidermis, in which, on the capsule, stomata of various forms are generally present ; either scattered all over, as in the Sphagnacese ; or confined to a par- ticular region of the capsule, generally the neck or the apophysis, in the Bryinese. The operculum and the peristome (Bryinese) show considerable complexity of structure. The structure of the seta in the higher Bryinese, where alone it is elongated, very much resembles that of the stem : in many forms, even in such in the stem of which no central strand is present, there is a central strand in the seta, marked off from the ground-tissue by one or two layers of sheath-cells. In the Bryinese also, the struc- ture of the neck (or apophysis) is generally characterised by the GROUP II. — BRYOPHYTA : MUSCI. 335) presence of loose assimilatory tissue, rich in chloroplastids, the intercellular spaces of which communicate with the outer air by means of the stomata. The hypobasal cell undergoes relatively few divisions. In the Sphaguacese it gives rise to a bulbous foot. In the Bryinese (e.g. Qrthotrichum, Barbula, Atrichum where the hypobasal cell under- goes a single division by a transverse wall) the true foot is rudimentary, but it is functionally replaced (e.g. Phascum, Ephe- merum, Polytrichum) by the dilated lower end of the seta which constitutes a false foot. After fertilisation, the venter of the archegonium developes into the calyptra which, for a time, keeps pace with the growth of the contained embryo, but is eventually ruptured by the gradual elongation of the seta. In Sphagnacese, and in some of the lower Bryinese, the whole of the ruptured calyptra remains as a sheath, the vaginula, round the base of the short seta ; in the higher Bryinese the lower portion remains as the vaginula, whilst the upper portion is raised up like a cap (still called calyptra) on the top of the elongating sporogonium. The floor of the receptacle (i.e. the apex of the sexual shoot) is also stimulated to growth, forming in most cases a conical projection on which are borne the paraphyses and the unfertilised archegonia, whilst in Sphagnacese it elongates into the long pseudopodium (see p. 312). The perichse- tial leaves also grow up round the lower part of the seta or of the pseudopodium. The sporogonium, possessing, as it usually does, assimilatory tissue and stomata, can assimilate the carbon dioxide of the air, and can transpire actively. The supply of water necessary to meet the loss by transpiration is obtained, together with salts in solution, from the gainetophyte, being absorbed from it by the true (hypobasal) or the false (epibasal) foot, and travels to the capsule through the rudimentary xylem-tissue of the central strand present in the seta of the higher forms. It is a point of consider- able physiological interest that the absorption of water in the first instance by the gainetophyte is apparently effected for the most part by the leaves rather than by the rhizoids. The remarkable capacity for vegetative propagation manifested by the gametophyte is shared by the sporophyte. It has been ascertained that if portions of the capsule or of the seta, whilst the cells are still living, be kept under favourable conditions, the superficial cells will grow out into protonemal filaments. In 340 PART IV. — CLASSIFICATION. this way the gametophyte may be derived from the sporophyte 'by budding, without the intervention of spores. The principal orders of Mosses are the Sphagnacese and the "Bryinese. Onler I. Sphagnaceae (Bog-Mosses). A. The GAMKTOPHYTE. The spore gives rise on germination to a fila- mentous protonema ; when germination takes place in water, the proto- nema remains filamentous and branches, but when it takes place on a solid substratum the protonema assumes the form of a branched cellular expansion attached to the substratum by root-like protonemal filaments. In either case adult shoots are developed as branches upon the protonema. The Morphology of the Adult Shoot. The shoot is radially symmetrical, and is differentiated into stem and leaves. It consists of a main axis bearing numerous lateral branches. Growth is effected, in both the main axis and the lateral branches, by means of a growing-point in which there is a three-sided apical cell. The Sexual Organs are borne on specially modified lateral branches (gametophores), the antheridia and archegonia being borne on distinct branches, and in some species on distinct shoots. The branch which bears antheridia (antheridiophore) is elongated and is covered with small, closely packed, imbricate leaves, by the side of each of which an antheridium is developed. The antheridium, which is raised upon a long stalk, is spherical ; it opens by the splitting of the wall into valves from the apex downwards. The branch which bears archegonia (archegoniophore) is short ; it bears at its apex a group of (1-5) archegonia, surrounded by rather large involucral leaves with rudimentary perichsetial leaves. The Structure of the Adult Shoot. The main axis has no central strand ; it consists of a mass of elongated thin-walled parenchymatous cells, which gradually passes over into an external zone of prosenchymatous cells, the walls of which are thick and brown 5 externally to this is a cortex, consisting of 1-5 layers of cells which are usually empty, and have large holes in their walls (except the Sphagna cuspidata) ; in some cases (Sphagna cymbifolia) the cortical cells have spiral thickenings. The leaves vary in form according to their position ; thus stem-leaves, branch-leaves, small scaly leaves, and involucral leaves may be dis- tinguished. They are sessile, and have a broad insertion ; in most cases the leaf is connected with the prosenchymatous tissue of the stem, the leaf-tissue extending through the cortex. The stem-leaves have, at the base, a pair of lateral appendages, the auriculae. The leaf consists of a single layer of cells, and has no midrib. It is made up of two kinds of cells : large empty cells of various forms with perforated walls frequently with spiral or some similar form of thicken- ing (Fig. 73, C}: small cells, arranged between the preceding, containing protoplasm and chloroplastids. The relative arrangement of these two kinds of cells affords a means of classification. Nostoc and other Algae are frequently found in the large empty cells. GROUP ii. — BRYOPHYTA: MUSCI. 341 The Sphagnacese have no special organs for vegetative propagation; but they multiply vegetatively by the dying away of the main stems so that the lateral branches become separate and constitute distinct plants ; consequently these plants are found in considerable masses. They in- habit damp, boggy spots, and retain a considerable quantity of water in the open cells of the branches and leaves. Masses of Sphagnum thus saturated with water form peat-mosses or peat-bogs, the water being raised to the surface by means of the capillary system formed by the open cells. B. The SPOROPHYTE. The oospore, as in other Mosses, is divided by the transverse basal wall into an epibasal and hypobasal half. The epibasal half gives rise to the capsule : it grows at first apically, segments being formed by transverse walls, each segment being divided into four cells (quadrants) by walls at right angles to each other: after six or eight FIG. 20(5. — Longitudinal section (diagram- matic : x 19) of the sporogonium of Spbag- num : p* pseudopodium ; / foot ; c calyptra FIG. 205.— Part of shoot of Sphagnum with neck of archegonium h ; a* arche- •icutiwl iiun (nat. size). /: Capsules. Bporium. segments have been formed, apical growth ceases, the further growth being intercalary. The cells of each segment become differentiated each into an external and an internal cell; the external cells constitute the amphithecium, the internal cells the endothecium. The amphithecium comes to consist of several layers by periclinal divisions, the first formed and most internal layer constituting, in its upper half, the archesporium ; the endothecium constitutes the columella. Thus the archesporium is a hollow hemisphere which covers the columella as a cap. There is no intercellular cavity formed in the capsule. The hypobasal half of the oospore undergoes but few divisions, forming a bulbous foot, the superficial cells of which grow out into short) papillse. The fully developed sporogonium consists of a capsule attached to the foot by a very short seta ; the wall of the capsule consists of a single 342 PART IV.— CLASSIFICATION. layer of cells, and has numerous stomata. The capsule opens by the throwing off of the apical portion of the wall as an operculum. There is no peristome. When the cal3*ptra is ruptured, it remains as a vaginula round the base of the capsule. No part of it is carried upon the top of the capsule. The growth of the archegoniophore is stimulated by fertilisation. It grows (Fig. 206 j?i) out at its apex into a long leafless stalk, the pseudo- podium, expanded at the top into a cushion of cells in which the foot of the sporogonium is embedded ; the perichsetial leaves grow and surround the base of the stalk. The order consists of the single genus Sphagnum, of which there are very many species. Order II. Bryineae. A. The G ^METOPHYTE. The protonema is filamentous, though in some cases (e.y. Tetraplris pellucida) it developes flattened cellular appendages which are assimilatory organs. The subaerial portion of the protonema is generally short lived, though in some cases it persists (e.g. Ephemerum) at least until the sporogonium has been developed and the spores are ripe : the subterranean portion frequently persists from year to year. The subaerial portion gives rise to the gametophores as lateral buds ; in some forms the subterranean portion produces lateral buds in the form of bulbils (p. 332) which, when* they are brought to the surface, give rise to gametophores either directly or indirectly with the intervention of protonema. It is commonly the case that, when protonema is kept dry, some of the cells grow larger and their walls thicker, whilst the other cells perish ; the persistent cells, when moistened, develope into filaments. The Adult Shoot does not present, with regard either to its morphologj- or its histology, any especially characteristic features ; it varies in size from a mere bud in such forms as Phascum and Ephemerum, where it is annual, to a shoot several inches long in such forms as Fontinalis and Polytrichum where it is perennial. In the latter case there is general 1}- a central strand, and frequently leaf-traces, in the stem. It may be either acrocarpous or pleurocarpous, a feature which is important in the classification of the group. The leaves have commonly a midrib : the lamina generally consists (e.g.. Funaria, etc.) of a single layer of cells. The leaves of Leucobryum resemble those of Sphagnum in that they consist of two kinds of cells, an internal layer of small living cells with chloroplastids, and external layers of dead cells with perforated walls ; the peculiar structure of the leaves of Polytrichum has been alread}- described (p. 336). B. The SPOROPHYTE presents features, both as to its morphology and histology, which are characteristic of the group. It is differentiated into a true hypobasal foot, a seta, and a capsule. The true foot is rudi- mentary. The seta is relatively short in the lower forms; a false (epibasal) foot is frequently developed from the lower portion of the seta. The neck of the capsule has nearly always stomata in its epidermis, and is developed into a distinct apophysis in some forms ( e.g. Splachnum, Polytrichum). Part of the external layer of the endothecium becomes GROUP II. — BRYOPHYTA : MUSCI. 343 the archesporium, which forms a hollow cylinder round the columella, but does not extend over the top of it : an air-chamber is developed in the amphithecium round the spore-sac, and is generally traversed by strands of cells (containing chloroplastids) stretching from the wall of the capsule to the spore-sac. In the lower forms the capsule is either indehiscent, its wall becoming eventually disorganised, or it ruptures irregularly ; in the higher forms, the apical portion of the wall is thrown off as an oper- culum, and a peristome is generally developed round the aperture thus formed. In all cases a portion of the calyptra is carried up as a cap on the top of the developing sporogonium. The Bryineae are classified as follows : Tribe I. Cleistocarpee. Tribe II. Stegocarpae. Tribe I. CLEISTOCARP^E. The adult shoots are generally minute, un- branched, annual, and always acrocarpous; there is generally a central strand in the stem, and a mid-rib in the leaf. With regard to the sporogonium, the seta is generally short, sometimes expanded at the base into a false (epibasal) foot (e.g. Phascum, Ephe- merum), without any central strand in some forms. The capsule does not open by means of an operculum, nor has it any peristome : it either ruptures irregularly, or the wall simply decays. Tribe II. STEGOCARPAE. The character- istics of this tribe are to be found in the sporogonium, which is distinguished by the formation of an operculum and, generally, of a peristome. FIG, 207.— a Ephemerum, sernituui The operculum is developed from the (x 3) . b 8hoot of AndreaM niraii,f apical portion of the capsule, either from with (K) capsule (nat. size). the epidermis alone (e.g. Georgiaceae), or from it and one or more of the subjacent amphithecial layers. The cell- walls become cuticularised and assume a yellow or brown colour. The outline of the operculum is circular; its form cap-like, more or less flattened in some cases, more or less conical in others, sometimes apiculate. The limit between the developing operculum and the rest of the capsule (urn) is generally marked by a slightly prominent zone, consisting of one or more rows of rather large epidermal cells, with cuticularised outer walls, termed the ring or annulus : its position is just above the level of the top of the spore-sac and of the air-chamber. The peristome is developed within the operculum, from the innermost layer or layers of the amphithecial cells; the entire walls, or only portions of the longitudinal and transverse walls, of larger or smaller plates of these cells, become thickened, cuticularised, and coloured yellow or brown. The unthickened cell-walls, or the unthickened portions of them, break a way as the capsule becomes ripe, leaving only the thickened portions forming, as it were, a skeleton attached to the urn just below the level of the annulus. The following are the principal varieties of peristome- formation. In the Georgiaceae (e.g. Tetraphis) the peristome is developed from the two layers of cells beneath the epidermis which forms the 344 PART IV. — CLASSIFICATION. operculum : the walls of these cells all become thickened ; when the oper- culum falls off this plate of tissue splits from the centre into four equal segments which are the teeth of the peristome. In most Mosses the peri- stome is formed from a single layer of cells, and consists of two rows of teeth, an inner and an outer. This double peristome is dependent upon the dis- tribution of the cuticul arisation of the walls : both the internal and the ex- ternal walls of the peristomial cell- layer become cuticularised, but the lateral and part of the transverse walls joining them remain unaltered and eventually break away, leaving the thickened internal and external walls as separate strips, which may be further divided longitudinally into teeth. The teeth of the outer peri- stome are generally larger than those of the inner which are sometimes dis- tinguished as cilia : their number is a power of two (4-8-16-32-64.) There is considerable variety in the structure and form of the peristome ; this affords a means of classification. The genus Polytrichum, for instance, is charac- terised by the fact that the teeth of the peristome consist of strands of thick-walled fibres, the tips of which are not free, as is usually the case, but are connected by a membrane stretched over the aperture of the urn, termed the epiphragm. A peristome is not present in several genera (e.g. Gymnostomum , H3rmenostomum, Schistostega, etc.); nor in some species (e.g. species of Pottia and Encalypta, etc.) belonging to genera in which a peristome is usually present. As the capsule matures, the cells(except the spores) lose their cell-contents ; and those whose walls have not become thickened and cuticularised, dry up and shrink, the shrinkage being necessarily accompanied by the tearing of the thin walls in various parts. The persistent cuticularised walls are highly hygro- FiG. 209. -Mouth of thethecaof { d k j j consequence of the Fontmalis antipyretic.', ap Outer peristome ; ip inner peristome ( x tensions set up by the unequal stretching 60.J and contraction of these walls, due to FIG. 208. — Funaria hygrometrica. A An adult shoot (g), bearing a calyptra (c). B A plant (g) bearing a nearly ripe sporogonium ; s its seta ; / the capsule ; c the calyptra. C Median longitudinal section of the capsule : d operculum ; a annulus ; p peristome; cc'columella; h air-cavity ; s the arche- sporium. GROUP II. — BRYOPHYTA : MUSCI. 345 X J- Fm. 210.— Sporogoninm of Funaria hyyromelrica. A, s seta; b capsule; e calyptra (x6). 15 Section of a half-developed capsule (x 10): c columella ; « archesporium ; I air-cavity; d sub-opercular tissue ; p peristome. C Apical portion of the same capsule ( x 40) j d oper- culum; id sub-opercular tissue; ap outer peristome; tp inner perisiome ; r ring; I air- fpace; c coliimel'a ; s spore-sac. variations in their moisture, that the splitting off of the operculum is effected. The Stegocarpse are classified as follows : — Sub-tribe AciWCARPM : archegonia terminal on the main shoots ; but ths sporogonia are some- times apparently lateral in consequence of the growth of lateral branches (innovations) which force the apex: of the main shoot to the side. Sub-Tribe PLEUROCARP& .- archegonia (and subsequently the sporogonia) borne terminally on short lateral branches. The following are among the more familiar species of acrocarpous Mosses : — Dicranuni scoparium, with sickle-shaped leaves, is common in woods. Leucobryum glaucum has leaves consisting of several layers of cells, which. resemble those of Sphagnum in their structure ; it occurs in Pine-woods and on moors. Cerato- don purpureus, with a red seta and a short stem, is very common in various localities. Barbula muralis grows in patches on walls and rocks; the midrib of the leaves is prolonged into a hair, so that the patches of Moss look greyish. Tetraphis pellucida has bright green leaves ; it u;rows on decayed tree-trunks, and bears gemmae of peculiar form. Grimmia pulvinata occurs on walls and stones in round greyish-green patches 5 the capsules have very short setae. Orthotrichum affine and other species have also shortly-stalked capsules, and are common on tree, Funaria ^< hyyrometrica (Figs. 208, 210) has an oblique, jDear- c calyptra. FIG. 211.— Two plants of P'tyMch™ /onnowtm bear- 346 PART IV. — CLASSIFICATION. shaped capsule*, the long setae have the peculiarity of contracting into a spiral on being wetted and dried ; it is common on walls and paths. Pol ijtricJium formosum (Fig. 211) and other species are the largest of our indigenous acrocarpous Mosses ; they have large dark green leaves and long hairy calyptrse an'd are common in woods and on heaths. The following are among the more familiar species of pleurocarpous Mosses : — Fontinalis antipyretica floats in water. Neckera crispa, with flat out- spreading leaves, grows on rocks. Tliuidium ahutimim and other species grow on banks and in woods; they have regular, piniiately-branched stems, and very small, closely-set leaves. Leucodon sciuroides is common on tree-trunks. Bracliytliecium rutabulum is common in woods. Eurhyncliium prcelonyum, with long creeping stems, occurs in woods and damp gardens. Hypnum cupressiforme is very common on tree-trunks, and H. cuspidatum and giganteum in bogs and ditches. Hylocomium triquetrum is very commonly used for garlands; this and H. splendens, with remarkably regular ramification, are both common in woods. GROUP III. PTEEIDOPHYTA (Vascular Cryptogams). The distinguishing characteristics of the plants forming this group are the following : — The life-history presents a well-marked alternation of generations, as in the Bryophyta ; but here it is the sporophyte which is the more conspicuous form, constituting " the plant." The sporophyte becomes quite distinct from the gameto- phyte at an early period : it is differentiated (with but few* exceptions) into root, stem, and leaf ; and in all cases it contains well-developed vascular tissue. The gametophyte, generally termed the protliallium, is a relatively small thalloid body, usually short-lived, containing no trace of vascular tissue. The group includes the three classes, Filicinse, Equisetinre, Lycopodinse. The SPOROPHYTE is developed from the oospore, which undergoes division, in all cases, into an epibasal and a hypobasal half, by a basal wall which is either more or less nearly parallel to the long axis of the archegonium (Leptosporangiate Filiciuse) or more or less nearly transverse to it : the epibasal half usually faces the neck of the archegonium, but in the Lycopodinse the hypobasal half occupies this position. In the Filicinae and Equisetinse, the formation of the basal wall is followed by the formation of another wall at right angles to it (qiiadrant-walT) so that the embryo now consists GROUP III.— PTERIDOPHYTA. 347 of four cells which are quadrants of a sphere, and this is followed by the formation of a third wall (octant-icall), at right angles to both the preceding, so that the embryo now consists of eight cells which are octants of a sphere. In the Lycopodinse the segmentation leading to the formation of quadrants and octants is confined to the epibasal half, the hypobasal half remaining un- divided or undergoing a few irregular divisions. From the epibasal half, the primary stem and one or two primary leaves (cotyledons) are developed in all cases. The hypobasal half gives rise, in the Filicinse and Equisetinse, to the primary root and to the foot, with but few exceptions (e.g. Salvinia in which there is no primary root) : in the Lycopodinse the hypobasal half gives rise to a filament consisting of a few cells, termed the suspensor (compare Phanerogams). The foot (as also the suspensor) is an embryonic organ, no trace of which persists in the adult. It is the organ of attachment of the embryo-sporophyte to the gametophyte ; and it is also the absorbent organ by which the embryo, until it is able to absorb and assimilate food for itself, obtains its nourishment from the pro- thallium (compare Bryophyta, p. 314). The development of a suspensor in the Lycopodinse is an adap- tation correlated with the fact that the nourishment of the embryo in that group depends upon its coming into direct contact with the tissue of the massive gametophyte, the cells of which are filled with nutritive substances. A primary root, that is, a root developed from the hypobasal half of the oospore, and so situated at its origin that its growing- point is in a straight line with that of the stem, only occurs in the Filicinse and Equisetinse ; but even here it does not persist as a tap-root in the adult : in these plants numerous adventitious roots are developed. In the Lycopodinse, where there is no primary root, all the roots are adventitious. Some adult forms are altogether without roots : as Salvinia, and some species of Trichomanes, among Filicinse; Psilotum and Tmesipteris, among Lycopodinse. The functions of the root are discharged, in Salvinia by modified leaves, in the others by modified branches. The branching of the root is generally lateral in the Filicinse and Equisetinse; it is dichotomous in the Lycopodinse and in Isoetes. In the former case, the lateral rootlets are developed, in the Filicinse, from cells (rlnzogenic} of the endodermis which 348 PART IV. — CLASSIFICATION. are opposite to the xylem-bundles of the stele ; in the Equi- setinse, from the cells forming the inner layer of the two-layered endodermis. The stem is generally short and un branched in the Filicinse ; generally elongated and much branched in the Equisetinse and Lycopodinse. The leaves are differentiated into foliage-leaves and sporophylls in the Equisetinse and generally in the Lycopodinse, but not in the Filicinse as a rule. The foliage-leaves are relatively large in pro- portion to the stem in the Filicinse, relatively small in the Lyco- podinse, reduced to cataphylls in the Equisetinse. The growth in length of root, stem, and leaf, is effected by an apical growing-point : the growing-point has generally a single apical cell in the Filicinse (except root and stem of Marattiacese and Isoetes) and Equisetinse ; in the Lycopodinse (as also in the exceptional Filicinse) there is generally a group of initial cells. The anatomy of the stem presents considerable variety. The primary stem is in all cases monostelic (pp. 102, 116) : it may con- tinue to be monostelic (e.g. Lycopodiacese, Isoetes, Osniundacese, etc.), but more commonly it becomes polystelic (most Filicinse). The vascular tissue of the wood consists of lignified spiral (protoxylem) and scalariform tracheides, or- less commonly vessels ; the bast contains no companion-cells. The bundles are generally closed and cauline. The relative arrangement of wood and bast in the stele is generally concentric (see p. 124) in the Filicinse and Selaginellacese, and radial in the Lycopodiacese : or the bundles may be conjoint and collateral as in the Equisetinse and some Filicinse. The reproductive organs are sporangia, generally borne on the leaves (sporophytte) but sometimes directly on the stem (e.g. Selaginella). Each sporophyll may bear many sporangia on its inferior (dorsal) surface, as generally in the Filicinse and Equise- tinse ; or a single sporangium on its upper surface (e.g. Lycopodium, Isoetes), or in its axil (Selaginella). When the sporophyll bears many sporangia, they are usually arranged in groups ; each group is termed a sorus, and the more or less well-developed cushion of tissue from which the sporangia spring is termed the placenta. The sorus may be naked ; or it may have a membranous covering, the indusium (e.g. many Filicinse). In the Filicinse the sporophylls are not confined to any special GROUP III. — PTERIDOPHYTA. 349 portion of the shoot, so as to constitute a flower : but in some cases (e.g. Osmunda, Ophioglossacese, Marsileaceae) they differ in form and structure from the foliage-leaves. .In the Equisetinse the sporophylls are highly specialised, and are grouped into cones (flowers) at the ends of the fertile branches : similar cone-like flowers, with less specialised sporophylls, occur in various Lycopodinse. The sporangia are unilocular, though in Isoetes they are incom- pletely chambered by trabeculse : they are developed singly or in groups (sori) ; in the latter case they are usually distinct, but in some cases they are coherent (Marattiacese, except Angiopteris ; Psilotacese) forming a synangium (see p. 52) : the synangium should not, however, be regarded as the result of the cohesion of originally distinct sporangia, but as a group of sporangia which have not separated. The sporangium is developed either from a single superficial cell (leptosporangiate) ; or from a group of super- ficial cells (cusporangiate), and sometimes from deeper cells as well : the mother-cells of the spores are derived from an arche- sporium which is either a single hypodermal cell or a group of hypodermal cells. The spores produced in the sporangia, are single cells, with generally two coats, endospore and exospore. Many of the Pterido- phyta produce spores which are all quite alike, whence they are said to be homosporous ; whereas others produce spores of two kinds, small spores (micros pores) and large spores (macrospores or mcgaspores), and are said to be heterosporous. The sporangia of the heterosporous forms are distinguished as microsporangia and macrosporangia, according to the kind of spores which they develope : and when the sporophylls bear either only microsporangia or only macrosporangia they are distinguished as microsporophylls and macrosporophylls. The number of macro- spores produced in the macrosporangium is generally small, though they are numerous in Isoetes : thus there are four in Selaginella, only one in the Hydropterideae. The spores are generally set free by the dehiscence of the sporangia : but in Salvinia the whole sporangium falls off and the spores germinate within it. B. THE G-AMETOPHYTE. The spore, on germination, gives rise to a prothallium which is the gametophyte. It is very small and inconspicuous, as compared with the sporophyte ; its body is, generally speaking, thalloid ; there is no vascular tissue in its 350 PART IV.— CLASSIFICATION. structure, and in many cases it does not become free from the spore. It usually lives through but one short period of growth. In any one of the homosporous forms, the prothallia developed from the spores are all essentially alike ; generally speaking, any one prothallium bears both male and female reproductive organs. The morphology of the prothallium varies widely in these forms : it may be a branched cellular filament (some Hymenophyllacese), or a flattened expansion (Equisetinse, most Ferns), containing chlorophyll abundantly ; or it is tuberous (Ophioglossacete, Lyco- podiacese), either wholly or in part destitute of chlorophyll. It becomes entirely free from the spore. In the heterosporous forms the gametophyte is represented by two individuals — a male and a female prothallium ; the former is the product of the germination of a microspore, the latter of the germination of a macrospore. As compared with those of the homosporous forms, the prothallia of the heterosporous forms are relatively small ; moreover they do not become independent of the spores from which they are developed. The male prothallium is reduced to little more than a single male organ (antheridium) ; the female prothallium is a small, usually green, cellular body pro- jecting more (e.g. Salvinia) or less (e.g. Selaginella) through the ruptured outer coat of the macrospore. Generally speaking, the symmetry of the prothallium is dorsi- ventral ; in the free-growing forms, the under surface generally bears numerous unicellular root-hairs. The distribution of the sexual organs on the prothallium varies ; they are frequently confined to one surface, but are occasionally scattered over the whole surface. The number of the sexual organs on a pro- thallium is in some cases only one, in others it is consider- able. The sexual organs are antheridia (male) and archegoma (female). The structure of the antheridium is simple ; it consists of a wall, a single layer of cells, enclosing the mother-cells of the spermato- zoids. The antheridia are developed from single superficial cells of the prothallium ; when the prothallium is thin, the antheridia project on the surface ; when the prothallium is tuberous, the antheridia become sunk in the tissue. The archegonium consists of a venter and a neck. As the venter is, in all cases, sunk in the tissue of the prothallium, it has no proper wall of its own, and is, in fact, simply a cavity in the tissue ; the short neck consists of a single layer of cells GROUP III. — PTERIDOPHYTA. 351 in four rows. The mature archegonium contains, in the venter, the female cell (oosphere}. The archegonium is developed from a single superficial cell of the pro thallium. This cell divides transversely into two, an upper and a lower ; the former, by growth and division, forms the neck of the archegonium ; the lower cell projects into the developing neck, and the projecting portion becomes cut off, constituting the neck-canal-cdl which sometimes divides again into two (Maratti- acese, Lycopodium) ; the remainder, now termed the central cell of the archegonium, divides transversely into two unequal parts, the upper and smaller being the ventral canal-cell, the lower and larger being the oosphere. As the archegonium becomes mature, the canal-cells become mucilaginous, the neck opens by the separation of the cells at the apex, and the archegonium is ready for fertilisation. The male cell is a naked motile cell, a spermatozoid ; it is a spirally coiled filament, pointed at the anterior end which bears the cilia, becoming thicker towards the opposite end : the cilia are numerous in Filicinse and Equisetinse ; two in Lycopodinse. Each spermatozoid is developed singly in a mother-cell in the antheridium. The whole of the contents of the mother-cell are not, however, devoted to the spermatozoid : a portion remains unused, and is discharged together with the spermatozoM, to which it adheres for a time as a protoplasmic vesicle containing, amongst other constituents, a portion of the nuclear substance of the mother-cell (see Fig. 222). The female cell, or oosphere, is a naked spherical cell lying in the venter of the archegonium. Its development is described above. Fertilisation is effected by the entrance of spermatozoids into the open neck of the mature archegonium, and the subsequent fusion of one of them with the oosphere. When, as is usually the case, numerous prothallia are developed near together on the ground, and become wetted by rain or dew, the ripe antheridia burst and set free the spermatozoids which, swimming actively in the water, are attracted to the mature archegonia by means of an acid excretion which is discharged from the neck of the arche- gortium when it opens. The effect of fertilisation on the oosphere is that it at once surrounds itself with a cell-wall becoming the oospore, and then begins to develope into the young sporophyte. In a few cases (e.g. species of Trichomanes and Lycopodium) the 352 PART IV. — CLASSIFICATION. gametophyte (prothallium) multiplies vegetatively by means of gemmae, which are short spindle-shaped rows of cells in the one case, and globular multicellular bodies in the other. The Life-History of the Pteridophyta presents in all cases, a perfectly clear alternation of generations, the sporophyte and the gametophyte being completely distinct. The oospore developes into "the plant," be it Fern, Equisetum, or Lycopod, which bears the sporangia and spores, and is the sporophyte. The spores, when shed, germinate to form the gametophytes (prothallia) bear- ing the sexual organs. The Pteridophyta are classified as follows : — Class V. FILICINJ3. The sporophyte is characterised by having relatively large and few leaves ; the sporophylls are gener- ally similar to the foliage-leaves and are not aggregated into flowers ; the sporangia are numerous on the sporophyll (except Isoetes) and are arranged in sori ; the archesporium is a single cell (except Isoetes); the embryo has a primary root (except Isoetes, Salvinia, and possibly some species of Trichomanes) but no suspensor. The characters of the gametophyte vary widely. The sper- matozoids are multiciliate. Sub-Class Eusporangiatae. Each sporangium is developed from a group of superficial cells. HOMOSPORE.E. Order 1. Opliioglossacece. Order 2. Marattiacea?. HETEROSPORE.E : Order 1. Isoetacese. Sub-Class Leptosporangiatae. Each sporangium is developed from a single superficial cell. (Filices in limited sense.) HOMOSPORE^. Order 1. Osmundacece. Order 4. PolypodiacecK. „ 2. Schizceacece. ,, 5. Cyatheacece. ,, 3. Glcicheniacece. ., 6. HymenophyllacecK. HETEROSPORE^E. Order 1. Snlviniacecr,. Order 2. Marsileaccai. GROUP III.— PTERIDOPHYTA. 353 Class VI. EQUISETIN.E. The sporophyte is characterised by the well-developed branched stem, with small whorled leaves forming a sheath at each node ; the small peltate sporophylls are aggregated into a cone-like flower at the apex of each fertile shoot, and bear a few sporangia on the inner (inferior) surface ; the archesporium is a single cell ; the embryo has a primary root and no suspensor. All the existing forms are homosporous and eusporangiate. The gametophyte is a free, green, membranous prothallium, generally dioeoious ; the spermatozoids are multiciliate. Order 1. Equisetacece. Class VII. LYCOPODIN^E. The sporophyte is characterised by the well-developed branched stem with numerous small scattered leaves ; the sporangia are borne singly either on the upper surface of a sporophyll, or on the stem ; the sporophylls resemble the foliage-leaves, but are sometimes aggregated into cone-like flowers; the archesporium is multicellular ; the embryo has a suspensor, but no primary root. All the existing forms are eusporangiate. The characters of the gametophyte vary widely. The sperma- tozoids are biciliate. Sub-Class HOMOSPOREJE : the sporophyte produces spores of one kind only ; the prothallia are free, more or less tuberous, mon- oecious. Order 1. Lycopodiacece. Order 2. Psilotacea. Sub-Class HETEROSPORE^: : the sporophyte produces microspores and macrospores ; the former give rise to male, the latter to female, prothallia ; the prothallium does not become free from the spore. Order 1. Selayinellacece. The relations of these various groups may be simply expressed as follows : — FILICIN^E. EQUISETIN.E. LYCOPODIN.S:. Homosporous — Filices — \Lepto- Heterosjiorous — Hydropterideae - ' tporangiate. Homotporous fOphioglossacese J_ E isetacese _ fLycopodiace* j } iMarattiacese ) IPsilotacese ' K — Eusporangiate Heterosporous— -Isoetacese —(none existing)— Selaginellacese ) M.B. A A 354 PART IV. — CLASSIFICATION. CLASS V.— A. HOMOSPOROUS EUSPORANGIATJE. Order 1. Ophio'glossaceae. This order includes the three genera Ophioglo'ssum, Botrychium, and Helminthostachys. SPOROPHYTE. The stem is a subterranean rhizome (except in epiphytic Ophioglossums), which does not branch at all in Ophioglossum, and but little in Botrychium and Helminthostachys; it is usually short and erect. The rather thick and fleshy roots are unbranched in Ophio- glossum, but they give rise to adventi- tious buds 5 they are branched in Bo- trychium and Helminthostachys, and produce no buds. The leaves are de- veloped close together at the apex of the rhizome, and are not circinate, or only slightly so, in vernation ; their growth is so slow that a leaf does not appear above ground until the fifth year after its first development ; generally, only a single leaf appears a'bove ground each year, when more are developed some of them are sterile. The sporophylls are remark- able for their peculiar branching (see p. 34) ; they are petiolate, and the petiole branches into two, the one bearing a sterile and the other a fertile lamina (Fig. 212), the fertile branch being situated on the ventral surface of the sterile ; the sterile lamina is leafy, whilst the fertile lamina consists of little more than the sporangia. In Ophioglossum the sterile lamina is entire, and the fertile lamina is spicate with two lateral rows of sporan- gia ; in Botrychium the sterile lamina is pinnate, and the fertile lamina is bi- pinnate with marginal sporangia. The sporangia are embedded in the tissue of the sporophyll in Ophioglossum, but are free in Botrychium and Helminthosta- chys : they are not arranged in sori ; they are globose, have no annulus, but dehisce into two equal valves by a transverse (Ophioglossum, Botrychium) or vertical (Helminthostachys) slit; the wall of the sporangium consists of several laj'ers of cells ; the spores are numerous and tetra- hedral. FIG. 212. — Botrychium Lv.na.ria (nat. size) : w roots ; st stem ;bs leaf-stalk ; x point where the leaf branches ; the sterile lamina (6) separating from the fertile branch GROUP III. — PTERIDOPHYTA : FILICIN^E. 355 GAMETOPHYTE. The germination of the spores has not been observed, but the mature prothallium has been described in the case of Ophioglosmm pedunculosum and Botrychium Lunaria. In both cases it is tuberous, sub- terranean, destitute of chlorophyll, monoecious ; the antheridia are sunk in the tissue, and the short necks of the archegonia project but little. It appears that the prothallium is saprophytic, though possibly it may possess chlorophyll in the early stages of its development. In Botrychium it is a somewhat ovoid body not more than half a line long, with long scattered root-hairs, bearing the antheridia chiefly on its upper surface, the archegonia chiefly on the lower. Ophioglossum vulgatum (the Adder's tongue) is the British species of this genus; O. lusitanicum has, however, been found in Guernsey. The epiphytic species are O. pendulum and 0. palmatum, both tropical forms ; the latter has palmately-lobed sterile fronds. Botrychium is represented in the British Flora by B. Lunaria (the Moon-wort) which occurs in hilly districts. Helminthostachys includes the single, species H. zeylanica which occurs in the Eastern tropics.- Order 2. Marattiaceae. This order includes the genera Marattia, Angiopteris, Kaulfussia, and Danaea, none of which are European, but are mainly tropical. SPOROPHYTE. In its general morphology the sporophyte agrees with that of the Ophioglossaceae ; but the leaves are more numerous, much larger, compound, and circinate in vernation, and each bears a pair of stipules. Branching of the stem occurs only in Danaea; in Kaulfussia the stem is a subterranean, creeping, dorsiventral rhizome. The roots are somewhat fleshy, and are much branched. The apical growing-point of both root and stem consists of a group of a few (four or more) initial cells. The sporophylls are not differentiated into a sterile and a fertile portion, but have the appearance of foliage-leaves. The numerous sporangia are borne in sori on the ribs of the under surface of the sporophyll ; in Angiopteris the sporangia of a sorus are free, whilst in all the other genera they are coherent, forming a synangium (see p. 349). The spores are numerous, and are either tetrahedral or radial. GAMETOPHYTE. On germination the spore gives rise to a dorsiveutral green prothallium resembling that of the Leptosporangiate Ferns. B. HETEEOSPOROUS EUSPORANGIATJE. Order 3. Isoetaceae. This order includes the single genus Isoetes which comprises about fifty species belonging to all parts of the globe. Some of these are terrestrial (/. Duricei and Hystrix}, whilst others are either altogether aquatic (e.g. I. lacustris, eckinospora, etc.), or amphibious (e.g. I. velata, setacea, boryana). The British species are /. lacwtris, echino- spora, and Hystrix. Isoetes has, of recent years, been generally included among the Lycopodinse ; but it betrays a relationship to the Filicinae in so many features, such as its general habit, its embryogeny, the absence of any cone-like fructification, the form of its spermatozoids, that it appears to be more natural to place the plant in that group. 356 PART IV.— CLASSIFICATION. SPOROPHYTE. The stem is small, unbranched, short and tuberous, with either two or three longitudinal furrows which give it a lobed appearance. It is closely covered with numerous, relatively long (1-12 in.), sessile •leaves. From the furrows of the stem there spring numerous, dicho- itemously branched, somewhat fleshy roots. The growth in length of the stem, which is very slow, is effected by an apical growing-point consisting of several initial cells. The growing- point of the root consists of small-celled meristem, and presents a similar differentiation to that of the root of Dicotyledons (see p. 102). The leaves are either fertile or sterile ; the fertile leaves each bear a single sporangium, and are termed macrosporophylls or microsporophylls in accordance with the nature of the sporangium which they severally bear. The order of development of the leaves in each year is that first of all macrosporophylls are produced, then microsporophylls, and finally a few sterile leaves in some species. Hence, when the development is com- pleted, the macrosporophylls are external in the rosette, the sterile leaves (when present) internal, and the microsporophylls intermediate. The sterile leaves persist during the winter, and form a protection in the next spring to the young leaves developed internally to them at the growing- point. The fertile leaves, whether macro- or micro-sporophylls, consist of a broad, sheathing base, with membranous margins, which bears a narrow subulate lamina, flattened somewhat on the upper (ventral) surface. Close above the insertion, on the upper or inner surface of the leaf-base, is a pit, the fovea, in which the single sporangium is situated. In some species the margin of the fovea is prolonged into a membrane, the velum. which either partially (e.g. 1. lacustris}, or completely (terrestrial species), covers the sporangium. Above the fovea, in the middle line, is another smaller pit, the foveola, occupied by the somewhat swollen base of a pro- jecting flattened membranous structure, the ligule, which is developed from a single superficial cell of the young foveola, and is relatively much larger in the quite young leaf than in the adult. The sterile leaves are less highly developed than the fertile ; they are smaller, especially as regards the leaf -base. In the terrestrial species they are reduced to scaly cataphyllary leaves of a brown colour. The sporangium is developed from a group of cells in the fovea. The archesporium consists of a layer of hypodermal cells in the young sporangium. In a microsporangium all the archesporial cells grow and divide so as to form rows radiating from the free surface to the attach- ment of the sporangium. Some of these rows of cells soon cease to grow, and are not sporogenous, but remain as plates of tissue, termed trnbecufa; which imperfectly chamber the cavity of the microsporangium. Of the remaining cells, the majority constitute the mother-cells of the micro- spores invested, towards the wall of the sporangium, by sterile cells forming the tapetum. In a macrosporangium, the fertile archesporial cells undergo but a single division, whilst the trabeculae are formed as in the microsporangium. The large mother-cells of the macrospores are isolated, and each is invested by a tapetal layer. Each spore-mother-cell gives rise, finally, to four spores. GROUP III.— PTERIDOPHYTA : FILICINVE. 357 GAMETOPHYTE. As Isoetes is heterosporous, the gametophyte is repre- sented by distinct male and female individuals, which remain connected with the spores producing them: they resemble those of Selaginel la ami of some Gymnosperms (q. r.). The male individual is developed from a microspore. The microspore — which has the form of the quadrant of a sphere and is consequently of the bilateral or radial type — undergoes,, on germination, division by a. transverse wall, formed near one of its somewhat pointed ends, into two cells, a large and a small : the latter is the vegetav- tive cell, and un- dergoes no further change ; the former is the mother-cell of the male organ or antheridium. The prothallium here is thus very much re- duced, consisting of a single antheri- dium and of a single purely vege- tative cell. The antheridium, de- veloped by the growth and divi- sion of the mother- cell, consists of four peripheral cells forming the wall, and of four central cells, each of which gives rise to a single spirally coiled multiciliate spermatozoid. The female indi- vidual is developed from a macrospore. The macrospores are much larger than the microspores, and are nearly globular in form ^ though they belong to the tetrahedral type, as can be seen by the three ridges on the spore where it was in contact with the other three developed from the same mother-cell. On germination, the nucleus of the macro- -J Fie. 213.— Itoetes lacustris (after Luerssen). A Plant, half naLsize: r dichotomously branched roots. B Inner (ventral) surface of base of a sporophyll : I ligule ; / fovea. C Longi- tudinal section of base of a sporophyll : «p the sporangium in the fovea ; tr the trabeculae ; e the velum ; I the ligule. D Transverse section of the base of a sporophyll : letters as in C. 358 PART IV. — CLASSIFICATION'. spore undergoes repeated division ; this is followed by free cell-formation in the apical region (the pointed end where the three ridges meet) of the macrospore, the result being the formation of a small-celled tissue ; sub- sequently cell-formation extends into the basal portion of the spore, a tissue being formed there consisting of relatively large cells with coarsely granular contents. Thus the macrospore becomes completely filled with a mass of cellular tissue which constitutes the female prothallium : the xipper small-celled tissue is the essentially reproductive portion, whilst the lower large-celled tissue simply serves as a depository of nutritive substances. The female organ, the archegonium, is developed from one of the super- ficial cells of the small-celled prothallial tissue, after the manner described on page 351. It appears that two or three archegonia are usually formed : but if none of these primary archegonia are fertilised, a small number of additional archegonia may be subsequently developed. The ai-chegonia are exposed, for the purpose of fertilisation, by the splitting of the coats of the macrospore along the three ridges already described : the prothallium does not, however, project from the spore, nor does it become green. After fertilisation, the qospore developes into the embryo: the foot of the embryo grows down into the large cells of the basal portion of the prothallium, absorbs the nutritive substances which were stored up in them, and thus supplies the embryo with food until such time as its leaves and roots are sufficiently developed to enable it to nourish itself in the usual way. C. HQMQSPORQUS LEPTOSPORANGIATvE (Filices). The orders constituting this group have so much in common that they may be advantageously considered all together. SPOROPHYTE. The body is differentiated into stem, leaf, and root (generally) : the leaves are large in proportion to the stem, and are relatively few in number. The stem has either radial or dorsiventral symmetry. In the former case it is commonly short and straight ; it grows into the air erect, or at any degree between the vertical and the horizontal ; its surface is generally completely covered by the insertions of the spirally arranged leaves, and by adventitious root$ : it becomes, however, elongated, to a considerable height sometimes, in the Tree- Ferns. In the latter case, the stem grows as a rhizome either on or in the soil, or on the surface of some tree upon which the plant lives as an epiphyte : the leaves are borne on its dorsal surface, either in two rows (e.g. species of Aneimia and Polypodium), or in a single row (e.g. Lygodtum palmatum, Polypodium Heracleum and P. quercifolium} : from the lower (ventral) surface, spring the adventitious roots. GROUP III. — PTERIDOPHYTA : FILICIN^E. 359 The growth in length of the stem, is effected by a growing-point with a single apical cell (with the occasional exception of Osmunda) : the apical cell is, as a rule, a" three-sided pyramid with its spherical base at the surface : but in Pteris aquilina it is usually a two-sided lenticular cell, with its longer axis in the dorso- ventral plane. The radial stems branch but little, least of all when the stem is elongated, as in the Tree-Ferns ; and such branching as there is appears to be mainly adventitious, the buds springing from the bases of the leaves. In the dorsiventral stems there is normal lateral branching, which takes place in the transverse plane : the branches are borne on the flanks of the stem, and are frequently (e.g. some Hymenophyllaceae) axillary in their origin. The leaves are for the most part fojiage-leaves, though scaly leaves are found on the subterranean rhizomes of Onoclca Stru- thioptcris and Osmunda regalis, and in some cases the sporophylls are more or less differentiated from the sterile leaves. The foliage-leaves are relatively large, sometimes entire (e.g. Scolopendrium), but generally more or less deeply and repeatedly pinnately lobed or branched; sometimes dichotomously branched (e.g. Platycerium, species of Schizaea). The leaves in all cases have apical growth ; the growing-point has, in most of the orders, a two-sided apical cell, whilst in the Osmundaceae the apical cell is tetrahedral. In Lygodium; where the leaf is a climbing organ, the apical growth is long continued. The leaf arises from a single superficial cell of the growing- point of the stem. When young it is strongly hyponastic (see p. 211), so that, as it elongates and branches, both the main axis of the leaf (phyllopodium) and the lateral branches become inrolled upon themselves like a crosier ; in other words, the vernation is circinate: as it grows older the growth becomes epinastjc, and thus the leaf becomes expanded. In the great majority of these Ferns the sporophylls are simply foliage-leaves bearing sporangia on the dorsal surface, b,ut in certain cases they are more or less specialised. Thus, in Onocleq Struthioptcris, the sporophylls are smaller than the foliage-leaves, and have narrower pinnae : in the Hard Fern, Blechnum borealc, the sporophylls are longer and have narrower pinnae than the foliage-leaves : in Osmunda regalis the pinna? of the upper branches of the sporophyll are reduced to little more than the midrib, the pinnules' being represented by clusters of sporangia: in Aneimia 360 PART IV. — CLASSIFICATION. (e.g. A. Phyllitidis) generally the lowest pair of pinnae of the sporophyll alone bear sporangia ; these pinnse consist merely of the nervature bearing numerous sporangia, and are erect on much elongated secondary petioles : in Platy cerium alcicorne there is a curious instance of specialisation ; the foliage-leaves are broad, and closely appressed to the substratum, whereas the sporophylls are erect, narrow, and branched. The sporangia are but rarely borne on the superior (ventral) surface of the sporophyll (e.g. Olfersia cervina) ; more commonly FIG. 2U.— Sori (s) of the most important groups of Leptosporangiate Ferns, all seen from balow. A Pinna of Trichomanes sinuosum, one of the HymenophyllacesB : r projecting placenta; s sporangia; i indu&ium ; at a half of the indusium is removed. B Pinna of Davallia (Leucostegia)— at s the one-valved indusium (i) is turned back. C Part of a leaf of Pteris eerrulata -. 8 the sporangia; m the inverted margin. D Pinnule of Nephrodium— at s the indusium is removed, and at r the sporangia also. E Pinnule of an Asplenium— at a the indusium is turned back. F Pinna of Polypodium vulgare with naked sori — at r the sporangia are removed. (All a-e x 3 to 6.) on the margin (e.g. Hymenophyllacese, Dicksonia, Davallia) ; but as a rule, on the dorsal surface, either near to the margin (e.g. Pteris, Adiantum), or distant from it (e.g. Asplenium, Aspidium, etc). They are usually developed in connexion with the nervature of the sporophyll, but sometimes also from the intervening tissue GROUP III. — PTERIDOPHYTA : F1LICINJE. 361 of the lamina (Acrostichese, such as Polybotrya, Chry sodium) : in the former case they occur in groups, termed son, which are commonly isolated, but occasionally (e.g. Pteris) a continuous marginal sorus is formed. The sorus generally consists of a large number of sporangia : in the Gleicheniacese, however, the number is small (2-8); and in some cases (e.g. Lygodium) there is only a single sporangium. The sporangia of the sorus are borne on a projection of tissue, the placenta or receptacle, which presents various forms : it may be a slight rounded elevation (e.g. Aspidium) ; or more elongated and conical (e.g. Cyathea, Hymenophyllum) : or very long and filiform, bearing sporangia only at its base (e.g. species of Trichomanes, Tig. 214 ^4) ; or a ridge (e.g. Pteris, Blechnum). FIG. 215. — A dehisced spor- angium of Asiridium Fillx-mas (xOO): a the stalk, with a glandular hair p ; r tho annu- lus ; » the stomium. FIG. 216.— Sections of young sporangia; A of a Fern (Mohria), B of Equisetum (x 150): w wall; t tape- turn; a s archesporium. The sorus is quite bare in many forms (Gleicheniacese ; Osmundacese ; Alsophila among Cyatheacese ; Schizseaceae, except Lygodium ; Polypodiese) ; in others it is more or less covered by a protective membrane, the indusium, which is an outgrowth of the tissue of the leaf, generally of the epidermis alone. When it springs from the placenta below the sorus, it is somewhat cup- shaped : thus it is urceolate and entire in Trichomanes (Fig. 214 A), Lygodium, Cyathea, Davallia. When it springs from the apex of the placenta, above the sorus, the indusium has the general appearance of a peltate scale, either orbicular in outline (Aspidium) or reniform (Nephrodium, Fig. 214 D). When it is developed on one side of the sorus, the indusium is a long narrow scale, attached 362 PART IV. — CLASSIFICATION. along its length, and overlying the sorus (e.g. Asplenium [Fig. 214 .E], Blechnum. Scolopendrium [Fig. 217];. In some cases, where the sori are near the margin, they are protected by a false indusium, which is merely the incurved margin of the leaf (e.g. Cheilanthes, Adiantum, some species of Pteris [Fig. 214 C]}. In Pteris aquilina, and some other species, in addition to the false indusium, there is also a membrane along the inner side of the FIG 217. — Scolopendrium vulgare (Hart's-tongue Fern). A Transverse section of a sorus ; » indusinm; * g sporangia. B-E Sporangia ; B and E seen sideways ; C in front ; D from the back ; F a spore. (A x 50 ; B-E x 145 ; F x 510 : after Strasburger.) sorus, which is a kind of lateral indusium, adapted to protect a continuous marginal sorus. In some forms (e.g. Aspidium Filix-mas, Fig. 215) the stalks of the sporangia bear glandular hairs ; sometimes even the sporangia themselves. GROUP III. — PTERIDOPHYTA : FILICIDE. 3G3 With the possible exception of the Osinundaceae, each sporangium is developed from a single superficial cell. The cell grows so as to project more or less : it is then divkied-krte-fcwo eells— an outer, the mother-cell of the sporangium ;_aa inner, the stalk-cell — by a horizontal waH. As the mother-cell of the sporangium grows, it undergoes division by the successive formation of three oblique walls, intersecting one another below at an angle of about 60°, and reaching above to the wall of the motherrcell ; at this stage the sporangium consists of three latero-basal external cells surrounding the pointed lower end of a tetrahedral cell, the spherical base of which occupies the summit of the sporangium. A wall is now formed in the tetrahedral cell, parallel to its spherical free surface, and intersecting the three oblique walls ; so that the sporangium now consists of four peripheral cells, forming the wall, and a central cell. From the central cell are cut off, by successive walls parallel to its sides, four cells which give rise to the tapctum by subsequent growth and radial, and sometimes tangential, division ; the remain- ing internal tetrahedral cell constitutes the unicellular arche- sporium from which the spores are derived. As the young sporangium grows, it gradually assumes its definitive form, which is mo^t commonly ovaL-lenticular. The four primary peripheral cells undergo repeated radial division, and form the wall of the sporangium, which ultimately consists of a single layer of cells with cuticularised walls : a portion of the wall is in all cases developed to form the ring or annulus, by means of which the dehiscence of the sporangium is effected, the walls of which are specially thickened and cuticularised, coloured yellow or brown, and are elastic. The form and position of the annulus varies in the different groups : thus in the Polypodiaceae (Fig. 217), where the sporangium is attached to the stalk by the margin, the incom- plete annulus is a projecting row of cells with their longer axes transverse, extending round the margin in the plane of the stalk, with which it is connected on one side, but not quite reaching it on the other. As the development proceeds, the formation of the spores takes place in the interior of the sporangium. The archesporial cell under- goes repeated division, with the result that usually sixteen cells are formed, which are the mother-pells of the spores (Fig. 216). At this stage the tapetal cells undergo disintegration, so that the mass of spore-mother-cells floats freely in the liquid thus produced. Each mother-cell then undergoes division to form four spores. 364 PART IV. — CLASSIFICATION. The sporangium may be sessile (Gleicheniaceae, most Schizseacese. Hymenophyllacese) ; or shortly stalked (Lygodium, Cyatheacete, Osmundacese * ; or it may have a usually rather long slender stalk consisting of two or three longitudinal rows of cells (Polypodiacese) ; this is dependent upon whether the originally-formed stalk-cell developes further or not. The spores are set free by the dehiscence of the sporangium ; this takes place at a certain part which, though different in the various forms of sporangia, is always closely connected with the annulus and is termed the stomium (see Fig. 215) ; dehiscence begins by a split between (not through) the cells of the stomium. In the Polypodiaceae the plane of dehiscence is at right angles to the long axis of the sporangium, and the stomium is situated on the margin between the end of the annulus and the stalk. A striking feature in the general morphology of these plants is the presence on the stem and the bases of the leaf-stalks, especially when young, of numerous scaly hairs (ramenta or palece), which consist usually of a single layer of cells, with more or less thickened brown walls ; they are of various shape, and frequently have marginal glandular hairs secreting tannin or mucilage, the latter generally in the neighbourhood of a growing-point or stem or leaf. Less commonly, glandular hairs are developed on the leaves, as in species of Gymnogramme (Gold and Silver Ferns), in which the under surface of the leaves is covered with a yellowish dust, consisting of minute needles of resinous and waxy substances, secreted by the hairs. Root-hairs occur on subterranean stems and leaf-stalks. A primary root is developed, probably in all forms, but in no case does it persist in the adult. In the full-grown plant all the roots are adventitious; they spring in great numbers from the stem or the leaf-stalks. The roots are small and branched ; the branching is lateral, and the growing-points of the young roots are developed each from a single cell of the endodermis, termed a rhizogenic cell, situated opposite to a xylem-bundle of the central cylinder. In most cases the growing-point of the root has a single pyramidal apical cell (see Fig. 86) with three flat sides and a spherical base directed outwards. Adventitious buds, subserving vegetative propagation, are com- monly produced ; they arise most frequently on the subterranean portions of leaf-stalks (as inPteris aquilina, Aspidium Fili.i--mas\ and sometimes, as in Onoclea Struthiopteris, the bud grows into GROUP III. — PTERIDOPHYTA : FITJCINJE. 365 a subterranean stolon which eventually throws up at its apex a whorl of green leaves, thus constituting a new plant ; but also frequently from the lamina, as in Asplenium (Diplazium} celtidi- folium. A. bulbiferum, and other species. The bud originates from a single epidermal cell. General Histology. — The structure of stem, petiole, and root, is characterised throughout by the presence of hypodermal layers, and, generally, of scattered strands of sclerenchymatous tissue, consisting of more or less elongated ground-tissue cells with more or less thickened brown-coloured walls ; and by the predominance of scalariform vascular tissue in the xylem which consists, with but few exceptions, of tracheides. The stem is, at its first development, monostelic, with a single axile stele : in some forms this structure obtains (with or without pith) throughout the whole stem (e.g. Hymenophyllaceae, Lygodium, Schizaea) : in the Osmundaceae also the stem is monostelic through- out, the stele eventually consisting of a ring of bundles enclosing a pith : in the other families the stem becomes polystelic. In the monostelic stem the bundles are sometimes conjoint and collateral (e.g. Trichomanes among Hymenophyllaceae, Osmun- daceae) : in all other cases the arrangement of the bundles in the stele is concentric, or, more strictly speaking, bicollateral (p. 123), since the phloem does not quite completely surround the xylem- bundles. The concentric steles are cauline and usually consist of two wood- and two bast-bundles, with usually an endodermis and a pericycle: in some cases, however, where the stele is small (e.g. some species of Polypodium) there is no pericycle, its place being taken by a layer of cells formed by the division of the primitive endodermis (p. 115) into two layers. In the polystelic stem the course of the steles is such that they form a meshwork, each mesh corresponding to the insertion of a leaf : the bundles of the leaf join those forming the corresponding mesh in the stem. The form of the mesh is determined by the number and insertion of the leaves : when the leaves are numerous and closely arranged, the meshes are relatively short and broad ; when the leaves are few and scattered, the meshes are long and narrow. In a monostelic stem, such as that of Osmunda, though the bundles are numerous, no such meshwork is formed. The bundles are here common. A single bundle enters the stem from each leaf, runs straigh-t through several internodes, and then curves to join 366 PART IV. — CLASSIFICATION. Fm. 218.— Embryogeny of the sporophyte of Pteris serrulata ( x 235 : after Kienitz-Gerloff). A In longitudinal section: B transverse section, at right angles to the preceding : C older embryo in longitudinaj Section. The vertical arrows indicate the long axis of the arche- goniurn, pointing to the neck : the horizontal arrows indicate the longitudinal axis of the prothallium, pointing to its organic apex. I-l Bsisal wall; 11-11 transverse wall; Ill-Ill median wall : r apical cell of root ; I apical cell of cotyledon ; s apical cell of stem ; /foot. with the bundle of a'n older leaf, seven leaves intervening between the two. Embryogeny of the Sporophi/te. The sporophyte is developed from the fertilised female cell, the oospore : the development has only been studied in species of Polypodiacese, and has been found to be as follows. The oospore is first of all divided into two cells by the formation of a wall, the basal icall, which nearly coincides with the long axis of the archegonium : a second wall is then formed, the transverse wall, at right angles to the preceding, with the result that the spherical embryo now consists of four cells or quadrants : then a third wall, the median wall, is formed in a plane at right angles to both the preceding walls, the embryo now consist- ing of eight equal cells or octants. Of these octants, four belong to one half of the embryo, which is termed the cpibasal half; and four to the other half, the hypobasal half: from these octants the primary organs of the sporophyte are developed. Beginning with the four epibasal octants, the two apical octants (i.e. nearest to the neck of the arche- FIG. 219,-^Adiantum CapiHus- Venerit. The prothallium (pj>) seen from below with young Fern attached to it by its foot ; i its first leaf or cotyledon ; w' its primary, to" secondary, roots; h root-hairs of the prothallium (x about 3). (After Sachs). GROUP III. — PTERIDOPHYTA : FILICIX.E. 367 gonium) give rise to the growing-point of the first leaf or cotyledon : of the two deeper (towards the venter of archegonium) octants, the one constitutes the growing-point of the stem, whilst the other gives rise to nothing beyond possibly some hairs. Of the four hypobasal octants, one of the two apical octants gives rise to the growing-point of the primary root, which is diametrically opposite to the growing- point of the stem; whilst the other gives rise to no special member : the two deeper hypobasal octants give rise to the embryonic absorptive organ, the foot. The gradual development of these members is dependent upon growth and corresponding cell-division, and at an early stage histological differentiation into cortical and stelar tissues is apparent in them. For a time the tissue of the venter of the archegon- ium keeps pace by growth with the increasing size of the em- b r y o : but eventually the primary root and the cotyle- don become free, and ultimately also the stem (Fig. 220). In the meantime the embryo is nourished by means of the foot which has become a mass of tissue filling the venter of the archegonium : it absorbs from the adjacent cells the organic substances formed in the prothallium by means of the chloroplajStids which most of the cells contain. The primary root and the cotyledon are both small and short-lived : the former is succeeded by the numerous adventitious roots, the latter by the true foliage -leaves. The foot is a merely embryonic organ : it disappears when the young sporophyte has become firmly attached FIG. 220. — Section of young plant of Pteris aquilina still attached to the prothallium by its foot : p prothallium ; / foot ; r primary root ; s growing-point of primary stem ; I primary leaf or cotyledon. (Magnified: after Hofmeister.) 368 PART IV. — CLASSIFICATION". to the substratum, and is capable of independently absorbing and assimilating food. GAMETOFHYTE. The gametophyte is a prothallium, always con- taining chloroplastids, generally a dorsiventral, flattened, cellular expansion, or sometimes filamentous, which is developed from a spore, but which becomes completely free from the spore. In the dorsiventral prothallium the reproductive organs, as also the root-hairs, are confined to the inferior (ventral) surface. The prothallium is typically rnonoscious : the male organs, or antheridia, are developed first, and are consequently situated towards the posterior or basal end of the prothallium; the later- formed archegonia lie towards the anterior or apical end. It sometimes happens, however, that, owing to imperfect nutrition, the growth of the prothallium does not proceed beyond the stage Fia. 221.— Diagram of the prothal- lium of a Leptosporangiate Fern : under side (x 10). ar Archegonia; an anthe- ridia ; fc root-hairs. FIG. 222. — Antheridium of Adiantum Capillus-Teneris (x550). p Prothallium ; a antheridium ; s spermatozoid ; b the vesicle containing starch-grains. necessary for the formation of the antheridia, so that exclusively male prothallia may be sometimes found ; less commonly, well- nourished prothallia fail to produce antheridia, and consequently exclusively female prothallia are found. The practical result of this successive formation of the antheridia and archegonia is that but few of them can possibly mature at the same time on one and the same prothallium ; and, consequently, cross-fertilisation is almost certainly ensured. GROUP III.— PTERIDOPHYTA : FILICIJUE. 869 The development of the prothallium commences with the rupture of the outer coat (exospore) of the germinating spore, which takes place either along three lines meeting at an angle, when the spore is tetrahedral, or by a longitudinal slit when the spore is bilateral, the contents covered by the inner coat (endospore) being exposed. Most commonly this cell grows out into a filament, cell-divisions taking place in the transverse plane only, so that the prothallium consists of a longitudinal row of cells. At length a longitudinal wall is formed in the terminal cell of the filament ; cell-division then proceeds in two planes, giving rise to a flattened plate of cells, further growth being effected by means of a two-sided apical cell. After a time the activity of the apical cell ceases, a periclinal wall being formed in it ; whatever further growth takes place is effected by the marginal cells. At this stage the prothallium becomes somewhat heart-shaped, the anterior depression indicating the posi- L" FJG. 223. — Polypodium. vulgare. A Youug arcbegonium, not yet open: K' neck-canal- 11 ; K" ventral canal-cell : o young oosphere. B Mature archegonium open. ( x 210 after Strasburger.) tion of the organic apex (Fig. 221). The cells lying anteriorly in the middle line now begin to divide in a plane parallel to the surface, with the result that the prothallium becomes thickened in this region, and eventually a " cushion :> of tissue, several layers of cells in thickness, is produced, which projects on the lower (ventral) surface, and bears the archegonia. The sexual organs. The antheridium is developed from a single superficial cell. The free surface of this cell grows out into a blunt protuberance, which is cut off by a transverse wall. The projecting cell thus formed generally undergoes division by the formation "of a transverse wall near its base, so that it comes to M.B. B B 370 PART IV. — CLASSIFICATION. consist of two cells, the lower of which is the stalk-cell, the upper, the anther-id ial cell. The latter grows, becoming more or less spherical, and undergoes repeated cell-divisions which result in the formation of a wall, consisting of a single layer of cells, sur- rounding a large central cell from which, by further division, the mother-cells of the spermatozoids are formed. When mature, absorption of water causes the rupture of the antheridium ; the mother-cells of the spermatozoids are now set free, and the spermatozoids soon escape from the mother-cells as coiled ciliated filaments, each having usually attached to it posteriorly a vesicle of granular protoplasm, the remains of the contents of the mother- cell (see Fig. 222). The archegonium. The general description given above (p. 351) of the development and structure of the archegonium, and of the process of fertilisation, will suffice for tnis group of the Pterido- phyta. It should, however, be mentioned that only a single neck- canal-cell is developed. The root-hairs retain in all cases their typical unicellular structure. They arise as tubular outgrowths from single cells, having, at first, colourless walls, which eventually become thickened, and assume a brown colour ; the cavity of the hair is cut off by a septum from that of the cell from w_hich it springs ; their form is most commonly elongated and cylindrical, but some- times (e.g. Hymenophyllacese) they are short and slightly branched. The development of the root-hairs begins at the earliest stage in the formation of the prothallium. Generally speaking, the root- hairs are developed laterally, and as the prothallium assumes the flattened expanded form, the development extends inwards from the margin, over the inferior surface, and forwards as far as the posterior part of the cushion. The life of the gametophyte is, as a rule, short, being limited by the fertilisation of an archegonium. If, however, fertilisation does not take place, the prothallium continues to grow for several months, or even years in the case of Osmunda. Propagation of the gametophyte by means of a gemmae is common in the Hymenophyllacese, but it has also been observed in certain Polypodiacese (Vittaria, Monogramme). In Hymenophyllum, the gemmae are small flat plates of cells ; in species of Trichomanes, Vittaria, and Monogramme, they are short spindle-shaped filaments, consisting of a single row of (6-9) cells, borne on a unicellular stalk or sterigma ; in Trichomanes, the gemma is attached at its GROUP III.— PTERIDOPHYTA : FILICIN.E. 371 centre to the stalk, so that its long axis is at right angles to the stalk ; in Vittaria and Monogramme, the gemma is attached to the stalk by one end. The sterigmata are developed either singly or several together, from a single cell of the prothallium ; and the gemmae may be borne singly or several together on one sterigma. It will have been observed that the gametophyte of the homo- sporous leptosporangiate Ferns presents, in its development, its root-hairs, its propagation by gemmae, remarkable and suggestive resemblances to the gametophyte of the Hepaticse. In the general morphology, too, of the gametophyte, there are striking corres- pondences between the two groups : thus, in both groups (with certain exceptions in both) the first stage in the life of the game- tophyte is a filamentous protonema, which is, however, relatively small and short-lived, except in the Fern Trichomanes where the gametophyte does not develope beyond the protonematous stage. The protonema in both groups gives rise to a single flattened, ex- panded shoot, the adult sexual shoot of the Hepaticae, the prothallium of the Ferns, bearing the sexual organs. Order 1. Hymenophyllaceae ; this order contains the simplest forms The leaf-blade almost always consists of a single layer of cells ; the sorus is always marginal (Fig. 214 A) and indusiate, the sporangium sessile or shortly-stalked, and the annulus entire and horizontal. Almost all the species are tropical. Trichomanes radicans and Hymeno- phyllum Tanbridgense and unilaterale (or Wilsoni) alone occur in Britain. Some species of Trichomanes have no true roots. Order 2. Polypodiaceae. The annulus of the stalked sporangium is incomplete and vertical (Fig. 215 r), that is to say, it is not continuous at the base : indusium present or absent. Almost all our native Ferns belong to this order, which is exceptionally rich in genera. The following are the chief families : — (a) Pteridece. Sori coalescent along the margin of the leaf (Fig. 214 6')," with a spurious indusium. Pteris (Pteridium) nquilina, the Bracken, has a stem which grows at some depth below the surface of the soil, and throws up every year a single large, much-segmented leaf (frond) : it has also a true lateral indusium. Adiantum, the Maiden-hair Fern, belongs to this group, as also Cheilanthes. (6) Aspleniece. The sorus, which is situated on the under surfaca of the leaf, is elongated or linear, and the lateral indusium springs from the vein to which it is attached (Fig. 214 E). Aaplenium Ruta muraria, the Wall- B,ue, is nob uncommon on walls and rocks ; A. Trichomanes is also abund- ant, with simple pinnate leaves and a shining black rachis. Athurt:im F'dix foemina, the Lady Fern, is common in damp woods. Scolopendritim vulgare, the Hart's-tongue, with entire leaves, is common in damp hedge- raws and woods (Fig. 217). Blechnum (Lornaria), the Hard Fern, as also 372 PART IV.— CLASSIFICATION. Ceterach, may be included here, though the indusium may be rudimen- tary or absent. (c) Aspidiect'. Sorus on the lower surface of the leaf, orbicular in form and covered \>y a peltate or reniform superior (Fig. 214 _D) or inferior indusium. Nephr odium (Lastrcea) Filix mas, the male Fern, and other species resembling it, with a thick tufted crown of leaves, are not rare in •woods. Aspidium is the Shield-Fern : A. (Polystichum) Lonchitis is the Holly-Fern : "Woodsia, Onoslea, and Cystopteris (Bladder-Fern), with an inferior indusium, also belong to this group. (d) Polypodies. The sorus, which is on the under surface of the leaf, is naked (Fig. 214 F). In the section Polypodium the leaves are articulated to the stem, so that when they die and fall off they leave a roundish scar ; the leaves are usually borne in two rows on the dorsal surface of the rhizome. Polypodium vulgare, with simple pinnate leaves, is common on tree-trunks, rocks, etc. In the section Phegopteris the leaves are not articulated to the stem, so that when they die, fragments of the leaf -stalks remain attached to it : Cryplogramme crispa is the Parsley-Fern : Pliegop- tsris Dryopteris and polypodioides are the Oak- and Beech-Ferns. Order 3. Cyatheaceae. Distinguished from the Polypodiaceae only by the presence of a complete annulus. The Tree-Ferns belong to this family. Cibotium and Dicksonia have marginal sori with two-valved inferior indusia : Cyathea, Hemitelia, and Alsophila have their sori on the under surface of the leaf : Alsophila alone has no indusium; in Cyathea it is cup-shaped, and in Hemitelia one- valved. Order 4. Gleicheniaceae, including the genus Gleichenia with a hori- zontal annulus ; no indusium : all tropical. Order 5. Schizaeaceae, including the genera Schizaea Aneimia, Mohria and Lygodium, with a projecting apical annulus to the almost sessile sporangium, occur only in the tropics. Lygodium is the most remarkable genus ; its pinnate leaves grow to a great length, and twine round supports by means of their midribs : it alone has an indusium, and the sorus is usually unisporangiate. Order 6. Osmundaceae. The shortly-stalked sporangia have a rudi- mentary annulus consisting of a group of cells just below the apex; they burst open by a longitudinal slit on the side opposite to this. Osmunda re/jails, the Fern-Royal, is a not very common but well-known Forn. Only the upper pinnae of the leaves are fertile, and develope little or 110 mesophyll ; the sori are marginal, and consist of a great number of sporangia; they have no indusium. The only other genus is Todea, belonging mainly to Australasia. GROUP III.— PTERIDOPHYTA : FILICIN^. 373 D. HETEROSPOROUS LEPTOSPORANGIAT.E. (Hydropteridese or Rhizocarpae.) This group includes the four genera, Salvinia, Azolla, Marsilea, Pilularia ; of these the two former constitute the order Salviniacese, the two latter the order Marsileacese. They are all more or less aquatic in habit, Salviuia and Azolla being free-floating fugacious- plants, whilst Marsilea aud Pilularia are perennials growing in bogs and marshes. SPOROPHYTE. — The stem is a horizontal dorsiventral rhizome. It generally bears foliage-leaves in alternating longitudinal rows (four rows in Salvinia ; two rows in the other genera) on the dorsal (superior) surface ; and roots in one (Marsileaceae) or two (Azolla) longitudinal rows on the ventral (inferior) surface. In Salvinia, however, there are no roots, but the stem bears in place of them two rows of submerged leaves on its ventral surface. The lateral branches, sometimes very numerous, are borne on the flanks. The foliage-leaf presents a considerable variety of form. In Salvinia it is broad and flat, sessile and entire, with a well- marked midrib ; in Azolla the leaf is small and two-lobed, the lower lobe being submerged, whilst the upper floats on the surface of the water : in Marsilea the leaf has a long erect petiole bearing a paripinnate bijugate compound lamina of four leaflets: in Pilularia the leaf is cylindrical and erect. Circinate vernation obtains in the Marsileacese, but not in the Salviniaceae : in Salvinia the vernation is conduplicate, and in Azolla the lamina is expanded from the first. In Salvinia the leaves are borne in a whorl of three at a node, two being a pair of opposite foliage-leaves, and the third a sub- merged leaf (p. 17) ; in the other genera the phyllotaxis is alternate. The suljmcrged leaf of Salvinia consists of a number of long filamentous branches, springing from a short petiole, and densely covered with multicellular hairs. The sporangia are of two kinds, microsporangia and macro- sporangia : they are bsrne in sori enclosed in structures termed sporocarps. The morphology of the sporocarp is, however, altogether different in the two orders, and the same term ought not to be applied to both : it would be well to restrict the term " sporocarp " to the more complex fructifications of the Marsileacese. 374 PART IV. — CLASSIFICATION. In the Salviniacese the so-called sporocarp is simply a sorus of sporangia, either micros porangia or macrosporangia, but not both, surrounded by an inferior indusium (Fig. 224). In Salvinia the sori are borne at the apices of the basal branches of a submerged leaf, and may be comparatively numerous (4-20) on one leaf : in Azolla the sori are borne at the apices of the segments of the lower (ventral) submerged lobe of a leaf, and that leaf is always the first (basal;i leaf of a fertile branch which is sometimes less vigorously developed than the purely vegeta- tive branches ; each leaf usually bears only two sori, but in A. nilo- tica there are four. In Salvinia the sori are all of the same size, whereas in Azolla the sori contain- ing microsporangia are consider- ably larger than those containing macrosporangia. In both genera the tip of the fertile leaf -segment expands into a cellular cushion, the placenta, from the superficial cells of which the sporangia are de- veloped : the indusium is developed as an annular outgrowth from the base of the placenta, becoming cup- shaped, and eventually closing over the sorus : it consists of two laj-ers of cells which, in Salvinia, are separated by large air-chambers and are connected by longitudinal cellular trabeculse. In both genera the microsporangia of a sorus are numerous (about 40 in Azolla, more in Salvinia) : the macro-sporangial sorus consists, in Salvinia, of many (up to 25) macro-sporangia, whereas in Azolla there is but one. Both kinds of sporangia are borne by the same plant. In the Marsileacese the sporocarp consists of a leaf-branch enclosing a number of sori, and each sorus includes both micro- and macrosporangia. In Marsilea the fertile leaf -branches spring from FIG. 224. — A Apical portion of the stem of Salvinia natans, seen obliquely from below (nat. size) : 1 1 aerial leaves ; ic >c aquatic leaves, with sori, *s; fc ter- minal bud of the stem. B Longitudinal section through three fertile teeth of an aquatic leaf ( x 10), forming two sori with microsporangia, (a) one with macrosporangia; t indusium. (After Sachs.) GROUP III. — PTERIDOPHYTA : FILICIXJE. 375 the ventral surface of the petioles of the foliage-leaves (compare Ophioglossacese), and each bears a sporocarp at the end of a longer or shorter stalk : the petiole bears a single fertile leaf-branch near its base in some species, or two or more adnate branches springing from the same point ; or it bears (e.g. M. polycarpd) a series of 10- 20 branches, one above the other, each bearing a single sporocarp. In Pilularia the fertile leaf-branches ap- pear to be also developed from the ventral surface of the foliage-leaves : each leaf has at its base a single almost sessile sporo- carp. The sporocarp of Marsilea (Fig. 225) is dorsiventral, somewhat pod-shaped, with its dorsal margin directed upwards ; the stalk is continued along the dorsal margin as a midrib : the sporocarp may, in fact, be regarded as being developed from the laminar portion of the leaf-branch. The sporocarp of Pilularia is globular, though it is slightly pointed at the apex : it may be likewise regarded as being a leaf- branch, four (usually) leaflets or segments being concerned in its construction. In both genera, especially in Marsilea, the wall of the sporocarp is composed of several layers of cells with thick hard walls : vascular bundles, springing from the stalk, are distributed in the wall. The number of sori in the sporocarp of Marsilea varies from five to twenty-three in the different species : they are developed in tubular cavities, extending from the ventral margin of the sporocarp for some distance towards the dorsal margin, which are disposed in two longitudinal rows, one row on each side of the middle line ; when young, these cavities are open at the ventral margin, but the apertures become closed as the sporocarp matures ; the external wall of each cavity developes into a projecting ridge of tissue, the placenta, which bears the sorus, consisting of a single median row of macrosporangia and a double row of microsporangia FIG. 225.— Stem of Xarsilea Salvatrix with leaves (reduced one-half). K Terminal bud ; b b leaves ; / / sporocarps borne on petioles. 376 PART IV. — CLASSIFICATION. on each flank. The cavities are surrounded by parenchymatous tissue. The globular sporocarp of Pilularia contains four (sometimes two or three) cavities, extending longitudinally from the base to the apex, enclosed by parenchymatous tissue. The placenta is a ridge of tissue on the external wall of the cavity, bearing the sorus which consists, in its upper part, of microsporangia, and in its lower of one or more macrosporangia. The cavities at first communicate with the outer air at the apex of the sporccarp, but eventually be- come completely closed. In their development, the sporangia of the Heterosporous Lepto- sporangiatse resemble those of the Homosporous Leptosporangiatse (see p. 363) in all essential points : but no annulus is developed. In each sporangium sixteen spore-mother-cells are developed from the single tetrahedral archesporial cell, and each of these mother-cells undergoes division to form four spores : but whereas in the micro- sporangia all these sixty-four spore-rudiments develope into micro- spores, in the rnacrosporangium only one developes into a macrospore, the others being abortive. The development of the spores in this group, is remarkable on account of the important part played by the multinucleate proto- plasmic mass (epiplasm), derived from the disorganisation of the tapetal cells, in which the free spore-mother-cells are embedded at the time when the development of the spores is commencing. Taking first the Salviniacese : the microsporangium of Salvinia contains, when mature, a number of microspores embedded in a spongy mass of a substance, which gives some of the reactions of corky cell-walls and is derived from the protoplasm of the tapetal cells : in Azolla the microspores are likewise embedded in this substance, but in more than one group or massula (2-8) according to the species ; each massula is surrounded by a membrane, bearing in some species a number of anchor-like hairs, the glochidia. In Salvinia the macrospore also is invested by a layer of this spongy substance, constituting the episporc or perinium. This is also the case in Azolla, but here the perinium is remarkably developed : over the rounded dorsal surface of the radial macrospore, the perinium is a thick membrane, firm at the surface, spongy within, with warty projections bearing filaments of the same substance : on the three flattened surfaces of the ventral aspect of the macrospore the perinium forms three or more large spongy masses which constitute the so-called floats of the spore ; at the pointed apex of the spore, GROUP III. — PTERIDOPHYTA : FILICIX.E. 377 between these masses, the perinium usually terminates in a tuft of delicate filaments. In the Marsileaceae the spores become invested by a perinium, secreted by the epiplasm, consisting of an inner layer made up of prisms placed with their long axes perpendicular to the surface of the spore, and of an outer layer which is homogeneous in the case of the microspore ; but in the case of the macrospore it is stratified, swells up enormously on being wetted, and gives the cellulose- reaction. In all cases the spore has its own proper coats, the exospore and the endospore, of the usual constitution. It contains a mass of granular protoplasm, with a nucleus, and encloses numerous starch- grains, oil-drops, and proteid granules. The root is altogether absent in Salvinia ; in the other genera the primary root is of but short duration, and the root-system con- sists of numerous adventitious fibrous rootlets which have an apical growing-point with a tetrahedral apical cell. In Azolla the root- cap is but imperfectly developed, and in A. caroliniana it is completely thrown off after a time. General Histology. In the Salviniacese the stem is monostelic ; in the Marsileacese it is polystelic. On the whole the histology of these plants generally resembles that of the allied homosporous Ferns, though in consequence of their more or less aquatic habit the intercellular spaces of these plants are more conspicuous, especially in Salvinia and in the root of Pilularia where they form large air-chambers. The Einbryogeny of the Sporophyte. The early divisions of the oospore are essentially the same as in the allied homosporous Ferns. The individual peculiarities of subsequent development are briefly as follows. In Salvinia the whole of the hypobasal half goes to form the foot, no primary root being developed ; of the four epibasal octants, one gives rise to the growing-point of the stem, and two to that of the first leaf or cotyledon, and from the basal region of these octants a transverse layer of cells is cut off which eventually elongates forming a cylindrical hypocotyl, sometimes termed the caulicle ; the cotyledon is termed the scutiform leaf on account of its form and mode of attachment ; the young stem first produces one or two isolated foliage- leaves, and then the regular development of the whorls of two foliage-leaves and one water-leaf (see p. 373) begins. In the other three genera, two of the epibasal octants give rise to the first cotyledon, one to the growing-point of 378 PART IV.— CLASSIFICATION. the stem, and the fourth to a second cotyledon, so that here there are two cotyledons : the hypobasal octants give rise to foot and root in the usual manner. THE GAMETOPHYTE. As these plants are heterosporous, the gametophyte is represented by distinct male and female indivi- duals : these remain connected with the spores producing them. The male individual is developed from a microspore : it consists of a rudimentary prothallium, represented by a single cell, bearing a single male organ (antheridium). In Salvinia the germination of the microspores takes place with- in the microsporangium ; the inner coat (endospore) of the spore grows out as a longer or shorter tube through the ruptured outer coat (exospore), and eventually makes its way through the wall of the sporangium so that its free end is in the water outside : a trans- verse wall is formed within it which cuts off the apical portion of the tube as the fertile portion of the prothallium. In Azolla the microspores germinate within the massula. The exospore of the tetrahedral spore ruptures along the three edges, and the endospore protrudes as a papilla at the apex. A transverse wall is formed across the base of the papilla, which now becomes the mother -cell of the single antheridium. The spermatozoids probably escape from the massula on the deliquescence of its substance. In the Marsileaceae the -male prothallium is formed altogether within the microspore ; the spore divides into two cells : a small basal vegetative cell and a larger one which is the mother-cell of the antheridium. In all cases the prothallium has no chlorophyll. The male organ, or antheridium, is developed from the antheri- dial mother-cell mentioned above. It generally undergoes divisions so as to form a central cell surrounded by a single layer of cells forming the wall of the antheridium. But Salvinia is peculiar in that the central cell of each antheridium is not completely sur- rounded by a parietal layer of cells, but comes to the surface of the antheridium. The central cell then undergoes further divisions to form the mother-cells of the spermatozoids, of which there are eight in the Salviniaceae and thirty-two in the Marsileacese. The male cells are spermatozoids, which resemble those of the homosporous leptosporangiate Ferns in all essential features of their form and development, as also in their extrusion from the au- theridium. In the Marsileacese, the male prothallium is enclosed within the microspore until the antheridium is mature, when the GROUP III. — PTERIDOPHYTA : FHJCINJE. 379 spore-walls are ruptured by the swelling of the cells of the con- tained prothalliura, and the spermatozoids are set free. The female individual is a small multicellular prothallium of a green colour, and is developed from a macrospore to which it re- mains attached. The development of the prothallium begins inside the macrospore at its pointed apical end, where there is an aggregation of protoplasm in which the nucleus lies. The nucleus divides, and this is followed by the formation of a cell- wall between the two nuclei, cutting off the apical portion of the spore, as a small cell, from the basal larger portion ; this first wall is termed the diapltragru, and it marks off that portion of the macrospore which gives rise to the prothallium from that portion which takes no part in the process. The small cell then un< repeated division to form the pro- thallium which consists, in the Salvi- niacese, of several layers of cells (at least in the middle region), and in the Marsileaceae of two layers only. As the prothallium developes, the coats of the macrospore split into three valves at the apex, so that now the prothal- lium is in direct relation with the exterior. Whilst in the Marsileacese the prothallium projects but little from the spore, in the Salviniaceae (especially Salvinia), where it is larger, the greater part of it is outside. R FIG. 226.— Gametophyte of Sal- vinia (x 60). A Macrosporangium m with a germinated macrospore sp No cell-formation takes place in the (dotted outline) ; pt female pro- larger basal portion of the macrospore SEyJSfiLT 71 below the diaphragm, though nuclear microspore isolated from a micro- division has been observed in Azolla. sporangium ;8p spore -,pt male pro- rn,. -n T_ „„ . , thallium ; a antheridia. Ihis eventually becomes filled with starch and other nutritive substances for the nutrition of the embryo. The female organ, or archegonium, differs in no essential feature of structure or development from that of the homosporous lepto- sporangiate Ferns. In the Marsileacese, only a single archegonium is developed ; it arises from a cell in the middle of the upper sur- face of the prothallium : in Azolla, if the archegonium does not become fertilised, a few more archegonia may be subsequently de- veloped : in Salvinia, at least three archegonia are always formed, 380 PART IV. — CLASSIFICATION. the first anteriorly in the middle line, the second and third one on each side of the first. If none of these become fertilised, the pro- thallium resumes its growth both in breadth and length, and a second row of archegonia is formed in front of the first. Similarly a third and a fourth row, with an increased number of archegonia in each (seven or more), may be successively produced. In Pilu- laria also the prothallium grows to a considerable size if the arche- gonium is not fertilised, though no more archegonia are formed. The female cell, or oosphere, developed in each archegonium, does not require any special description. Order 7. Salviniaceae : microsporangia and macrosporaiigia in distinct sori, each sorus being covered by an indusium ; the spores are surrounded by spongy mucilage, forming an episporium or perinium round the in- dividual macrospores, and holding all the microspores together within the sporangium (Salvinia) or in several groups or massulse (Azolla) ; the female prothallium is relatively large and bears several archegonia. No species of Azolla is European: Salvinia natans occurs in Southern Europe. OrderS. Marsileaceae : microsporangia and macrosporangia in the same sorus, a number of sori being enclosed in the specially modified sporo- phyll-segment, the whole forming a sporocarp ; each spore is invested by a mucilaginous prismatic perinium : the female prothallium is relatively small, and bears but a single archegonium. Pilularia globulifera is the one British species belonging to this order. The European species of Marsilea are M. pubescens, which occurs in the Mediterranean region, and M. quadrifoliata, in Central Europe. CLASS VI. EQUISETIN^. This class includes, among existing plants, only the homosporous order, Equisetacese ; but there are many extinct fossil forms, some of which are undoubtedly heterosporous. Order 1. Equisetaceae. This order includes the single genus Equisetum (Horsetail). Of the twenty species of this genus, about half are British (E. arvense, maximum, silvaticum, paluntre, limosum, hyemale,trackyodon,varieya- tum, pratense, iitorale). THE SPOUOPHYTE. The utern consists of a horizontal, subterranean, much- branched rhizome ; some of the branches come to the surface, and are the sub-aerial shoots. The rhizome and its branches are very distinctly seg- GROUP III. — PTERIDOPHYTA : EQUISETIN.E. 381 mented into nodes and internodes. At each node is borne a whorl of scale- leaves forming a continuous sheath. The branches, as also the adventi- tious roots, spring from the nodes, a bud being developed in the axil of each leaf, and an adventitious root from the base of each bud ; but in the sub-aerial shoots the rudimentary roots do not grow out, whilst in the subterranean shoots relatively few of the buds grow into branches. In certain species (E. arvenie, silvaticum, maximum, palmfre, etc.) some of the subterranean branches become tuberous. The sub-aerial shoots generally live for one season only, and are her- baceous in texture, but in some species they persist (e.g. E. hiemale, trachyodon, variegatum). They are generally green in colour, and their sur- face is more or less strongly ribbed. Some of these shoots are sterile, whilst others are fertile, bearing the sporangia. In most species the sterile and fertile shoots are alike (Equiseta ho- mophyadica), but in the four species E. arvense, maximum, silvaticum, pratense (Equiseta hete- rophyadica) they are more or less widely dif- ferent. Thus in E. arvense and maximum, the fertile shoots are developed early in the spring, and are un- branched, whereas the copiously branched sterile shoots are not developed till the sum- mer ; moreover the fer- tile shoots are not green. In E. pratense the dif- ference between the PIG. 227. Median longitudinal section of the apical por- tion of a vegetative shoot of Equisetum arvense ; p» apical growing-point ; g bud-rudiment ; g'-g'" stages in the de- velopment of lateral buds; v r> developing adventitious roots on the buds ; TH central ground tissue ; vs develop- ing (common) vascular bundle; n nodal diaphragms, (x 26: after Strasburger ) fertile and sterile shoots is le.^s marked, the former bearing a few whorls of simple branches. In E. silvaticum the fertile shoot has no branches at first, but alter the [shedding of the spores the terminal cone dies off, the 382 PART IV.— CLASSIFICATION. shoot throws out branches, and thus comes to be a sterile shoot. In some species the sub-aerial shoots are generally unbranched (e.g. E. hiemale trachyodon, variegatuni). The leaves are either cataphylls or sporophylls. The cataphylls, repre- senting the foliage-leaves, are borne in whorls at the nodes, having a common attachment, so that they form a leaf-sheaf at each node. They are small and brown in colour. The sporophylls, like the cataphylls, are developed in whorls, but owing to the fact that the inter- nodes between the whorls do not elongate, the sporo- phylls are aggregated into a cone-like flower situated terminally on the shoot (Fig. 228), or less common- ly (e.g. E. palustre) on some of the lateral branches. The leaf-sheath below the cone, marking off the re- productive from the vege- tative region of the shoot, is peculiar, being gener- ally very much reduced, and is termed the ring (see p. 57). Each sporophyll has a small hexagonal lamina which is inserted on the axis of the cone by a short stalk attached to the centre of the inner surface of the lamina. Thus the sporophyll is peltate. It bears on its inner (dorsal) surface a small number (5-10) of sporangia ar- ranged round the leaf- stalk. The sporangia are some- what elongated in form, and are sessile. The wall of the sporangium consists of a single layer of cells with spiral thickening. Dehiscence takes place longitudinally on the side facing the leaf-stalk. The archesporium is usually a single cell from which are derived the mother-cells of the spores, each of which give rise to four spores. The spores, which are all of one kind, are developed tetrahedrally, but lv FIG. 228. — A Upper portion of a fertile branch of Equisetum palustre. v Leaf-sheaths, below which the branches (r) spring; w the uppermost sterile sheath (ring); a the flower; s the peltate sporophylls. B Transverse section of the stem ( x6) : c central cavity ; s the vascular bundles arranged in a circle, each having a carinal cavity, fc ; / the vallecular cavities ; r the ridges. C Sporophyll with sporangia ( x 10) : «t the stalk ; sp the sporangia. I) Diagram of the course taken by the vascular bundles where two internodes meet ; i i the internodes ; fc the node. GROUP III. — PTERIDOPHYTA : E^UISETIfOE. 383 are nearly spherical when ripe. Each spore has two coats, exospore and endospore, and originally a perinium is present. The perinium, as it developes, becomes irregularly thickened in such a way that, when the thin portions are destroyed, the thickened portions remain as four fila- ments, the elaters, all attached at one point only to the spore. These elaters are very hygroscopic. When the air is dry they expand, and stand out stiffly from the spore ; when moistened, they suddenly roll up spirally round the spore. The spores become entangled by their elaters, so that when set free from the sporangium a number of the spores fall to the ground and germinate near together. The roots are all adventitious, though a short-lived primary root is developed. They are developed at the nodes of the rhizome. General Histo- logy. A striking feature in the anatomy of the stem is the pre- sence of large, mainly lysigen- ous, air-cavities : thus, in some species, the rhi- zome has a large centra] cavity in each inter- node (Fig. 230 C, a) ; a similar cavity is present in the internodes of the aerial shoots of nearly all the species (Fig. 230 A, «); the central cavi- ties of successive internodes are shut off from each other by diaphragms at the nodes (Fig. 227 n) : a series of similar cavities occurs always in the cortex, alternating with the vascular bundles internally and with the surface-ribs externally, hence termed vail ecu1 nr cavities (Fig. 230 1>) ; finally, in connexion with each vas- cular bundle, there is a large cavity, the carinal cavity (Fig. 230 c), extend- ing, like the others, from one node to another. The growing-point of the stem, and of its branches, is apical, and has a tetrahedral apical cell (Fig. 229). In the aerial shoots (except specialised fertile shoots of E. arvenie, etc.) FIG. 229.— Growing-point of the stem of ISquisetum. arvensc, in longitudinal section; t apical cell; «'«" successive segments; p anticlinal segment-wall ; m wall dividing the segment into an upper and a lower cell ; pr penclinal wall dividing the segment into an inner and an outer cell ; fff" successive whorls of leaves ; 9 initial cell of a lateral bud. (After Strasburger: x 240.) 384 PART IV.— CLASSIFICATION. there is a considerable development of assimilatory tissue in the cortex, strands of this tissue corresponding in position with the furrows on the surface in which the stomata are developed ; whilst opposite the ridges on the surface there are cortical strands of sclerenchyma. The development of assimilatory tissue in the shoots and branches is correlated with the absence of foliage-leaves, the functions of foliage-leaves having therefore to be discharged by the shoots and branches. The epidermal cell-walls are impregnated with silica. Within the cortex, and almost abutting upon the large central cavity, is a ring of distinct vascular bundles which run down the internode from -End. FIG. 230. — Portions of transverse sections of stems of species of Equisetum (after Pfltzer: x 36). S rhizome of E. litorale ; C rhizome of E. sihaticum ; A aerial stem of E. palustre, in which the structure is the same as in C, but the markings of the internal en- dodermal layer are not developed; a central cavity ; 6 vallecular cavities in the cortex ; c carinal cavities in the vascular bundles ; JGnd. endodermis. the leaves at the node. Each bundle is collateral, closed and common, with very rudimentary xylem consisting of the few annular vessels of the protoxylem and of two small groups of scalariform tracheids. The root grows in length by means of a tetrahedral apical cell ; in its mode of growth, development of root-cap, etc., it essentially resembles that of the Ferns. Its structure is rather peculiar : — the vascular cylinder consists (usually) of three wood-bundles and three bast-bundles, and is invested by two layers of sheathing-cells, the outer of which has the characteristic marks of an endodermis, whilst the inner appears to be a pericycle and gives rise to the growing-points of the lateral roots ; how- ever, the inner layer is, as a matter of fact, not a pericycle, but the inner- GROUP III. — PTERIDOPHYTA : EQUISETIN^E. 385 most layer of the cortex, the endodermis being the last layer but one of the cortex (p. 115). Hence it appears that here, as in the Ferns, the lateral roots spring from the innermost layer of the cortex. There is no pericycle in the root of Equisetum. Embryogeny of the Sporophyte. The oospore is divided by a transverse basal wall, and then becomes segmented into octants, as in the Filicinae. Of the four epibasal octants, one gives rise to the growing-point of the primary stem ; two to the first cotyledon ; and the fourth to the second cotyledon : the two cotyledons cohere to form a leaf-sheath round the young primary stem. Of the four hypobasal octants, one gives rise to the growing-point of the primary root, and two to the foot. The primary stem grows erect, and its leaf-sheaths are three-toothed, the three leaves corresponding to the three segments cut off from the apical cell of the stem ; it branches at its base ; stouter shoots with an increasing number of teeth in the leaf-sheaths are successively produced, and eventually a branch is produced which becomes the perennial sub- terranean rhizome. The GAMETOPHYTK is a green, dorsiventral, lobed prothallium which be- comes quite free from the spore. The prothallia are generally dioecious, the female prothallia being larger than the male ; but the distinction of sex is not absolute, for the female prothallia may eventually bear male organs, and the male prothallia female organs ; it appears to depend largely on conditions of nutrition. The germinating spore divides into two cells : one of these contains no chloroplastids, and grows out into a root-hair, the other contains chloro- plastids, and grows and divides to form the first lobe of the prothallium, which branches as its development proceeds, some of the branches of the female prothallia growing erect. On its under surface the prothal- lium bears numerous root-hairs. At first the prothallium consists throughout of a single layer of cells ; in the female prothallium, however, one of the lobes becomes thick and fleshy, consisting of several layers of cells formed by repeated horizontal cell-division, and this constitutes the archegoniophore. The male organ, or antheridium, is developed from a single cell of the margin of the male prothallium : this cell undergoes repeated division, with the result that a wall, consisting of a single layer of cells, is formed surrounding a central cell from which, by subsequent divisions, the numerous mother-cells of the male cells are developed : the antheridium eventually dehisces by the separation of the cells forming the roof, in consequence of the swelling-up of the contents of the antheridium. The male prothallium bears several antheridia, one being developed terminally on each lobe, and others in succession on the lateral margins. The male cell is a spermatozoid, which is larger than that of any other Pteridophyta ; it has only two or three coils, and bears a tuft of short cilia at its anterior pointed end. The/e»i«?e organ, or archegonium, resembles, in all essential features of its structure and development, that of the typical Filicinse : a distinctive peculiarity -is afforded by the long recurved terminal cells of the neck, M.B. C C PART IV. — CLASSIFICATION. and by the relatively small neck-canal-cell. Each prothallium bears a number of these organs : they are developed each from an anterior marginal cell, and, as the prothallium continues to grow, the organs come to lie on its upper surface. The female cell is an oosphere, and calls for no special remark. CLASS VII.— LYCOPODIN.E. SUB-CLASS HOMOSPORE^E. Order 1. Lycopodiaceae. This order contains the two genera Lyco- podium and Phylloglossum : the former is a large genus, six species being British (L. Selago, inundatum, annot- inum, davatum, alpinum, complan- atum} and com- monly termed Club-mosses : the genus Phylloglos- sum has a single species (P. Drum- mondii) found in Australia and New Zealand. THESPOKOPHYTE. The stem. In Ly- copodium the.stem is slender and much branched, erect (e.g. L. Sela- go), or growing FIG. 231.-Portion of Zycopodium vlavatum, somewhat smaller horizontally on than nat. size : s, the cone-like flower. B a single sporophyll (b) the surface of tJ from the cone, bearing a sporangium sp, which has dehisced ( x 10). ground (e.g. L. davatum [Fig.28i] annotinum), or beneath the surface as a rhizome (e.g. L. alpinum, com- planatum) : the branching is in some cases dichotomous, in others mon- opodial ; it may take place in all directions, or in one plane only (L, complanatum). The leaves. In some species of Lycopodium all the leaves are alike (e.g. L. Selago) ; but in most species the foliage-leaves and the sporophylls are more or less widely different. The foliage-leaves are small and very numerous in Lycopodium ; their arrangement is various, whorled, or spiral, or in decussate opposite pairs (L. complanatum, etc.) : in the last case there is heterophylly (p. 41), as the de- cussate leaves on the flattened sterile shoots vary in size, those on the lateral GROUP III. — PTERIDOPHYTA : LYCOPODIX.E. 387 margins of the shoots being larger than those on the flattened surfaces. The leaves are sessile, simple, and have a rudimentary midrib. The sporophylls present considerable variety. _ In L. Selago and its allies they are quite like the foliage-leaves ; in most species of Lycopodium (e.g. L.inundatum, clacatum, Phlegmaria, etc.) the clearly differentiated sporophylls are aggregated into terminal cone-like flowers, and in some cases the branch bearing the cone grows out into a long peduncle (L. dauatum, cotnplanatum, etc.). The sporangia are borne singly on the upper surface of the sporophylls near their base. The archesporium consists of a single row or of a few rows of cells which, by their division, give rise to the mother-cells of the spores. The sporangia are unilocular, somewhat reniform in shape, and (in Lycopodium) seated on a short broad stalk ; they dehisce by a longi- tudinal slit. The spores are all of one kind, and are tetranedral in form ; they have the ordinary structure. The roots are all adventitious. In the erect species of Lycopodium they spring as a tuft from the basal end of the stem : in the procumbent species they are born singly on the under surface of the stem. The roots branch dichotomously in alternate planes. General Histology. The growing-point of stem and root alike consists, in Lycopodium, of small-celled meristem, no apical cell being present. Both stem and root have an axial vascular cylinder consisting of alter- nating bundles of wood and of bast arranged radially (p. 125) : thus the stem is monostelic, and its structure only differs from that of the stouter roots in respect of the larger number of bundles present: in smaller roots there is only one wood- and one bast-bundle. In the stem the bundles fre- quently anastomose, more especially in the erect-growing species, so that transverse sections taken at different levels present diverse pictures. The wood-bundles consist of scalariform tracheids, with the exception of the protoxylem. Neither stem nor root grows in thickness. In both stem and root there is an endodermis, the cells of which have the characteristic marking when young, but eventually become thick-walled and corky. The leaves of Lycopodium are of verj- simple structure ; they usually have stomata on both surfaces. Embryogeny of the Sporophyte. The early stages have only been observed in L. Phlegmaria, where the oospore is divided by a transverse basal wall, the cell next to the neck of the archegonium being the hypobasal cell, and the lower cell the epibasal. The hypobasal cell developes into a short, cylindrical, usually unicellular, suspensor (p. 347). The somewhat hemi- spherical epibasal cell becomes segmented into four octants "by two walls at right angles to each other and to the basal wall ; and then the octants are divided transversely, by a wall at right angles to the two preceding, into two tiers or stages of four cells each. Of these two tiers the lower forms a short hypocotyl (as in Salviuia, see p. 377) which is commonly (but erroneously) called the foot, though it is morphologically quite different from the foot of the Filicinse and Equisetinse. The upper tier of cells gives rise to the first leaf or cotyledon, and to the primary stem. The 388 PART IV.— CLASSIFICATION. first root eventually springs adventitiously from cells belonging to the upper tier, below the cotyledon : its origin is exogenous. (Compare em- bryogeny of Selaginella, p. 393). Vegetative Propagation. In the creeping forms, as the main stems die off, the younger branches become independent and constitute new individuals. In some species there are gemmae, as in L. Selago, where they are borne on the stem, and are modified leafy branches ; and as in L. cernuum, where they are small tubercles borne on the roots. THE GAMETOPHYTK. In so far as the gametophyte has been investigated, it is a monoecious prothallium, either containing chlorophyll (L. inun- datum and cernuum), or destitute of chlorophyll (L. annotinum and Phleg- maria) and saprophytic. The morphology of the prothallium offers considerable variety. In some species (e.g. L. inundatum) the prothallium is tuberous ; its base is embedded in the soil, and bears root-hairs ; its apex projects above the surface and bears a tuft of green leafy lobes : the sexual organs are de- veloped in a zone from a layer of tissue which long remains merismatic and which is situated just below the apical tuft of lobes ; occasionally some antheridia are developed on the lobes. Whilst in these species the prothallium is very small and simple, in L. PJilegmaria and some other species it is considerably larger and more complex. It consists here of a cylindrical, monopodially-branched body, with apical growing-points similar in structure to those of the sporophyte. The superficial layer of cells, representing an epidermis, gives rise to a number of root-hairs. The sexual organs are developed on special branches, yametophores, though antheridia sometimes occur on the vegetative branches ; the gametophores are shorter and thicker than the vegetative branches, sometimes even tuberous, and on the upper surface bear the sexual organs surrounded by stout multicellular hairs, paraphyses. The male organs (antheridia) are sunk in the tissue of the prothallium : they resemble those of the Eusporangiate Filicirise. Their development precedes that of the female organs. The male cells are spermatozoids, oval in shape, and have two cilia. The female organs (archegoma) have short necks which project but little above the surface of the prothallium ; they have the structure usual among Pteridophyta. The female cell (oosphere) requires no special description. In consequence of its position and of its mode of development, the embryo is forced downwards into the tissue of the fleshy prothallium, being anchored, as it were, at one end by the suspensor (see p. 347). As it grows it destroys the cells of the prothallium with which it comes into contact, and absorbs the nutritive substances stored in these cells by means of the so-called foot, the superficial cells of which grow out into short papillae. In its further growth the embryo becomes more and more curved until it regains the surface of the prothallium and projects. In L. Phleymaria the embryo remains for some time enclosed in a sac, the calyptra, formed by active growth of the prothallial tissue. The life of the prothallium is short and closes, in most cases, with the GROUP III. — PTERIDOPHYTA : LYCOPODIN.E. 389 development of an embryo from the oospore, but in L. Phleymaria it seems to persist from one season to another. In the latter species the prothallia are multiplied vegetatively by the isolation of branches, as also by small multicellular bulbils. Order 2. Psilotaceae. This order consists of the two genera Psilotum and Tmesipteris ; of these the former is widely distributed in the tropics ; the latter is confined to Australia, New Zealand, and Polynesia, and lives epiphytically, and perhaps parasitically, on the trunks of Tree- Ferns. THE SPOROPHYTE. The most striking feature in the morphology of these plants is the total absence of roots, the functions of these organs being performed by specially adapted stem-branches bearing minute scale- leaves, and covered with root-hairs. The stem. In Psilotum the subterranean shoots have unlimited apical growth : they are much branched, apparently dichotomously, but it seems probable that the branching is really lateral. The subaerial shoots generally arise as lateral branches on the subterranean shoots : they have limited apical growth; they are branched, the mode of branch- ing being probably the same as that of the subterranean shoots ; and they bear small scattered leaves. The stem of Tmesipteris appears to agree in all essential morphological points with that of Psilotum ; but with this conspicuous difference, that it is much less branched. The leaves. In both genera the leaves of the subaerial shoots are of two kinds. In Psilotum the vegetative leaves are minute scales, whereas in Tmesipteris they are relatively well-developed as foliage-leaves: they are simple and sessile. The sporophylls, on the contrary, are petiolate and bilobed in both genera, a single sporangium being borne on the upper surface of each sporophyll just at the junction of the basis of the two lobes : they are not borne in cones. The i>poran, ' or hermaphrodite ; but it frequently contains but one kind of sporangium 'unisexual) : in the latter case there are two kinds of flowers, microsporangiate and macrosporangiate, which may be borne by the same individual, when thev are said to be diclinous and monoecious; or by two distinct individuals, when they are dioecious (see p. 61). Occasionally the same plant produces both monoclinous and unisexual flowers, when it is said to be polygam- ous. The microsporangiate flowers are frequently termed stamin- ate, and the macrosporangiate flowers carpellary < p. 56 > : the former are indicated by the sign $ , the latter by the the sign £ , and monoclinous flowers by the sign £ . In the Gymnosperms the flower always has bnt one kind of sporangium : in the Angio- sperms it generally, though by no means always, has both kinds. The flower of the Gymnosperms is nearly always destitute of a perianth. The special morphology of the Perianth is dealt with under the Angiospermae, in which class it attains its highest develop- ment. The Sporophyfo are of two kinds : microsporophylls. otherwise known as stamens ; and macrosporophylte, otherwise known as ctirpete : the former bear exclusively microsporangia, the latter exclusively macrosporangia. The sporophylls present considerable varietv of form, and are on the whole more highly specialised than in any of the Pteridophyta. The microsporophyll, or stamen (see p. 56 1, in its most highly specialised form, consists of a stalk of varying length, theJUament bearing a terminal structure, the an*h( r, which is a sorus of one or more microsporangia embedded in more or less placental tissue. lu the less highly organized Phanerogams \e.g. most Gymno- s perms;, the microsporophylls are morphologically simpler, having the general character of sessile or shortly-stalked scaly leaves. The macrosporophyU, or carpel, bears usually macrosporangia see p. 5»3 . In the Angiosperms the carpel, either by itself or by cohesion with others, forms a closed cavity, the onary, which is frequently prolonged at its apex into a longer or shorter process, the ftyh\ bearing at its summit a glandular surface, the stigtaa : sometimes the style is absent, so that the stigma is sessile on the ovary : within the ovary the macrosporangia are developed. In the Gymnosperms, the macrosporophylls ) : this is frequently found in Monocotyledons, such as Hemerocallis, Ornithogalum, Alstroemeria, Juncacese. It has been ascertained, however, that in many cases (various Solanaceae and Boragiiiacese) the so-called scorpioid cymes are monopodial : the axis is therefore not a pseud-axis but a true one, and the inflorescence must be regarded as a unilateral raceme. II. Compound cymose inflorescences arise on the one hand from the reduc- 442 PART IV. — CLASSIFICATION. tion of the ramification in the higher orders, as, for instance, when the secondary members of a cyme are not cymes, but dichasia ; these are dichasial cymes ; they occur in many Euphorbias : again, when dichasia terminate in scorpioid or helicoid cymes. On the other hand it sometimes occurs that helicoid cymes are combined to form scorpioid cymes, as in Geranium. C. Compound racemose and cymose inflorescences. It may occur that a compound inflorescence changes in type in the different ordei-s of ramification. Thus the branches of the first order may exhibit a race- mose arrangement, and those of the second a cymose arrangement, as in the dichasial racemes of many Euphorbias (e.g. E.Esula, amygdaloides], in the scorpioid racemes of the Horse-Chestnut, and in the helicoid capitula of many species of Allium. On the other hand the branches of the first order may have a cymose, and those of the second a race- mose arrangement ; for instance, the helicoid cymes of capitula in Cichorium. Finally, there are certain terms used in describing inflorescences which refer only to the general external appearance rather than to the mode of formation of the inflorescence : thus, the panicle is a pyramidal inflorescence generally of the racemose type, at least in its first ramification : the corymb is a racemose inflorescence of which all the ultimate ramifications lie in one plane and bear flowers, e.g. the Elder, many Cruciferse : the amentum (catkin) is a simple or compound spicate inflorescence, usually pendulous and elongated, bearing inconspicuous unisexual flowers, which falls off entire from the plant when the flowering is over. Of cymose inflorescences there is the fascicle, consisting of a number of flowers on pedicels of equal length (Sweet William); the glomerule (Nettle and Box) or verticillaster (many Labiatse), consisting of a few sessile or shortly pedicillate flowers ; and the anthela, which is a compound inflorescence, in which the branches of the first order are gradually shorter from below upwards (or rather from without inwards), as in Juncaceae. To a floral axis arising from the ground, with no leaves, or with only a few bracts, bearing a single flower or a more or less complex inflorescence, the term scape is applied. The Bracts (p. 43) are leaves borne on the inflorescence, in the axils of which the flowers are developed : there may be a single large bract, termed a spathc, enclosing the whole inflorescence, as in Palms and in the Arum Lily (Richardia cKtliiopica) where the bract is white ; or the bracts may be brightly coloured (petaloid), as in Poinsettia and other Euphorbiacese where they are red, and in Leycesteria formosa, Melampyrum, etc. ; or the bracts may be scaly, forming an involucre round the inflorescence as in the Com- positae : the glumes of the Grasses are scaly bracts. The bracts are frequently not very unlike the foliage-leaves, differing from them mainly in form and size. GROUP V. — ANGIOSPERM.E. 443 The portion of the floral axis below the flower (i.e. the peduncle or the pedicel) commonly bears one or more bracteoles or prophylla . In most Monocotyledons there is a single posterior prophyllum, whilst in most Dicotyledons there are two lateral prophylla. In some cases several bracteoles are arranged in a whorl, forming an epicalyx, either close beneath the flower (as in Malva, Anemone Hcpatica, Dipsacus, or at some distance below it (other species of Anemone). In some plants (Nyctaginaceae) the epicalyx may become an involucre enclosing several flowers ; this is due to the fact that flowers are developed in the axils of some of the bracte- oles of the terminal flower. Though they are generally green, the bracteoles are sometimes brightly coloured, as in some Amarantacese and Nyctaginacese ; or scaly, as the lodicules of Grasses. The Flower (p. 55) is a shoot of limited growth, with un- developed or but slightly de- veloped internodes, bearing, as a rule, both perianth-leaves and sporophylls on the some- what shortened and expanded terminal portion of the axis which is the receptacle or torus. The perianth -leaves are generally differentiated into two series : an outer, of usually rather small green leaves, the sepals, constituting the calyx : an inner, of usually conspicuous brightly coloured leaves, the petals, constituting the corolla. The flower is usually mono- clinpus (hermaphrodite) ; but is not infrequently unisexual, when it is diclinous, or even dioecious. The sporangia, with but few exceptions, are borne upon sporo- phylls (see p. 56) : the microsporophylls (stamens) constitute the andrcecium, the macrosporophylls the yyna:ceum, of the flower. It occasionally happens, that one or more of the internodes within the flower may be developed to some extent : for instance, PIG. 260.— Diagram of an angiospermous flower: Ke calyx; K corolla ; / filament of stamen; a anther with two pollen-sacs in each half which are opened, showing the pollen- grains (p). On the stigma (n) are pollen- grains (p) which have germinated; the pollen-tube (p») penetrates the style (g) as far as the cavity of the ovary (F), reaching the ovule (S) ; t the integument of the ovule ; em the embryo-sac ; E the oosphere. 444 PART IV.— CLASSIFICATION. the internode (termed anthophore) between the calyx and the corolla, as in Lychnis and some other Caryophyllacese ; that (termed gono- phorc) between the corolla and the androecium, as in the Passion- Flower, and in Orchids where the styles adhere to it forming the gynostemium or column ; that (termed gynophore) between the andrcesium and the gynseceum, as in some Gentians and some Cruciferse. When the axis grows, as is usually the case, equally in all parts, the gynseceum, being nearest to its apex, is the uppermost part of the flower. When this is the case its insertion is above that of the androecium and perianth (Fig. 261 fl), and the ovary is said to be superior and the flower hypogynous, as in Ranunculus, Papaver, Lilium, and Primula. But in a great number of plants the perianth and androecium are raised by the intercalary growth of a FIG. 261.— Diagram of H hypogynous; P perigynous; E epigynous flowers; a axis fc calyx ; c corolla ; sstamsns; /carpels; n stigma; sfc ovule. lower portion of the axis (as represented by the outer portion of the torus) and stand on a circular rim surrounding the apex of the axis which lies at a lower level. Of this condition two different forms occur : — in the one, the carpels are inserted in the depression at the apex of the axis (Fig. 261 P), and there form one or more ovaries free from it, primarily at least, though they may sub- sequently become adherent to it ; in such cases, as in the Rose and Apple, the flower is said to be perigynous : in the other, the car- pels spring from the upper rim of the cavity which is formed by the axis itself and simply cover it in at the top ; such flowers are said to be epigynous, and the ovary to be inferior, e.g. Gfourds and Umbelliferae (Fig. 261 E). Many transitional forms between these two extremes are found. Stipules are sometimes developed in connexion with the floral GROUP V.— ANGIOSPERMJE. 445 leaves ; thus in sDme Rosaceae (Poteatilla, Comarum, Geum, Al- chemilla) the stipules of the sepals form a calyculus or epicalyx : stipules are developed in connexion with the petals of some Sapotacese (Dipholis, Mimusops) : and in connexion with the stamens of Allium, Ornithogalum, some Zygophyllaceae, etc. The Phyllotaxy of the Floicer. The floral leaves, like the foliage-leaves on the stem (see p. 10), are frequently arranged spirally, (e.g. Calycanthus, Anemone, Trollius) when the flower is acyclic. The most common divergence is f, but higher divergences also occur, especially in the androecium, when numerous small organs are inserted upon an expanded axis (e.g. Ranunculus). In the spiral or acyclic flower there is either no well-marked dis- tinction of the various series, that is, the members of the calyx, corolla, and androecium, are connected by intermediate forms (e.g. Nymphsea) ; or the various series are sharply defined, each series taking up one or more turns of the spiral. In most cases the floral leaves are arranged in whorls, that is, the flowers are cyclic. Cyclic flowers are connected by inter- mediate forms with the acyclic, especially through pentamerous forms. Thus some pentamerous flowers are hemicyclic, that is, some of their floral leaves are arranged spirally, and the others in whorls. Instances of a spiral perianth combined with cyclic sporophylls are afforded by those flowers in which the members of the perianth, calyx, or corolla are developed in f succession, and the prefloration is quincuncial (see p. 43) ; the perianth is spiral in the flowers of the Cannabinacese and Chenopodiacese ; the calyx is spiral in the flowers of the Bindweed (Calystegia Septum), the Rose, some Boraginacese (Cerinthe, Echium, etc.), Geraniacese, Oxalidaceae, Linaceae, Caryophyllacese, and many other dicotyle- donous orders ; both calyx and corolla are spiral in Camellia, though the phyllotaxy is not -£. In other cases, the sporophylls are spirally arranged, whilst the perianth-leaves are cyclic. For instance, in Magnolia, Ranunculus, and Helleborus, both stamens and carpels are spirally arranged ; and in Delphinium and Acoui- tum, the stamens only. Closely related to the foregoing cases of £ phyllotaxy— occurring in fact not only in flowers of closely allied species, but also in flowers of the same species — are certain of the typical forms of cyclic arrangement in which each series (whether perianth, calyx, corolla, or androecium), instead of consisting of five floral leaves, taking up two turns of a spiral with a divergence of -* , consists of 446 PART IV. — CLASSIFICATION. four or six leaves arranged in two whorls, consisting respectively of two or three leaves. For purposes of comparative description, it is convenient to re- gard each turn of the spiral in an acyclic or a hemic}7clic flower as equivalent to a whorl : thus a well-defined series with f arrange- ment would represent two whorls. As in the case of the foliage-leaves (see p. 9), so in that of the floral leaves, the order of development is as a rule acropetal : hence each whorl of the flower is developed later than the one external to it, and earlier than the one internal to it. When, however, a series of floral organs is becoming degenerate, its development is retarded ; for instance, in the Compositse, Valerianaceae, and Um- belliferse, the degenerate calyx is developed after the corolla, or even after the androecium. The members of each whorl may be developed either simultaneously or successively. In their arrangement, also, the floral leaves resemble the foliage- leaves. When, in an acyclic or hemicyclic flower, the spiral is continuous with the same divergence from one series of floral organs to another, the members of the successive series lie on the same radii drawn from the centre of the flower, that is, they are directly superposed. A good example of this is afforded by the ter- minal flower of the inflorescence of Berberis (Fig. 262 ; occasionally in Epimedium, and also in Gagea among Monocotyledons), where the stamens, petals, and sepals are all directly superposed. When, on on the other hand, the divergence varies from one series to another, direct superposition does not occur, but some form of alternation, as is generally the case in acyclic flowers : for instance, the calyx of certain (pentamerous) forms of Anemone and other Ramin- culacese is arranged with a % divergence, whereas the divergence of the stamens is y\ or T8T. In hemicyclic flowers with a simple spiral perianth and cyclic stamens (e.g. Cannabinacese, Cheno- podiacese), the stamens are superposed on the perianth-leaves. When the floral leaves are in whorls consisting of equal numbers of members, the general rule is that the members of the successive whorls alternate with each other : thus, in a flower with calyx, corolla, androecium, and gynaeceum, each consisting of a single whorl of five members, the petals alternate with the sepals, the stamens with the petals, and the carpels with the stamens ; and if radii be drawn from the centre of the flower, it will be seen that the stamens are opposite to the sepals and the carpels to the petals, or more briefly, that the stamens are antiscpalous and the carpels GROUP V.— ANGIOSPERM.E. 447 are antipetalous. This is not, however, a case of direct super- position, since the corolla intervenes between the androecium and the calyx, and the androecium intervenes between the gynseceum and the corolla. There are, however, certain cases in which this law of alter- nation does not prevail, in which, that is, the members of successive whorls are directly superposed. Tor instance, the (4-5) stamens are directly antipetalous in several natural orders (Primulacese, Plumbaginaceae, Ampelidaceae, Rhamnaceae) ; again, in some Campanulacese (e.g. Campanula Medium, Fig. 263) the (5) carpels are directly superposed on the stamens. The Floral Diagram. — These various arrangements of the floral leaves, like those of the foliage-leaves, are most clearly represented FIG. 262.— Floral diagram (ground-plan) of an acyclic flower, with \ divergence in the calyx, corolla, and androeciam (ter- minal flower of Berberis : after Eichler). Fio. 263.— Floral diagram of Cam- panula Medium : the five carpels are directly superposed on the stamens. (After Eichler). by means of diagrams (see p. 13). In & floral diagram, the calyx lies externally, and the gynseceum, as being the uppermost series of organs (even in epigyhous flowers) lies most internally. In order to be able readily to distinguish the various series, symbols are used which recall some peculiarity of their form : thus the mid-rib of the sepals is indicated, and, in the case of the stamens, the anthers. If only such relations of position as can be actually observed in a flower are indicated in the diagram, a simple empirical diagram is the result. If, however, the results of the investigation of the development of the flower and of the comparison of it with others be borne in mind, a general plan of arrangement will be detected, and the individual peculiarities of arrangement, quite apart from any variation in the form of the organs, will be seen to be due either to the suppression of one or more whorls or of one or more 448 PART IV. — CLASSIFICATION. members of a whorl, or, more rarely, to a multiplication of the whorls or of their members. If, however, the organs which are absent, but which should typically be present, be indicated in the empirical diagram by dots, it becomes a theoretical diagram. In this way it is possible to arrive at general types on which large numbers of flowers are constructed. Fig. 264, for instance, is the empirical diagram of the flower of the Lily, and it is at the same time the type on which the flower of Grasses (Fig. 265) is con- structed in which certain members are suppressed. lu constructing a floral diagram the position of the main axis should be indicated by a dot placed above the diagram : the bract, which would of course be exactly opposite to it, may or may not be indicated : the side of the flower toward the main axis is said to be posterior, and that toward the subtending bract, anterior. A plane which passes through the flower and also through the main stem and the median line of the bract is termed the 'median FIG. 264.— Floral Diagram Fra. 265.— Floral Diagram FIG. 266.— Floral Dia- of a Lily. of a Grass. gram of a Crucifer ; the median stamens are duplicated. plane or section of the flower : the plane which cuts the median plane at right angles is the lateral plane or section : and the plane which bisects the angles made by the intersection of the median and lateral planes is the diagonal plane or section : any plane other than these is said to be oblique. By means of these conceptions the position of the parts of a flower may be accurately indicated : thus, in describing the flower of the Cruciferse (Fig. 266), the two external sepals lie in the median plane ; the two inner sepals, the two outer stamens, and the two carpels, in the lateral plane ; whilst the petals and the four inner stamens lie in the diagonal planes. The number and the relations of the different parts of the flower may be indicated not by diagrams only, but also by formulae in which, as in the diagrams, for the sake of clearness, all the GROUP V.— AXGIOSPERM.E. 449 peculiarities of form are overlooked. Thus the diagram Fig. 264 may be expressed by the formula KB, C\ A3 + 3, (?«, which means that the calyx jfiT, and the corolla C, each consist of a single whorl of three members, the androecium of two whorls each of three members, and the gynaeceum of one whorl of three members, all in regular alternation. When one whorl is superposed on another, the superposition is indicated in the formula by a line | between the whorls. If the number of members in any whorl is variable, the letter n is used instead of a number. Thus, for instance, Kn, C*n, An + n, Go. is the theoretical formula' of most Monocotyledons. The absence of a whorl is expressed by a cypher 0, and of individual members by the number of those actually present. Thus the formula for the flower of a Grass (Fig. 2G5) is AT), (70, A3 + 0, Gl. Superior and inferior ovaries are indicated by a stroke below or above the corresponding figure, and duplica- tion by the exponent 2 ; thus the diagram Fig. 266 is represented by the formula K2 + 2, C x 4, A2 + 22, G™, the x after C in- dicating that the position of the petals is diagonal, i.e. that the four petals alternate with the four sepals, as if the latter all belonged to the same whorl. The bracket in which the number of the carpels of the gynseceum G is enclosed, indicates that the members thus bracketed are coherent. Staminodia may be dis- tinguished by a -j- before the figure. When the perianth is not differentiated into calyx and corolla, it is expressed by the letter P: thus the formula for the flower of Chenopodium is P5 | A5 diplostemonous. Oligotaxy, or a decrease in the typical number of whorls in a flower, is frequently FIG. 268.— Diagram of due to suppression. For instance, owing to the suppression of one whorl of stamens in some Monocotyledons, either the outer (some Hfemadoraceae, also Cypripedium), or the inner (Iridaceae, most Orchidacea?\ the androecium is monocyclic. In some cases a whole series is suppressed : for instance the corolla may be absent (e.g. Glaux, among the Primulacese ; Alchemilla, Sanguisorba, among the Rosaceae : some Caryophyllaceae, such as Sagina apetala, Scleranthus, etc.) : or the androscium or gyna?ceum (diclinous or dioecious flowers, such as those of Sedum Rhodiola, Rhamnus cathartica, Hydrocharidaceae, ray-florets of Compositse, etc.) : or the whole perianth (Frajcinus excelsior}. Although it is true that both oligotaxy and oligomery are frequently due to sujjpression, in the one case of one or more whorls, in the other of one or more members of a whorl, it must not be assumed that this is the only possible explanation. On the contrary, it is very probable that the simple structure of the flower in some plants (c.g. Urticales and Amentales among GROUP V.— ANGIOSPERM.E. 453 Dicotyledons) is not the result of suppression, but is itself typical : in other words, these flowers are probably to be regarded, not as reduced, but as primitive, belonging to plailts which are, it may be, of a relatively low type among Phanerogams, but which are on the up-grade^ and not on the down-grade of organisation. The Symmetry of the Flower. The flower presents all the varieties of symmetry which are discussed in Part I. (p. 4) ; these are mainly determined by the number and the relative develop- ment of the floral leaves, and in a few cases by the development of the floral axis or receptacle. The symmetry may be radial or actinomorphic. When an eucyclic flower is also regular, that is, when the members of each whorl are similar to each other in size and form, it can be divided into symmetrical halves by sections made in two or more planes, the halves produced by section in one plane being similar to those produced by section in one or more other planes. Such a flower is poly symmetrical (see p. 6). The number of these planes of symmetry de- pends upon the numerical constitution of the flower. Thus a regular eucyclic trimerous flower (e.g. many Monocotyledons) can be so divided in three planes, the median and the two diagonals, that all the three pairs of resulting halves are exactly alike (Fig. 269 B). Similarly, the pentamerous flower of Primula, Geranium, species of Campanula, is divisible in five planes (Fig. 269 A}- But where the flower is tetramerous (e.g. Fuchsia, Euonynni* europceus), there are but two planes of section, the median and the lateral, which will give exactly similar halves, though the flower is also symmetrically but diversely divisible in the diagonal planes (Fig. 270 A); or, again, where the flower is hexamerous (e.g. species of SeduuVt it is symmetrically divisible in twelve planes, but the halves produced by the section in six of the planes are unlike those produced by section in the other six planes. A B FIG. 239.— A Diagram of the pcntameroua flower of Primula, showing the five planes of symmetry; the stamens are antipetalons ; there are no pro- phylla. B Diagram of the trimerons flower of Lilium, showing the three planes of symmetry. (After Eichler.) 454 PART IV. — CLASSIFICATION. The symmetry may be isobilateral ; in this case the flower is divisible into symmetrical halves in two planes, but the halves produced by section in one plane are unlike those produced by section in the other plane. Thus, a regular eucyclic dimerous flower (e.g. Circcea lutetiana, Fig. 270 B ; Fraxinus dipctala\ is symmetrically divisible in the median and lateral planes, but the halves produced by the median section differ from those produced ~by the lateral section. This is true also of some regular hetero- cyclic flowers, such as those of the Cruciferas, Jasminum, Olea euro- pcea, Cornus, Hamamelis, the whorls of which are 2- or 4-merous, and of the somewhat peculiar flower of Dicentra. The symmetry may be zygomorpMc, that is, the flower may be monosymmetrical, there being only one plane in which it is sym- metrically divisible. Monosymmetry is characteristic of irregular flowers, whether eucyclic or heterocyclic ; of flow- ers, that is, in which the members of one or more whorls differ in various respects among themselves, accompanied frequently by a reduction in the typical number of members in one or other of the whorls, fre- quently of the androacium : it is, in fact, to irregular flowers that the term zygomorphic is specially applied in Descriptive Botany. Such a flower usually presents a clear distinction into two diverse portions, an anterior and a posterior, separated by the lateral plane, whilst the two lateral halves about the median plane are symmetrical ; hence it is clearly dorsiventral (Fig. 271). Dorsiventrality is presented by some flowers which, so far as their early development is concerned, or even so far as is shown by their floral diagram, are actinomorphic, isobilateral, or simply zy- gomorphic, the dorsiventrality being due to the subsequent irregu- lar development of some of the floral leaves ; as in some eucyclic flowers (e.g. among Monocotyledons, Amaryllis, Gladiolus ; among Dicotyledons, Dictamnus, and other Rutese, species of Impatiens, Pelargonium), and in some heterocyclic flowers (e.g. some Scrophu- FIG. 270 — X Diagram of the tetramerons flower of Fuchsia, showing the four planes of symmetry. B Diagram of the dimerous flower of Circsea, show- ing isobilateral symmetry. GROUP V.— AXGIOSPERM.E. 455 lariacese, Labiatse, some Caprifoliacese, Violacese, Echium, Lobelia, Orchidaceae, the marginal flowers of the inflorescences in some Umbelliferae and the ray-florets of some Compositae). The degree of irregularity in these flowers varies widely; the irregularity may be very slight, due to the more active growth of the leaves (perianth-leaves only, or stamens also) of one half of the flower, either the posterior (e.g. Gladiolus), or the anterior (e.g. Amaryllis), which causes an upward or a downward curva- ture; this is more marked in Dictamnus where the calyx and corolla tend to form two lips, an upper and a lower ; this bilabiate form of flower is more fully developed in the calyx and corolla of the Labiatse, the corolla (personate, the lips being closed) of the Scrophulari- acese, and of the Orchidacese and Lobelia. In not a few cases the irregularity of the flower is increased by the development of spurs from some portion of the perianth (e.g. among Monocotyledons, Orchis, Rhinopetalum, from the corolla ; among Dicotyle- dons, Linaria, Viola, from the corolla ; Pelargonium, from the calyx). A remarkable morphological feature is offered by the flowers of Orchis and of Lobelia which are resupinate ; that is, in consequence of torsion of the pedicel, the posterior side of the flower becomes anterior. The plane of symmetry is generally median in these flowers. In some few cases the irregularity, leading to dorsiventrality, is due, not to the unequal development of the floral leaves, but to the configuration of the floral receptacle, so that the floral leaves are not developed in a radially symmetrical manner (e.y. Reseda, Papilioneae, Fig. 272). When in irregular flowers the single- plane of symmetry is the median plane, the flower is dorsiventral : but there are other cases (e.g. flowers of some Fumariaceae, Fumaria, Corydalis) in which the single plane of symmetry is the lateral ; these flowers are FIG. 271.— Dorsiventral flower of a Heracleu (mag.) 45G PART IV. — CLASSIFICATION. therefore not dorsiventral, that is, they have not an tero- posterior, but lateral, asymmetry. The zygomorphic symmetry of a flower is indicated in its floral formula by symbols ; when the plane of symmetry coincides with the median plane the symbol ^ is used, and when it coincides with the lateral plane the symbol ->. Sometimes regular flowers are developed by plants which usually produce irregular flowers : these exceptional flowers are termed pcloria. This is due in some cases to the fact that the primitive number and arrangement of the floral organs is not disturbed by the irregular development of the parts which usually takes place : such cases are distinguished as regular peloria (e.g. Viola, Gloxinia, Labiatse, etc.). In other cases the peloric flower is to some extent the result of the symmetrical development of the irregularity O o (e.g. the development of five spurred petals and five sta- mens in Linaria). Dorsiven- tral flowers are, generally speaking, such as are borne laterally on the inflorescence ; whilst the terminal flowers (which may be regarded as peloric) are frequently regular. Peloric lateral flowers are, however, known to occur. There remain to be con- sidered those flowers which cannot be symmetrically di- vided in any plane : such flowers are asymmetric. Amongst these are to be included most of the acyclic or hemicyclic flowers in which the number of members is high and the divergence vari- able (e.g. Calycanthus, some Ranunculacese, etc.) : the asymmetry of most of these is approximately, though not quite accurately, actinomorphic, but in some it is dorsiventral (e.g. Delphinium, (Fig. 273 A,) Aconitum). Asymmetry is rare in cyclic flowers, but is to be found in some heterocyclic flowers : for instance, in Tropseolum, (Fig. 273 5,) Canna and other Marantacese, Valeriana and other Valerianacese, where the asymmetry is dorsiventral and is due to oligomery and irregularity combined, whilst in other cases (e.g. some Paronychiese), it is due merely to oligomery. FIG. 272.— Diagram illustrating dorsiventral symmetry in leguminous flowers: A Vicia Faba, (Papilionese) : B Cercis SiUquastrum (Casalpinieae) : in both cases the odd sepal is anterior : the plane of symmetry is median. GROUP V.— AXGIOSPERM.E. 457 Tlie Floral Organs. The Perianth is completely absent, that is, the flower is achlamy- dcous, in a few families (e.g. Piperacese, Aracese, Graminaceae, many Cyperacese, Salicacese). When present, it is usually differentiated into calyx and corolla, the flower being termed dicldamydcous or biscriate : when the calyx and corolla clearly differ from each other in colour, texture, etc., the flower is said to be heterochlamy- dcous • for instance, when the calyx is green and the corolla highly coloured (as in most* Dicotyledons, and in some Monocotyledons such as Tradescantia and Commelyna) ; or when the calyx is col- oured (petaloid) and the petals reduced to nectaries (as in Helleborus and other Ranunculacese). When the perianth-leaves are all alike, the flower is said to be homochlamydeous. This condition may be due to different causes in different cases : the flower is sometimes homochlamydeous, even though calyx and corolla are differentiated, because the sepals and petals are very similar, as in most Monoco- tyledons where the sepals are often petaloid : in other cases the flower is homochlamydeous, because only one series of perianth- leaves is developed ; that is, because the flower is monoclilamy- dcous. The flower may be monochlamydeous, because, though typically dichlamy- deous, either the calyx or the corolla is suppressed (e.g. calyx suppressed in some Umbelliferse and Composite ; co- rolla suppressed in most Thymelseacese, Paronychiese, Glaux, some Rosacese such as Alchemilla and Sanguisorba); where the corolla is sup- pressed or rudiment- ary the calyx is frequently petaloid (e.g. Clematis, Anemone, Caltha, and other Ranunculacese) : or the flower may be monochlamydeous merely because the perianth is undifferentiated (simple), and is then generally sepaloid (e.g. Urticacese, Betulacese, Chenopodiacese, etc.), or petaloid '(e.g. some Amarantaceae). I.— Floral diagrams illustrating asymmetry. A Dorsiveutrally asymmetrical hemicyclic flower of Del- phinium Ajaels : B Dorsiventrally asymmetrical heterocy- clic flower of Tropoeolum majug : br subtending bract ; p-t>, prophylla. (After Eichler.) 458 PART IV.— CLASSIFICATION. The individual leaves of the perianth may be either perfectly separate (eleutlieropeialous or polypetalous corolla, eleutherosepal- ous or polysepalous calyx), e.g. Ranunculus ; or they may cohere from the base upwards, so as to form a longer or shorter tube, which divides at its upper end into as many teeth or lobes as there were originally leaves (gamosepalous calyx, gamopetalous corolla) (Fig. 274 A B C c and B k) ; e.g. the Primrose and the Tobacco plant. In Dianthus (the Pink) the sepals alone are coherent, as also in Daphne (Fig. 274 Z>) where the corolla is absent. More rarely all the leaves of the perianth cohere to form one tube, e.g. the Hyacinth and allied genera ; the six lobes of the tube corre- spond to the three sepals and the three petals. The simple perianth also may con- sist of separate leaves (eteutlieropliyllous or polypliyllous peri- anth^, e.g. Amarantus, or the leaves may be k 0 coherent (gamophyl- lous\ e.g. Aristolo- chia. D FIG. 274. — Cohesion of sepals and petals. A Flower of Convolvulus arvensis, with a funnel-shaped corolla (c) ; and a 5-partite calyx (k). B Nicotian* Tabacum, with a 5-cleft calyx (it) ; tubular corolla (r), with a distinct 5-toothed limb («). C The rotate corolla of Sambucns. D Gamose- palous calyx of Daphne Mezereum ; r the tube ; s the limb. The degree of division presented by gamophyl- lous perianths into teeth or lobes is indi- cated by the same terms which are used in de- scribing the incision of the leaf-blade (page 37). The form of the gamo- petalous corolla may be campanulale, as in the Campanula ; funnel-shaped (or infundibuliform), as in the Bindweed (Fig. 274 A) ; rotate, as in the Elder (Fig. 274 C). The upper and lower portions may frequently be distinguished, the lower as the tube (Fig. 274 B r), the upper expanded part as tlie limb (Fig. 274 B s). Other peculiarities of form are connected with the symmetry of the flower (page 453). The petal frequently consists of two parts, the daw and the limb, as in the Pink (Fig. 275 A B). The Corona (paracorolla) in the Narcissus and Lychnis is formed by ligular outgrowths GROUP V. — ANGIOSPERM^:. 459 from the claws (Fig. 275 B l\ Any segmentation of the petal, as in the Pink (Fig. 275 A) is unusual ; emarginate or obcordate petals are more common. In many cases the petals have spur- shaped appendages (Violet, p. 455), or they are prolonged at the base into tubes, as in Helleborus and Aconitum. This peculiarity is connected with the secretion of the nectar The Reproductive Organs of the Flower are sporangia of two kinds, microsporangia and macrosporangia, borne usually on sporophylls, though sometimes directly on the floral axis. The flower is usually monoclinous ( $ , hermaphrodite, see p. 395) ; but it is not infrequently unisexual, in which case the flowers are either microsporangia te ( £ , staminalj or macrosporangiate ( ? , car- pellary). The plants which have unisexual flowers may be either P , FIG. 275.— A Petal of Dianthus superbus, with (n) the claw and (p) the limb, much divided. B Petal of Lychnis : n claw ; p limb ; I ligula. C Flower of Potentilla, seen from below : c corolla; fc calyx ; a epicalyx. moncecious (e.g. Zostera, Arum, Carex, Typhaceae, Zea, Betnlaceae, Euphorbia, Buxus, Juglans, Quercus, etc.) : or dioecious (e.g. some Palms, Vallisneria, Cannabinacese, Salicacese, Mercurialis, Yiscum, etc.) : or polygamous. Of polygamy there are several varieties : thus, the plant may bear monoclinous flowers and staminate flowers (e.g. Veratrum, ^Esculus Hippocastanum, Celtis) ; or mo- noclinous flowers and carpellary flowers (e.g. Thymus vulgar is and T. Serpyllum, Parietaria dijfusa and P. officinalis) : or it bears monoclinous flowers and both staminate and carpellary flowers (e.g. Fraxinus excelsior, Saponaria ocymoidcs}. Some flowers are probably primarily unisexual ; that is, there is no reason to believe that the unisexual condition is due to the suppression of either micro- or macrosporangia (e.g. Hemp, Oak, Walnut, Poplar, Willow). Others are secondarily unisexual : that is, there is reason to believe, either from their development 460 PART IV. — CLASSIFICATION. and structure, or from their relation to allied hermaphrodite forms, that they are typically monoclinous, but have become unisex- ual by suppression : thus, in the Cucurbitacese some genera (e g. Cucurbita, Cucumis, Bryonia, etc.) have unisexual flowers, whilst in others the flowers are always hermaphrodite ; similarly, in the Caryophyllacese, the flowers are generally hermaphrodite, but in the species Lychnis vespcrtina and L. diurna they are unisexual. In some unisexual flowers traces of the missing organs are to be found, such as staminodes in carpellary flowers (e.g. Feuillea among the Cucurbitacese ; Laurus nobilis), or rudimentary pistils in staminate flowers (e.g. Rhamnus cathartica, Lychnis vespcrtina and diurna). It sometimes happens that typically dioecious plants become exceptionally monoecious (e.g. development of ? flowers on ^ plants of Cannabis sativa ; or of ^ flowers on ? plants of Cannabis sativa and Mercurialis annua) : or that a typically monoecious diclinous plant bears some monoclinous flowers (e.g. Ricinus). The Andrcecium comprises the microsporophylls (one or more) of the flower, the stamens. Each stamen usually consists of two parts ; a slender stalk called the filament (Fig. 276 s), and a placental portion which bears the pollen-sacs (Fig. 276 D p,) known as the anther (Fig. 276 a). The anther consists of two longi- tudinal halves, termed thecce, each of which usually contains two pollen-sacs ; these two halves are united by the placental portion of the filament which is known as the connective (Fig. 276 c). This is occasionally very narrow, so that the two halves of the anther lie close together (Fig. 276 Ala): in this case it may be that the anther is not sharply marked off from the filament, and is attached throughout its whole length to the filament (aclnatc, Fig. 277 C) : when the anther is sharply marked off from the filament, it may be attached to the filament by its base, when it is said to be innate or basifixcd (e.g. Tulip) ; or the filament is in- serted in the middle of its dorsal surface, when it is dors (fixed (Fig. 277 A) ; in the last case it may be articulated as by a joint, so that the anther with the connective can oscillate on the apex of the filament (versatile anther, Fig. 276 C), as in Grasses aucl some other plants. But the connective is often broader, so that the two halves of the anther are widely separated (Fig. 276 B) : it may be much elongated (distractile) and very delicate, so that with the filament it forms a T-shaped body (Fig. 276 C) ; in this GROUP V. — AXGIOSPERM.E. 461 plant, the Sage, the further peculiarity is exhibited that one- half of the anther is abortive and is modified for another purpose. It is only rarely, as in Herb Paris (Fig. 277 (7), that the con- nective is prolonged beyond the anther into a point, or into a bristle as in the Oleander. FIG. 276.— Stamen : .4, Of Lilium : s fila- ment; a the dorsiflxed anther. A., Side view. B Of Tilia : c connective. C Of Salvia, with dorsifixed versatile anther: b is the half of the anther that has been modified. J) Transverse section of the anther of Hypericnm (mag.) : p the 4 pollen-sacs ; c connective. FIG. 277.— A Stamen of Allium. H Of Faccinium Xyrtillus. C Of Part* quadrifolia (mag.) : / filament; c connective ; o anther; b appen- dages ; p the pores by which the anther opens. The filament is usually round and stalk-like, of a delicate coloured or colourless tissue, with a central vascular bundle ; it is occasionally flattened ; when it is very short or absent the anthers are sessile. In some plants, e.g. Allium (Fig. 277 .4), the filament has what appear to be stipular appendages ; in others, e.g. Erica (Fig. 277 B\ the anther is furnished with appendages, such as spurs and so forth : in Viola, the spurs borne by the two anterior sta- mens are glandular. In certain plants the stamens, that is to say the filaments, branch ; either, like most leaves in a plane perpendicular to the median plane, as in Myrtaceae and Fu- mariaceae, or in various planes, as in Ricinus (Fig. 278) and Hypericacese ; an anther is borne on each of the branches of the filament. Somewhat similar in appearance, but essentially different in structure are the coherent stamens of the Papilionese and other PAET IV. — CLASSIFICATION. plants. The stamens of each flower may be coherent into one or more bundles. The arrangement becomes complicated when the filaments are at the same time coherent and branched as in the Malvaceae. When the filaments are all coherent into a single bundle (e.g. Malvacese), they are said to be monaddphous ; when in two bundles (e.g. some Papilionese, Fumariacese), they are diadelphous ; when in several bundles (e.g. Hypericacese), they are polyadelphous. In the Compositse (e.g. Sunflower and Thistle), though the filaments are free, the anthers become coherent or syngenesious. When the stamens are quite free from each other they are said to be polyandrous. Besides these varieties of cohe- sion, adhesion frequently occurs ; that is the filaments adhere to other portions of the flower, par- ticularly of the perianth, so that they — or when they are very short, the anthers— appear to be inserted not upon the axis of the flower, but upon the leaves of the perianth (epipetolout or epiphyl- lous) : this condition is most fre- quently present when the petals themselves are connate and form a tubular corolla, e.g. Primula. The adhesion of the stamens to the carpels is of rarer occurrence (e.g. Orchidacese, Aristolochia) ; the flower is then termed gynan- drous. In many flowers it happens that certain filaments, occupying a definite position with regard to the other parts of the flower, are longer than ths others ; thus, of the six stamens of the Cruciferse (e.g. Wallflower), four are much longer than the other two ; of the four stamens of the Labiatae, (e.g. Lamium), two are longer than the other two. In the former case the stamens are said to be te- tntdynamous, in the latter didynamous. Stamens which bear no anthers are termed stanii nodes : they are frequently petaloid (e.g. Canna). In many acyclic flowers (e.g. Nymphsea), the stamens and the petals are connected by intermediate structures, of which it is difficult to say whether FIG. 278.— Part of a staminal flower of Ricinus communis cut through length- ways : / / the basal portions of the compoundly-branched stamens ; o the anthers. (After Sachs.) GROUP V.— ANGIOSPERM^E. 463 they are to be regarded as petaloid stamens or as staminoid petals. The Microsporangia or Pollen-Sacs are borne on the anther. There are commonly four of them (quadrilocular anther), two forming a sorus in each longitudinal half (or theca) of the anther, situated usually side by side, but sometimes (Lauracese) one above the other ; in the former case the typical arrangement seems to be that of each pair of pollen-sacs one belongs to the anterior or inner surface of the anther, the other to the posterior or outer surface. In some cases, however, there are but two pollen-sacs — (bilocular anther) ; this may be due to the non-development of one longi- tudinal half of the anther (e.g. Cucurbitacese, Salvia, Canna) ; or to branching (e.g. Adoxa, Malvaceae) ; or to the abortion of one pollen-sac, generally the posterior one, of ff each pair (e.g. As- clepiadaceae) ; or (some Lauracese) of the upper or lower one of each pair ; or, finally, to the early fusion of the arche- sporia of two adja- cent pollen-sacs (some Orchidacese). In the Aracese the process of fusion is carried to such an extent that all four archesporia fuss, so that the anther is uni- locular. Each pollen-sac encloses an archesporium from which the mother-cells of the microspores (pollen-grains) are developed by division ; each group of spore-mother-cells is invested by a layer of granular cells, the tapetum (Fig. 279 f), which eventually becomes disorganised : externally to this is the wall of the pollen-sac con- sisting of one or more layers of cells with usually reticulately thickened walls, followed by the epidermis at the surface. The pollen-sacs dehisce usually by a longitudinal slit which, when the anther is quadrilocular, is generally so situated that it at once opens into both the pollen-sacs of each half of the anther, and frequently the tissue separating each pair of pollen-sacs be- FIG. 279.— Transverse section of a yonng anther of Sam- lucus racemosa ( x 80) : c the connective with the vascular bundle ; ps the four pollen-sacs (microsporangiaj ; p the mother-cells of the pollen; t tapetal layer; tc the wall of the pollen-sac. 464 PART IV. — CLASSIFICATION. comes dried up and ruptured whilst the anther is ripening : some- times the dehiscence of the pollen-sac is transverse (Alchemilla) ; sometimes it is valvular (Berberidacese) ; or by apical pores (Ericaceae, Polygalacese). Though in a quadrilocular anther the pollen-sacs typically belong, two to the inner (ventral), two to the outer (dorsal), surface of the anther, it frequently happens that in the course of their development they become somewhat displaced, so as to appear all to belong to either the inner or the outer sur- face ; hence, when dehiscence takes place, the pollen is shed, in the former case, towards the centre of the flower, when the anthers are said to be introrse ; and, in the latter case, towards the periphery of the flower, when the anthers are said to be extrorse. These terms are similarly applicable in the case of bilocular anthers. Introrse anthers are the more common ; extrorse anthers occur in the Aristolochiacese, Iridacese, Juncaginese, Aracese, and in various genera of other orders. In rare cases some of the anthers of the flower are introrse, and others extrorse, as in some species of Polygonum (P. Bistorta, tataricum, aviculare, etc.), where the anthers of the outer whorl are introrse, and those of the inner whorl extrorse ; and as in most Lauracese, where the anthers of the innermost staminal whorl are extrorse, whilst those of the outer whorls are introrse. The Microspores or Pollen- grains. The essential features in the structure and development of the microspores have been already fully described (see pp. 85 and 396). The shapes of the pollen-grain are very various : it may be spherical, oval, triangular, etc., or long and cylindrical (confervoid) as in the Naiadacese. On germination the pollen- grain gives rise to one or more pollen-tubes, which consist of outgrowths of the intine : these penetrate the exine (when pre- FIG. 2so.— Germinating pollen-grain of sent), either rupturing it irregu- Epilobium (highly mag.) bearing a , , j , . . , pollen-tubes;* exine ;,' intine ; a b c the larlJ> Or at determinate points three spots where the exine is thicker in where the exine is thinner and Malvaceae), or where there are lid-like areas which are easily removed (e.g. Cucurbitacese, Fig. 235). These points are definite in number (1, 2, 3, 4, or more), sometimes very numerous (Malvaceae). GROUP V. — ANGIOSPERM^E. 405 The Gyna>ceum or Pistil is always the terminal structure of the flower, occupying the apex of the floral axis. It consists of the macrosporophylls or carpels, which, in the Angiosperms fprm the whole or part of the ovaries, that is, closed cavities containing the ovules. If in a flower where there are several carpels, each of them closes by the cohesion of its margins, they form so many ovaries ; the gynseceum is then said to be apocarpous (Fig. 281 .4), e.g. Ranunculus, Pseonia, and Butomus : if there is only one carpel (Fig. 281 J3), the pistil is said to be apocarpous and simple : if several carpels in one flower cohere and form a single ovary (Fig. 281 (7), the gynseceum is said to be syncarpous, e.g. Poppy and Lily. Intermediate forms occur in that the carpels may cohere by their lower ends whilst their upper ends remain free (Fig. 281 D). The ovary is said to be monomerous when it is formed of only FIG. 281.— 4 Apocarpous gynseceum of Aconite. B Simple apocarpous gynseceum of Melilotus. C Tetramerous syncarpons gynsecenm of B/iamnm cathartica. D Ovary of Saxifraga, formed of two carpels which diverge towards the top: t tonu ; /ovaries; g style ; n stigma ; b ventral suture. one carpel (Figs. 281 B and 282 A), the margins of which cohere on the side opposite to the midrib. The outer side along which the midrib runs is the dorsal surface (Fig. 282 A r), and the midrib itself is the dorsal suture; opposite to it is the line of cohesion, the ventral suture, which runs therefore along the ventral surface. The cavity thus enclosed (loculus) is not usually divided by dissepi- ments, but it is a simple cavity, as in the Vetch ; such an ovary is said to be unilocular. False or spurious dissepiments, formed by growths on the inner surface, occur in some few instances, as in Astragalus. When, on the other hand, several carpels cohere to form a syn- carpous ovary, it is polymerous (di- tri- or tetra-merous, etc). The .syncarpous ovary is unilocular (Fig. 282 B) when the individual M.B. H H 466 PART IV.— CLASSIFICATION. carpels cohere simply by their edges without any portion of them projecting inwards ; but if the margins project into the cavity so as to form incomplete longitudinal dissepiments, the ovary is chambered (Fig. 282 C), e.g. Poppy ; but since the chambers are open towards the centre, the ovary is still unilocular. When the margins form dissepiments which meet in the middle, the ovary is multilocular (Pig. 282 D) ; sometimes the margins turn outwards again towards the circumference. In the last case the individual loculi are completely separated ; but there are others in which the margins of the carpels do not extend so far towards the centre at the upper part as at the lower, but the two margins of each carpel simply cohere together above ; consequently the lower part of the ovary is polymerous and multilocular, while the upper part is composed of a number of monomerous ovaries, e.g. Saxifraga (Fig. 281 D). In all these cases the floral axis may grow up into the FIG. 282.— Transverse section of ovaries ; p placenta. A Monomerous and unilocular ; )• dorsal suture : b ventral suture ; placentation marginal. B Polymerous and unilocu- lar ; placentation parietal. C Pol.vmerous and many-chambered, but unilocular ; pla- centation parietal. D Polymerous and multilocular ; placentation axile. interior of the cavity of the ovary, and when the ovary is multi- locular the axis may coalesce with the dissepiments. False dissepiments may be formed in polymerous ovaries by in- growths from the internal surface of the carpels ; thus the ,ovar}r of the Boraginaceae and Labiatae is originally bilocular, but each loculus becomes divided into two by a false dissepiment, and when the fruit is ripe the four loculi separate completely ; similarly, the unilocular ovary of the Cruci ferae becomes spuriously bilocular. The inferior oYary of epigynous flowers (see p. 444) is com- monly polymerous, but it may be either unilocular or multilocular. In some bases the axis is prolonged between the carpels, con- stituting a carpophore, as in the Geraniacese and Umbelliferse (Fig. 287). The Style (Figs. 281 and 283) is the prolongation of the upper part of the carpel : it is commonly a slender cylinder, but some- GROUP V. — ANGIOSPERMJE. 467 times it is leafy and petaloid (e.g. Iris). Monomerous ovaries have but one style ; polymsrous ovaries have as many styles as there are carpels, which may cohere throughout their whole length, or at their lower parts only, the uppsr parts remaining distinct ; or they may remain quite free, and they may even branch. The style originally arises from the apex of the ovary, but it is frequently displaced forwards, by the vigorous development of the dorsal portion of the carpel, on to the inner side, so as to appear to be a prolongation of the floral axis (gynobasic style) : this is conspicuous in the Boraginacese and Labiatse, where it is surrounded by the four rounded loculi of the ovary which have been already men- tioned (p. 466). The style is sometimes very short, and appears only as a constriction between the ovary and the stigma, as in the Poppy. In some rare cases it is hollow, but it is usually filled with a loose tissue, called conducting tissue, through which the pollen-tube can easily pene- trate. The Stigma (Figs. 281 and 283 n) is usually terminal, but it may be lateral (e.g. Iris) ; it is distinguished by being covered with papillae, or frequently with hairs, and by the secretion of a sugary fluid which retains the pollen-grains which fall upon it, and which promotes the development of the pollen-tubes. The stigma is often evidently distinct from the style, appearing as a lobed ex- pansion ; in other cases it seems to be merely a portion of the style at its end or sometimes on its side. In the Poppy it is a sessile disk-shaped expansion on the upper surface of the ovary ; more rarely it is represented by bands of papillae on the ovary itself, when it is said to be pleurogynous. The number of the stigmata often affords a means of ascertaining whether the ovary is monomerous or polymerous ; for instance, the ovary of the Compositse seems, at first sight, to be monomerous ; but the two short branches of the style, each bearing a stigma, show that it is dimerous. On the other hand, this character may be misleading : for instance, in various Grasses the ovary bears two or three stigmata, either directly, or springing from the style ; hence it might be inferred that the ovary is di- or tri-merous, whilst as a matter of fact it is monomerous. In this respect some few Fio. 283.— Gj" naeceum of the Lily : / ovary ; g style; * stigma (nat. size). 468 PART IV.— CLASSIFICATION. other plants, belonging to the Naiadacese and other families, re- semble the Grasses. The Macrosporangia or Ovules are always enclosed in the cavity of the ovary, either singly or in larger or smaller number. Usually they may be readily seen to be developed on the carpels (Fig. 284 A, B, C), but in many cases they appear to be developed from the floral axis (Fig. 284 Z), F, G). However, from careful comparative examination, it seems that the apparently axial ovules may be re- garded in some cases as having been developed on the carpels, their position on the axis being merely the result of a more or less con- siderable subsequent displacement due to the coalescence of the carpels with the axis. That portion of the ovary which bears the ovules is called the placenta. The ovules, when borne by^ the carpels, are but rarely developed over the whole surface of the carpel, but are confined to the margin : in other words the placentation is rarely superficial but generally marginal. Superficial . placentation (Fig. 284 (7) is to be found in Butomus, Nymphsea, and Nuphar, the dorsal sutiTre (midrib) of the carpel being the only sterile portion of its internal surface. Of mar- ginal placentation there are two varieties : in the one the ovary is syncarpous but unilocular, and the contiguous placental margins of the carpels constitute so many placentas on the wall of the ovary, that is, the placentation is parietal (Fig. 282 S, (7), as in the Violacese, Cruciferse, Papaveracese, Ribesiese, Orchidacese, etc. ; in the other the ovary is syncarpous and multilocular, the margins of the carpels meeting in the centre and there bearing the ovules, so that each placenta is at the inner angle of each loculus, that is, the placentation is axile or axillary (Fig. 282 Z>, and Fig. 284 B) : in a monomerous ovary (Fig. 282 A, and Fig. 284 A) the placentation is essentially parietal, but it is simply termed marginal. The position of attachment is a point of descriptive importance, more especially where the number of ovules is small, or where there is but one, in the loculus. When the ovule is attached to the roof so that it down hangs into the loculus, it is said to be pendulous ; when it is attached high up, but at the side, it is suspended (Fig. 284 E) ; when it is attached to the side and projects straight, it is horizontal ; when it is attached at the side, but towards the base and stands up into the loculus, it is ascending. When the ovules are borne, either actually or apparently, by the axis, the placentation is said to be axial. When many ovules are borne on the axial placenta (as in the Primulacese, Santalacese, etc., GROUP V.— ANGIOSPERM.E. 469 Fig. 284 eing formed by free cell-formation. P The pollen- tut*, passing through the micropyle, n. 472 PART IV.— CLASSIFICATION. a multitude of distinct flowers situated inside the cavity of the receptacle, and the individual fruits appear as hard grains ; such a fruit is termed a syconus. Again when the ovaries and floral envelopes of closely crowded flowers, as in the Mulberry and the Pine-Apple, become succulent, a kind of spurious fruit is formed which is termed a sorosis. Iii other cases, a husk, called the cupule, is formed, which con- tributes to the formation of a spurious fruit : this is formed by the bracteoles and is not developed until after fertilisation ; it may surround either a solitary distinct fruit, like the acorn-cup, or several distinct fruits, like the four-valved spiky husk of the Beech-tree or the prickly husk of the edible Chestnut. W hen the fruit consists of one or more monomerous ovaries, it is said to be apocarpous : examples of this occur in Kanunculus, in the Raspberry, where the individual ovaries are succulent, and in the Star-Anise (Fig. 286). The individual fruits may be de- veloped in very different ways ; they may be dehiscent or indehiscent, dry or succu- r H — f lent- & When the fruit consists of a single polv- FiG.286.— FrnitoflZKcium ... .1,1 •onisatum -. st peduncle ; // merous ovary, it is said to be syncarpouft. the separate fruits, each When the carpels of such a fruit separate septicidally during the process of ripening, so that it ultimately appears as if a number of distinct fruits were present, it is termed a scMzocarp : it may thus split into only two distinct fruits, as in the Umbelliferae (Fig. 287) ; or, as in the Geraniacese and many Malvacese, into several distinct fruits : each of them is termed a coccus or mericarp ; the individual coccus is generally indehiscent (dehiscent in most Eu- phorbiacese). In various multilocular ovaries only one loculus becomes fully developed and bears seeds, as in Valerian, the Coco-Nut, and the Oak ; the others are abortive. It sometimes happens in cultivated plants that the fruit becomes perfectly formed without any develop- ment of seed, as in a particular seedless variety of Grape, the Banana, the Pine-Apple, etc. In all true fruits the wall of the ovary forms the pericarp or rind. In some more or less succulent fruits, the pericarp consists of three distinct layers ; the external layer is the epicarp, the -middle the mesocarp, and the innermost the endocarp. GROUP V. — ANGIOSPERM/E. 473 The following varieties of true fruits have been distinguished by the character of the pericarp, whether it is dry or succulent, hard or soft. — and by the dehiscence or indehiscence of the pericarp. A. DRY FRUITS. The pericarp is woody or coriaceous ; when ripe, the sap has usually disappeared from all the cells. I. Dry Jndehiscent Fruits. The pericarp does not rupture, but encloses the seed until germination; the testa is usually thin, and frequently coa- lescent with the pericarp. (1) One-seeded fruits : (a) The nut (glans), e.g. Acorn, Hazel-Nut (but not the Walnut) ; the dry pericarp is hard and sclerenchymatous : it is inferior and syncarpous. (b) The aakene is superior and monome- rous : the pericarp is thin and cori- aceous ; e.g. the Rose and the But- tercup. The similar fruit of the Compos itse is a cypsela 5 it is in- ferior and dimerous. The fruit of Grasses, termed a caryopsis, is very similar to the achene; it differs from it in that the testa and the pericarp closely adhere, whereas in the achene they are not adherent. (2) Many-sseled fruits: these (schizocarpn) commonly split into one-seeded fruits, which usually enclose the solitary seeds until germin- ation: e.g. the Umbelliferse (Fig. 287) and Maple, with two, the Euphorbiacese with three, meri- carps ; the Geraniacese, with five mericarps ; and most Malvaceae, where the fruit is termed a carcerule, and splits into many mericarps. The pericarp of dry indehiscent fruits is sometimes developed into a membranous wing (e.g. Ash, Elm, Birch) ; to such a fruit the term samara is applied : the fruit of the Maple is a double samara. II. Dry Dehiscent Fruit*. The pericarp rup- tures and allows the seeds, which usually have a firm and thick testa, to escape: — they are commonly many-seeded. (1) Dehiscence longitudinal. (a) The follicle, consisting of a single carpel which dehisces along the ventral suture, where also the seeds are borne, e.g. Pseonia and Illicium (Fig. 288) ; but sometimes (e.g. Magnolia) along the dorsal suture : it is superior. (b) The legume or pod likewise consists of but one carpel which dehisces along buth the dorsal and ventral sutures (Fig. 288 A, "transverse section Fig. 282 A) : e.g. the Vetch, Pea, Bean, and FIG. 287.-C the hypocotyl ; I thecpiblast; r the radicle; cp the root-cap of the radicle ; el the root-sheath (coleorhiza) ; m phice of exit of the radicle, corresponding with the micropyle of the ovule ; p the funicle ; vp vascular bundle in the funisle : / lateral surface of the furrow (x 11). (After Strasburger.) plemented by the development of adventitious roots in succession at higher and higher levels upon the stem. The stem of Monocotyledons is traversed longitudinally (Fig. 99, p. 122) by scattered closed vascular bundles (Fig. 103) , it has therefore no growth in thickness by the means of cambium. In a few genera only, as Yucca and Dracaena, it grows subsequently GROUP V. — ANGIOSPERALE : MONOCOTYLEDONES. 479 in thickness by the formation of meristem in the pericycle from which additional closed vascular bundles are developed (see p. 148, rig. in). The axis of the embryo in many cases continues to be the main axis of the plant ; at first it is thin and weak, and since no secondary growth in thickness of the stem takes place, and since the successive portions of the stem are thicker and more vigorous, the whole stem gradually assumes the appearance of an inverted cone ; but when the plant has reached a certain height it may then grow cylindric- ally : this is the reason why in Palms, in the Maize, and other similar erect stems, there is a diminution in thickness at the lower end. Frequently, however, the primary axis of the plant perishes when it has given rise to lateral shoots. The arrangement of the leaves is at first alternate : when the stem is well de- veloped this alternate ar- rangement often passes over into complex spiral arrange- ments, as in Fritillaria and in Palms, in which plants a crown of leaves is conspicu- ous. In the Grasses, and a few other families, the phyl- Fio. 293.— Longitudinal section of thegrain of Zea Mais ( x about 6) : c pericarp ; n re- mains of tbe stigma : /« base of the erain ; eg hard yellowish part of the endosperm ; tie whiter less dense part of the endosperm ; »c scutellum of the embryo; an its apex; « its epidermis ; fc plumule ; w (below) the primary root; ITS the coleorhiza: w (above) secondary roots springing from the epicotyl (st). (After Sachs.) lotaxis is permanently alter- nate. A whorled arrangement of the foliage-leaves occurs but rarely. The leaves commonly have a well-developed sheathing leaf-base : they may be described as exstipulate. The lamina is usually entire, simple in outline, often long and narrow, linear or ensiform, more rarely orbicular, cordate or sagittate. Branched leaves occur only in a few of the Aracese : the pinnate or palmate leaves of the Palms acquire this form by the splitting of the originally entire laminae, and the same is the case with the perforated leaves of many Aracese (see p. 37). 480 PART IV.— CLASSIFICATION. The venation of the leaves is characterized by the fact that the weaker veins do not usually project on the under surface. In linear leaves, and in such as are inserted by a broad base, the stronger veins run almost parallel ; in broader ones, e.g. Lily of the Valley (Convattaria majalis\ they describe a curve which is more or less parallel to the margin ; the weaker veins usually run at right angles between the stronger ones. In the Scitamine* and a few other plants, a number of parallel transverse veins are given off at various angles (sometimes acute, and sometimes nearly right angles") from the midrib. Reticulate venation of the leaves is unusual ; but it occurs in Aroids, in Paris quadrifolia, etc. (see p. 39). The flower of Monocotyledons consists typically of five alternat- ing and isomerous whorls, two belonging to the perianth, two to the androecium and one to the gynaeceum. Thus the typical formula is En, Cn, An + n, Gn, where n in most cases — 3, more rarely = 2, 4 or 5. The perianth-leaves are generally all much alike, and petaloid in both series : sometimes they are all sepaloid (e.g. Juncaceae) : more rarely those of the external whorl are sepaloid, those of the internal petaloid (e.g. Alismacese). This type is most closely adhered to in the Liliaceae. The simplest departure from it is exhibited in the suppression of the inner whorl of stamens in the Iridaceae, and in the inferior position of the ovary. This latter character occurs also in the Scitamineae and Orchidaceae, which are further characterized by the zygomor- phism of their flowers and the considerable reduction of the androecium. Other various and considerable deviations by reduc- tion from the Liliaceous type of flower occur among the Aracese, and in the Glumales, and Typhacese, and in certain water-plants (e.g. Naiadaceae, Lemnacese). On the other hand, the deviation may be due to increase in number, more especially of the members of the gynaeceum and to some extent of the androecium (e.g. Alismaceae). GROUP V.— AXGIOSPERiLE : MOXOCOTYLEDOXES. 481 I 482 PART IV. — CLASSIFICATION. SUB-CLASS I. SPADICIFLORvE. Inflorescence usually a spadix with a spathe, but flower some- times solitary : flowers frequently unisexual, sometimes dioecious : perianth, often wanting, never petaloid : anthers usually extrorse, or dehiscing by pores : ovary superior. Cohort I. Arales. The flowers are small and numerous ; the inflorescence a spadix or a panicle with thick branches, commonly enclosed in a greatly developed spathe ; the bracts of the indi- vidual flowers are frequently wanting ; perianth 0, or polyphyl- lous ; the flowers are usually diclinous, but both kinds of flowers frequently occur in the same inflorescence : gynseceum apocarpous or syncarpous : the seeds have a large endosperm : the embryo is straight and minute. Order 1. ARACE^E. Flowers monoecious or $ : perianth 0 or of 4—6 leaves : stamens 1—8, frequently coherent into a synandrium in the <$ flowers : ovary inonomerous, or polymerous and multilo- cular : fruit a berry : seed sometimes exal- buminous. Mostly tropical. In many of the genera the flowers are com- plete and conform to the monocotyledonous type, A"n, Cn, An + n G (n), where n may stand for 2, or 3, as in Acorus (Fig. 294), in which the flowers are exactly typical. In other Senera> h°wever, the flowers are reduced in a outer, i inner peri- various ways and degrees ; not only does the 3ns ' * perianth disappear, but the number of the stamens and carpels is frequently diminished. In many ? flowers staminodia are present, either in the typical or in a smaller number. An extreme case is offered by those diclinous flowers of which the <$ consists of only a single stamen (e.g. Arisarum), and the ? of only one monomerous ovary. These much-reduced flowers are disposed in regular order on the spadix : thus in Arum (Fig. 295) the numerous $ flowers, consisting each of one carpel (Fig. 295 /), are inserted on the base of the spadix ; and the £ flowers, each consisting merely of 3-4 stamens, are closely packed higher up on it (Fig. 295 a). The upper part of the spadix is covered with rudimentary flowers (b, c). When, as in this case, the perianth of the true flowers is wholly wanting, the whole inflorescence may assume the aspect of a single flower ; but GROUP V. — ANGIOSPERM.E : MOXOCOTYLEDDXES. 483 irrespectively of the numerous intermediate forms which are to be found, such a view is untenable when it is barne in mind that here the ovaries are invariably situated below the stamens, while in a flower they are invariably above them. The usually sympodial stem may be un- derground, a tuber, or a rhizome, or it may be aerial ; in the latter case it often climbs, clinging to trees by means of aerial roots. The leaves are either alternate and dis- tichous or, more often, spiral with a diver- gence of f . They are rarely narrow, linear, or ensiform, and commonly consist of leaf- base, petiole, and blade ; the venation is reticulate, and the leaf often exhibits a more or less complicated segmentation. Laticiferous sacs or cells (see p. 99) occur in some families of the order, as do also sclerotic cells (see Fig. 98 A, p. 120). The principal families are : — FIG. 295.-Spadixof ,4 runt maculatum (nat. size) : / macrosporangiate, a micro- sporangiate, and b rudi- mentary flowers ; c the up- per club-shaped end of the spadix. Fam. 1. Pothoidece : without either laticiferous or sclerotic cells : flowers usually $ , with or without a perianth. This family includes a number of genera, such as Pothos, Anthurium, Acorus. The only member which occurs in Britain is Acorus Calamus, the Sweet Flag, which grows on the margins of ponds and rivers : its subterranean rhizome bears long ensiform alternate leaves, crimped at the edges ; its flowering-shoot is triquetrous, bearing a terminal spadix which is, however, displaced to one side by the spathe which developes so as to form a continuation of the long axis of the flowering-shoot : the spadix is densely covered with flowers (Fig. 294). Fam. 2. Caltoidece: with straight rows of laticiferous cells: fli>\v.-r> usually $ , with or without a perianth : leaves never sagittate. No member is indigenous in Britain : Calla palustris occurs in the marshes of Northern Europe; it has a white spathe and parallel- veined leaves. Fam. 3. Philodendroidece : with straight rows of laticiferous cells: flowers diclinous, without a perianth : stamens usually connate : leaves generally parallel-veined. Zantedeschia (Calla or Richardia) cdhiopica, with a white spathe, is commonly cultivated under the name of the Trumpet Lily. Fam 4. Aroidece: with straight rows of laticiferous cells: flowers diclinous: usually without perianth. Arum maculatum, the Cuckoo-pint or Lords and Ladies, is a British plant, common in wood and hedges : the 484 PART IV. — CLASSIFICATION. large green spathe completely envelopes the spadix (Fig. 295). Dracun- culus and Arisarum are also European genera. Order 2. LEMNACE^E. Stem leafless. Each, inflorescence con- sists of two <$ flowers and one $ flower borne on a lateral branch of the stem : the £ flowers consist of a single stamen, and the ? flower of one carpel. Lemna trisulca, L. (Synrodela) polyrhiza, minor and ffibba, are known as Duck-weed ; they are common in tanks and ponds, floating on the water. The stem, which is leafless, is almost flat, resembling a thallus : it bears two rows of branches (Fig. 296), as also roots on its under FIG. 296,— Part of a plant surface which are suspended in the water, of Lemna trisulca, seen from Hoots are, however, absent in Wdffia arrhiza, above: a the young lateral which is algo devoid of vascular bundles; its flower has no spathe, and it bears only one row of branches : it is the smallest known flowering plant. Order 3. TYPHACE.E. Flowers diclinous ; the perianth repre- sented only by scales, or 0. Stamens usually 3. Ovary usually monomerous, containing one ovule. Inflorescence a spadix, without a spathe,. elongated or compact. In Sparganium, the Bur-Reed, the inflorescences are spherical spikes which are borne terminally and laterally in two rows on the upper part of the stem. The lower spikes bear only , and the upper only J flowers : the perianth consists of 3-6 scales ; stamens 3-8, free ; gynseceum some- times dimerous with an ovule in each loculus. Sparganium simplex and ramosum are not rare in ditches. Typha, the Reed-Mace or Bulrush, bears its flowers on a long terminal spadix ; the $ flowers are borne directly on the upper and thinner portion of the main axis ; on the lower and thicker portion are borne the ? flowers, partly on the main axis and partly on very short lateral shoots ; the perianth is replaced by long hairs *, stamens 1—5, monadelphous. Typha angustifolia and latifolia occur in bogs and wet places. Cohort II. Pal males. Order 1. PALMACE^E. The dioecious or monoecious, rarely moncclinous or polygamous, flowers are in- serted, with or without bracts, on the spadix or on the thick axis of a spicate or paniculate inflorescence (Fig. 297) : they generally conform to the type -BT3, C3, ^43 + 3, G -' : in some instances a larger or a smaller number of stamen's are present : anthers some- times introrse : carpels rarely more or less than 3, either free or connate ; when the gynseceum is apocarpous, the ovary is unilocu- GROUP V. — ANGIOSPERM/E : MONOCOTYLEDONES. 485 lar: when syncarpous, the ovary has from one to three loculi. Each loculus contains, typically, a single basal ovule ; but in tri- merous ovaries, two of the ovules are generally abortive : frequently not more than one of the carpels (whether the gynseceum be apo- carpous or syncarpous) developes into the fruit: the fruit is generally baccate or drupaceous, one-seeded : the seed is large, and the contained endosperm is horny. Their mode of growth is somewhat various. Most Palms bear their leaves closely arranged in a crown at the top of a tall or of a quite short stem, which is clothed for some distance below its apex with the remains of the older withered leaves. But in some genera, e.g. Calamus, the stems creep or climb and the leaves are inserted at some distance from each other. The blade of the leaf commonly splits in the course of its growth, assuming a compound palmate or pinnate form. The inflorescence is invested by bracts : there is usually a large bract (spathe) which en- velopes the whole inflorescence when young, and other, inner, bracts which either partially invest it or (when branched) its branches. Palms chiefly inhabit the tropics, par- ticularly the Moluccas, Brazil, and the region of the Orinoco, and the different genera be- long exclusively (at least originally) either to the Old or to the New World. FIG. 297. -Port of the panicle of ? flowers of Chauiaedorea: » the thick axis; a the ex- ternal; and j) the inter- nal whorl of the peri- anth ;/ ovary (x 3). PhcKnix dactt/lifera (the Date Palm) a native of Asia and Africa, has pinnatifid leaves. Of the three ovaries, one only developes to form the fruit which is known as the Date (p. 475, Fig. 291) ; the stone of the Date consists of a very thin testa en- closing the large mass of hard endosperm in which the embryo is imbedded. Chamcerops humilis is a frequently cultivated < .ma- mental plant, with fan-like leaves, which belongs to the Mediterranean region. Metroxylon (Ett-Sagus) Rumphii and teuc, growing in the Mo- luccas, are the plants from which Sago is obtained ; it consists of thw starch-grains obtained from the parenchyma of the trunk. The stems of species of Calamus, in the East Indies, supply Malacca-cane. Areca Catechu (Fig. 298 J) is the Betel-Palm of tropical Asia, Coco* nucifera (the Coco-nut Palm) has, as is well known, many uses. The fruit itself gigantic drupaceous fruit; the mesocarp is traversed by an immense number of vascular bundles, which are used to make ropes, etc. In- side the excessively hard innermost layer of the pericarp, the eiidocarp, lies a single large seed. When the fruit is mature, the endosperm forms a 486 PART IV. — CLASSIFICATION. layer only a few millimetres in thickness, which lines the hard shell ; the rest of the space (the remaining cavity of the embryo-sac) is filled with fluid, known as coco-nut milk. The embryo, which is small, is imbedded in the firm tissue of the endosperm, tinder the spot where there is a hole FIG. 298. — A Pait of the $ inflorescence of Phoenix reclinata (nat. size): B single $ flower: C two carpels: D floral diagram. J Fruit of Areca Catechu: one half of the fibrous pericarp has been removed. (corresponding in position to the style of the single fertile loculus of the ovary) in the endocarp. Elaia guineensis is the Oil Palm of West Africa ; the mesocarp of the plum-like fruit, yields the oil. Phytelephas grows in tropical America : the hard endosperm is known as vegetable ivory. SUB-CLASS II.— GLUMIFLOBJE. Flowers monoclinous, or unisexual and then mostly monoecious, usually in heads or spikelets invested by imbricate bracts : perianth absent, or scaly : ovary superior, uni- or multilocular, with one ovule in the loculus : seeds with endosperm. Cohort I. Glumales. Ovary unilocular : ovule erect. Order 1. GRAMINACE^E. True Grasses. The leaves are alter- nate on the stem, which is known as the haulm ; the embryo lies on the side of the endosperm (Figs. 292-3). The usually monoclinous flowers generally have the formula AD, CO, .43 + 0, 6rl ; they are enclosed by bracts here termed palecc, and are arranged in com- FIG. aW.-Diajjrams of Grass flowers. A plicated inflorescences ; the P.ambusa. B Common type of Graminacece. In A there are three, in B two lodicules. monomerOUS unilocular ovary GROUP V.— ANGIOSPERM.E : MONOCOTYLEDOXES. 487 contains only one ovule ; the grain is the fruit, a caryopsis, to which one (the inferior) or, less commonly, both, of the palese some- times adhere, e.g. Barley and Oats. The flower is sessile in the axil of a bract, which is termed the inferior or outer palea, sometimes also called the flowering- glume (Fig. 301 &j, &2,...), and there is also a bracteole opposite to and somewhat higher than this which is termed the superior or inner palea (Fig. 301 ps). The two palese completely enclose the flower. Within the inferior palea are usually two small (antero-lateral) scales, the lodicules (sometimes only a single anterior one, Melica), FIG. 300.— Single-flowered spikelet of Pant'cum miftaceum (mag.); C, and C, second and third glumes : D inferior palea: E superior palea. Fio. 301.-A spikelet of Wheat dis- sected (mag.) : * axis of the spikelet ; g glumes ; 6, 5, b, b. inferior pale«e bear- ing (gr) the awn ; 6, is sterile. B, J?, Bt the flowers raised (as indicated by the dotted lines) out of the azila of the in- ferior pale* ; pt superior palea.1 ; a an- thers ; /ovaries. and occasionally (e.g. Stipa, some Bambusese, Fig. 299 A), there is a third scale situated posteriorly within the superior palea. These lodicules are frequently regarded as rudimentary perianth- leaves, but it is more probable that they are bracteoles, the two antero-lateral lodicules representing the two halves of a single bracteole, present, as such, in Melica. They grow and become succulent at the time of flowering, thus forcing apart the palese and the glumes. Usually two or more flowers, thus enclosed by palese, are present on an axis (Fig. 301 a?), and constitute the spikelet of the Grass, and beneath the lowest flower there are usually two (or more) bracts which bear no flowers in their 488 PART IV. — CLASSIFICATION. axils and are known as the glumes (Fig. 301 g}. Thus a spikelet consists of a main axis bearing two rows of bracts of which the two first and lowest are barren, while the succeeding ones bear each a flower in its axil, and beneath each flower there is also a bracteole (superior palea) belonging to the floral branch itself. The inferior palese often have, either at the apex or else borne on the midrib, a spinous process called the arista or awn (Fig. 301 gr} . The number of flowers in each spikelet varies, however, accord- ing to the genus ; often there is but one, the lowest, with rudi- ments of others above it ; if, however, only one of the upper flowers is developed, then the lower palese bear no flowers in their axils and are regarded as glumes, several being therefore present in such a case. The spikelets themselves are in many genera, e.g. Rye and Wheat (Fig. 302 B), arranged in two rows on a main axis ; the inflorescence may then be designated a compound spike (see p. 440) ; in most of the other genera the main axis of the inflor- escence bears lateral branches which are slender, of various length, and often branched again, and which bear the terminal spikelets ; in this way a panicle is formed, as in the Oat (Fig. 302 A}. This may be either loose and spreading, with long lateral branches, or compressed, with very short branches, e.g. Alopecurus. The posi- tion of the branches of the panicle is more or less bilateral ; dorsi- ventral, when (e.g. Festuca) the branchlets of the main branches of the panicle all arise on the same side (unilateral or secund panicle). The androecium consists commonly of one (Fig. 299 5) or two (A) whorls of 2-3 stamens; when there is but one whorl of stamens, it corresponds to the outer whorl in those flowers in which two whorls are present. Sometimes (e.g. Luziola, Ochlandra, Pariana) the stamens are numerous (about 18-20;, or there may be but one or two. When there are normally only two stamens, they are usually situated in the median plane (e.g. Anthoxanthum), sometimes in the lateral plane (e.g. Coleanthus) ; but where this is the result of suppression (Diarrhena, Orthoclada) they are postero-lateral, the anterior stamen being suppressed : when there is only a single stamen, this is generally the anterior stamen (e.g. species of Festuca and Andropogon), the two postero-lateral stamens being suppressed. The monomerous gynseceum consists of a single median carpel (Fig. 299), bearing 1-3 styles (see p. 467) : the single, somewhat campylotropous ovule is sessile on the ventral suture of the carpel. GROUP V.— ANGIOSPERM^ : MONOOOTYLEDONES. 489 The stem is generally characterised by swollen or tumid nodes, to which the sheathing leaf -bases contribute. The long internodes are hollow : the sheathing leaf-bases are largely developed, and frequently extend over several internodes. A membranous ligule is developed at the junction of leaf-base and lamina (see p. 32 ; Fig. 19 A). The more common Grasses are classified as follows : — Series A. PAXICOIDE^ : spikelet one-flowered, or sometimes two-fiowered and then the lower flower is imperfect; articulated so that it falls off entire after flowering ; no prolongation of the axis beyond the flower. Tribe 1. Panicece : spikelets dorsally compressed, in compound spikes : glumes 3, of which the lowest is the smallest : inferior palea without an awn. Panicum fflabrum (Digitaria humifusa), P. (Echinochloa) Crus-gaUi, and P. (Setaria) viride occur occasionally on cultivated land. P. miliaceum yields Millet (Fig. 300). Tribe 2. Maydece: the diclinous flowers are in distinct spikelets; the two kinds of spikelets usually form distinct inflorescences, but sometimes they occur in different parts of the same inflorescence : the lowest glume is the largest. Zea Mais, the Maize Plant, cultivated in warm countries, is a native of Tropical America: the $ spikelets form a loose panicle at the apex of the haulm, and the ? flowers are borne laterally on a thick spadix, which is ensheathed by leaves. Tribe 3. Andropogonece : flowers monoecious or polygamous : glumes 3, of which the lowest is the largest. Saccharum Officinarum, the Sugar-cane, is a, native of the East Indies. Andropoyon Sorghum, in different varieties (vulgaris. Durra, etc.), yields a kind of Millet seed : the flour of this is known in Arabia and India as Durra. Tribe 4. Oryzece : spikes laterally compressed : glumes 2-4, often repre- sented only by bristles : stamens generally 6. Oryza saliva is the Rice- plant, from the East Indies; cultivated in marshy regions of Southern Europe. Leemia oryzoides, the Cut-grass, is found in ditches in the South of England. Series B. POOIDE^: : spikelet one- or many-flowered, with distinct inter- nodes between the flowers : when one-flowered, the axis of the spikelet is prolonged beyond the flower : the' ripe fruits fall, leaving the glumes behind. Tribe 5. Phalaridece: spikelets pedicillate in panicles, laterally com- pressed, 1-flowered : glumes 4, the inner pair being smaller. Phalaris arundinacea, the Reed-Grass, is common on the banks of streams, etc. : a variety with white-streaked leaves is cultivated in gardens. Anthox- anfhum odoratum, Vernal-Grass, which has only two stamens and a pani- culate inflorescence, is common in meadows : it gives the peculiar odour to fresh hay.' 490 PART IV.— CLASSIFICATION. Tribe 6. Agrostidece : spikelets l-flo\vered, in panicles : glumes 2. In Agrostis, the Bent-Grass,- the axis of the spikelet is glabrous, or it baars short hairs ; A. vulgaris and alba are common in meadows : Apera Splca Venti is common in fields : in Calamagrostis, the Small Reed, several species of which occur on the banks of rivers and in woods, the axis of the spikelet is covered with long hairs. Stipa pennata, the Feather-Grass, has a long hairy awn. Milium effusum, Millet-Grass, without an awn, is common in woods. Amongst the forms with dense cylindrical panicles, Alopecurus, the Fox-tail Grass, has the glumes coherent at the base, and one rudimentary palea. Phleum, the Cat's-tail Grass, has free glumes and two distinct paleae. Phleum pratense is commonly known as Timothy- Grass. Tribe 7. Avenece : the pani- culate, or rarely spicate, spike- lets consist of several (usually two) flowers one of which is sometimes $ ; the glumes (or one of them at least) are as long as the whole 'spikelet, longer than the inferior paleae, which usually have a long twisted or bent awn. A vena, the Oat-Grass, has loose panicles, and a two- toothed inferior palea ; of this genus there are many species ; A.fatua (Wild Oats, or Havers), pratensis and pubescens, are common in cornfields and meadows. The following species are cultivated : A. sa- tiva, the Oat (Fig. 302.4), with its panicles in various planes ; A. orientalis, with its panicles in one plane ; A. atrigosa, with a hairy floral axis ; and A, nuda, the spikelets of which usually consist of three flowers. Trisetum (Arena) flavescens, the yellow Oat-Grass, with a free fruit, occurs in pastures. Aira (Deschampsia) c.cespitosa, a.ndflexuosa, Hair-Grasses, have truncate inferior palese, and are common in meadows and woods. Holcus, the Honey-Grass, has spikelets consisting of two flowers, the upper of which is usually or <1> and are not sessile in the axils of the glumes (6 in Fig. 304 A and C), but a short branch springs from the axil of each of these leaves bearing a second bract (s in FIG. 303.— A Flower of Scirpus (magnified): FIG. 304.— Flower of Carex (masr.). p the bristly perianth ; a the three stamens ; A $ flower with (b) bract (elume) ; * the ovary : n the three stigmata. B Its second bract (utriculus) ; / ovary ; n floral diagram. stigma. B inner whorl of the perianth ; n stigma of the $ flower (mag.). Order 1. URTICACELE. Ovary monomerous: ovule central, ortho- tropous. Seed containing endosperm. They are mostly herbs or shrubs without milky juice and frequently provided with stinging hairs : leaves alternate, stipulate. Flowers polygamous, monoecious, or dioecious, in paniculate or glomerulate inflorescences. Urtica urens and dioica (Stinging Nettles) are known by the stinging hairs which are distributed over their whole surface: perianth 2 + 2; the two outer segments of the perianth of the $ flower are larger than the inner segments (Fig. 323 B). In the former species the $ and ? flowers are contained in the same panicle, and the floral axis is but feebly de- veloped ; in the latter they are on different plants, and the axis is well GROUP V.— ANGIOSPERALE ; DICOTYLEDONES. 515 developed and bears leaves. Bohmeria nivea, a native of China and Japan, has strong bast-fibres used for weaving the material known in England as Grass-cloth. Parietaria officinalis, Wall-Pellitory, having polygamous flowers with a gamophyllous perianth, and destitute of stinging hairs occurs occasionally on walls, by roadsides, etc. Order 2. MORACE^E. Ovary generally dimerous, and sometimes bilocular (Artocarpus) : ovule suspended, anatropous or campylotro- pous, more rarely basal and orthotropous : seed with or without endosperm ; the fruit is enveloped by the perianth (usually 2 + 2), which becomes fleshy, or by a fleshy floral axis. Trees and shrubs with milky juice, scattered leaves and caducous stipules. Morus alba and n iyra (Mulberry) come from Asia ; the flowers' are dis- posed in short catkins ; the catkins are borne singly on shoots, which, at the time of flowering are still buds, and they contain the diclinous flowers ; the $ flowers give rise, as ripening take place, to a spurious fruit (sorosis, p. 472), consisting of spurious drupes formed mainly by the perianths. The leaves, particularly of the former species, are the food of the silkworm. Brousxonetia papyrifera (Paper Mulberry) has flowers like the preceding, but they are dioecious : the bark is made into paper in China and Japan. Madura tindoria, in Central America, yields Fustic, a dye. Fiats Carica is the Fig-tree of Southern Europe ; the fig itself (termed a syconus) is the deeply concave axis of the inflorescence, on the inner surface of which the flowers and subsequently the fruits, in FIG. 32*.— Longitudinal the form of hard grains (achenes), are borne (Fig. 8ection of a Fi* (nat 8ize>: 321 »/); the cavity is closed above by small ^eel""/ 1 .^ T, bracts (Fig. 324 6). Ficus elastica is the India- flowers ; 6 bracts, rubber tree ; it is frequently cultivated in rooms. F. religiosa and other East Indian species yield Caoutchouc, which is their inspissated milky juice (latex). Ficus indica is the Banyan. Artocarpus incisa is the Bread-fruit tree of the South Sea Islands ; the large spurious fruit (sorosis) of this tree is roasted and eaten as bread. Galadodendron riile, the Cow-tree of Columbia, has a nutritious latex, while that of Anti- aris toxicaria (Java) is poisonous. Order 3. CANNABINACELE. Ovary dimerous, unilocular: ovule suspended, campylotropous : seed with endosperm. Flowers dioecious : the <£ flowers (Fig. 325 A) have a 5-partite perianth and 5 short stamens; the $ flowers have a tubular entire perianth (Fig. 325 S, p\ enclosed in a bracteole (Fig. 325 B, tf). Herbs with decussate leaves— at least the lower ones— and persistent stipules ; devoid of latex. 516 PART IV.— CLASSIFICATION. FIG. 325.— A perianth ; a stamens. B $ flower magnified : d bract ; c cnpule ; y the epijrynous perianth; g the style; n the stigma. C The same, still more magnified, in longitudinal section ; / ovary ; a ovules. GROUP V. — ANGI08PERlt£ : DICOTYLEDOXES. 521 the pendulous . Ajacis ; see Fig. 273 A) or 4 (D. Staphisagria) posterior are developed ; the spurs of the two posterior petals pro- ject into that of the posterior sepal : D. Staphisagria has 3-5 carpels; D. Consolida and D. Ajacis, common garden plants, have usually but one carpel. In Aconi- tum, the Wolf's-bane or Monk's-hood, the posterior of the 5 petaloid sepals is large and hooded ; the two posterior of the 8 petals have long claws and are covered by the posterior sepal, the others being in- conspicuous (Fig. 342 E, c). Tribe 3. PcKoniece. The perianth con- sists of calyx and corolla, and the petals FIG. 343,-Diagram of flower of are not glandular : ovaries with numerous Aquilegia. ovules, surrounded by a disc : fruit of several follicles. In Pseonia, the Peony, the flower is acyclic: the calyx consists of 5 sepals which gradually pass into the foliage-leaves ; the petals are 5 or more. P. officinalis, corallina, and others are cultivated as decorative plants ; P. Moutan has a woody stem and a tubular disc. Order 2. MAGNOLIACELE. Perianth cyclic, consisting usually of three alternating trimerous whorls, one of sepals and two of petals, stamens and carpels numerous, arranged spirally : seed containing endosperm. Woody trees or shrubs. Tribe 1. Magnoliece. Carpels very numerous on an elongated cylindrical receptacle : flowers invested by a spathoid bract ; stipules connate. Mag- nolia grandiflora and other species, and Liriodendron tulip/era, the Tulip- tree, from North America, are ornamental trees. Tribe 2. Illiciece. Carpels in a single whorl on a flat receptacle (Fig. 286). Illicium anisatum, the Star-Anise, is a native of China. Order 3. NYMPH.EACE.E. Flowers usually acyclic without any sharp demarcation between the petals and the stamens : pistil either apo- or syn-carpous. Water-plants, generally with broad floating leaves. Tribe 1. Nymphceina;, Carpels connate, forming a polymerous multilo- cular ovary which may be either superior or inferior. Ovules numerous, placentation superficial: seeds numerous, containing both endosperm and perisperm, sometimes arillate (p. 415). The rhizome grows at the bottom of the water and throws up broad, flat, cordate leaves with long petioles which float on the surface. The flower also reaches the surface, borne on a long peduncle. GROUP V.— AXGIOSPERM.E ; DICOTYLEDONES. 531 Xgmphaa alba, the white Water-Lily, has four green sepals, a great number of white petals which, together with the very numerous stamens, are arranged spirally, and a semi-inferior ovary. Xuphar luteum, the yellow Water-Lily, has a calyx consisting usually of five greenish-yellow sepals ; the petals, which are smaller and yellow, are usually 18 in number, and form a continuous spiral with the indefinite stamens ; the ovary is superior. Victoria regia, a Brazilian species, has peltate leaves of more than a j-ard in diameter. Tribe 2. Nelunibiece. Ovaries numerous, distinct, imbedded in the fleshy receptacle : seeds solitary, exalbuminous. Nelumbium upeciosum is the Lotus of Egypt and Asia. Tribe 3. Cabombete. Flowers cyclic. Calyx and corolla each three- leaved. Stamens 3-18 or x. Ovaries 3-18, monomerous, each with two or three ovules attached to its walls or to the dorsal suture of the carpel. Seeds containing endosperm and perisperm. The submerged leaves are much divided, the floating leaves peltate. Cabomba occurs in tropical America : Brasenia is widely distributed. Order 4. BERBERIDACE.E. The calyx, corolla, and androecium, each, consist of two di- or tri-merous whorls. Gynaeeeum mono- merous ; ovary with numerous marginal ovules. Fruit capsular or baccate. Seed with endosperm. Berberis vulgar is is the Barberry, its floral formula is /T3 + 3, 6*3-1-3, A3+3, G± ; the flowers are in pendent racemes, usually without terminal Hewers; when a terminal flower is present it is acyclic and its formula is Kb | Co | Ab (see Fig. 262, p. 447). Fruit an oval berry. The leaves of the ordinary shoots are transformed into spines (Fig. 29), in the axils of which are dwarf-shoots bearing the foliage-leaves and the inflorescences. Epi- medium has a dimerous flower ; calyx of 4-5 whorls ; petals spurred. In Berberis, sub-genus Mahonia. there are 3 whorls of sepals, and in Nandina many whorls, the inner of which gradually become petaloid. Podophyllum has sometimes 3 whorls of petals (though the number of petals varies in consequence of oligomery), and shows duplication of the stamens of the inner whorl. The anthers usually dehisce by valves, but in Podophyllum and Nandina the dehiscence is longitudinal. Cohort II. Caryophyllales. Flowers cyclic, generally actino- imrphic and pentamerous, sometimes monochlamydeous : calyx often gamosepalous : stamens usually definite : ovary unilocular, with basal placenta : seed with perisperm. Order 1. CARYOPHYLLACE.E. Flowers generally pentamerous, with calyx and corolla, though the latter is suppressed in some cases; sepals distinct or coherent: stamens in two whorls of which the inner is often wanting; ovary 2-, 3-, or 5-merous, unilocular; or multilocular at the base, with a central placenta 532 PART IV. — CLASSIFICATIONS or with a single basal ovule : fruit usually a capsule : leaves opposite, decussate : stems usually tumid at the nodes. Tribe 1. Ahinea. The corolla and the inner whorl of stamens are usually present ; the calyx is eleutherosepalous ; fruit a capsule ; usually no stipules. The British genera are Sagina (Pearl-Avort), Arenaria (Sand-wort), Cerastium, Stellaria (Chick-weeds and Stitch-worts), Spergula (Spurrey), Lepigonum, Holosteum, Moenchia ; they are mostly small herbaceous plants with white petals, occurring in meadows, on roadsides, etc., but species of Lepigonum (Spergularia), the Sandwort-Spurrey, and Arenaria (Honckenya) peploides, Sea-Purslane, grow on the sea-coast ; they are dis- tinguished from each other principally by the number of carpels present, and by the mode of dehiscence of the fruit. Tribe 2. Silenece. The corolla and the inner whorl of stamens are always pre- sent : the calyx is gamo- sepalous ; stamens 10, fila- ments connate at base : the fruit is a capsule (in Cucu- balus a berry) : the leaves have no stipules ; the floral axis often elongated between the calyx and the corolla (Fig. 344 y} : the petals (as in Lychnis and Saponaria) often have ligular appendages (Fig. 344 x : see p. 459). The species of Dianthus, the Pink, which commonly Fie. 344.-Longitudinal section of the flower of oceur wild are D. deltoides, Lychnis Flos Jovis-. y prolonged axis (anthophore; ^ . and D Armeria . see p. 444) between the calyx and the corolla; * ligular appendages or corona. {After Sachs.) D- Caryophyllus, the Carna- tion, and D. chinensis, are well-known garden flowers ; there are two styles, and the calyx is sur- rounded at its base by bracteoles. The genus Saponaria has two stj-les but no bracteoles; S. officinalis, the Soap- wort, occurs on the banks of rivers. The genus Silene (Catchfly) has three styles; S. inflata, unions, and others, are common in meadows. The genus Lychnis (Campion) has five styles ; the species alba (vespertina) and diurna are dioecious; L. Githayo, the Corn-cockle, is common in fields. Tribe 3. Polycarpece. Leaves with scarious stipules : calyx eleutherose- palous; the corolla is present, but the inner whorl of stamens is wanting : style 3-fid. This group includes the British genus Polycarpon (Allseed) and others. Tribe 4. Paronychiece. Sepals distinct or coherent : the corolla and the GROUP v.— ANGIOSPERM.E; DICOTYLEDONES. 533 inner whorl of stamens are usually wanting: style usually bifid: ovary umlocular, with 1-4 ovules : fruit generally indehiscent. The British genera are Scleranthus (KnaWel), Herniariar Corrigiola (Strap- wort), and Illecebrum : they are small inconspicuous herbs, with scarious stipules (except Scleranthus). The Paronychieae have also been placed, as a distinct natural order, ILLECEBRACE^:, among the Monochlamydeae. There is no doubt that they have affinities with the Chenopodiales. and that they thus connect that cohort with the Caryophyllacese. Cohort III. Parietafes. Flowers cyclic, with calyx and corolla : sepals free : stamens definite or indefinite; gynaeceum of two or more carpels : ovary unilocular, sometimes many-chambered, with parietal placentation : seed with or without endosperm, Order 1. PAPAVERACE^E. Flowers usually actinomorphic, A'2, C2 + 2, A has a siliquose fruit which is generally spuriously bilocular Order 2. FUMARIACE^. Flowers isobilaterally symmetrical, FIG. 315.— Flower of Ctielidoniitr* mojijs (nat. size) ; fc calj-x ; ca outer, ct inner petals ; a Btamens ; n stigma. A Diagram of the flower of Chelidonium. a Many- chambered ovary of Papaver. 534 PART IV.— CLASSIFICATION. or zygomorphic with lateral symmetry : floral formula A"2, C'2 + 2, A2 + 2, G®. The three whorls of the perianth alternate ; one of the outer petals (rarely both) is usually furnished with a spur : in most genera there are three stamens on each side, a central one, with a perfect anther (the stamen of the outer whorl, Fig. 346 B a), and two lateral stamens, each with only half an anther (apparently the halves of the stamens of the inner whorl ; Fig. 346 B at a,). The fruit is siliquose and many-seeded, or one-seeded and indehiscent. Herbaceous plants without milky latex, sometimes climbing by means of their petioles which act as tendrils (Adlumia, Fumaria). Seeds containing endosperm. The flowers of Adlumia, Dicentra, and Hypecoum are isobilateralJy symmetrical. ])icentra spectabilis is a favourite ornamental plant; both the outer petals are spurred, the two inner petals are hollowed at their apices, so that they completely enclose the anthers. In Hypecoum the petals are not spurred, and there are four stamens, two lateral forming the outer whorl, and two antero-posterior forming the inner whorl : fruit usually indehiscent. In Corydalis and Fumaria only one of the outer FIG. 346<-^l Flower of Dicentra spectabilis : one of the outnr petals is removed : s pedicel ; ca the outer, ci the inner petals ; / stamens. B The three stamens of one side, seen from within : /filaments ; a the middle complete anther ; a, a, the lateral half-anthers. C Flower- bud, with the sepals, which soon fall off, still adhering (fc)i (nat. size). Diagram of Fumi- tory. petals is spurred, and consequently the flower is irregular and laterally zygomorphic (p. 455). In Corydalis the fruit is atwo-valved capsule with numerous parietal seeds : some species, e.g. C. cava and solida, have a tuberous rootstock ; others, as C. lutea and aurea, have rhizomes. Fumaria officinalis, and others (Fumitories) are common in fields ; the ovaries contain but few ovules, and of these only one ripens to a seed ; fruit globose, indehiscent. Order 3. CRUCIFER^:. Flowers regular, isobilateral : floral for- mula K2 + 2, C x 4, A2 + 22, 6P. The four petals form a whorl, alter- GROUP - V.— ANGIOSPERM.E ; DICOTYLEDONES. 535 nating with the four sepals as if the latter formed one whorl; there are, however, three perianth-whorls, as in the two preceding families ; but whereas in them only the outermost whorl is sepaloid, in this family the two outer whorls are sepaloid, and the innermost, which alone is petaloid, is a whorl consisting of four instead of two mem- bers. The two outer stamens are lateral, as in those families ; the two inner ones, which in most Fumariaceae are apparently divided, are here duplicate, having longer filaments (Fig. 348 B b b) than the outer ones (a) ; hence they are tetradynamous. There are usually four, sometimes more, nectaries at the base of the stamens (Fig. 348 B d). The ovary consists of two carpels with the ovules in two longi- tudinal rows on the connate margins of the carpels ; these two parietal placentae are connected by a membranous growth which, as it is not formed of the margins of the carpels, must be regarded o. 317. — Diagram of the flower of Cruciferae. Fi 343.- Flowers, fruits, and embryos of various Cruciferas. A Flower of Brassica (nat. sizeT's pedicel; l:k calyx; c corolla. B The same after removal of the perianth ( ma) ; or, thirdly, the radicle may be lateral to the two cotyledons (Fig. 348 //), when the cotyledons are said to be accumbentr Pleurorliizece (diagram Q =) : more rarely the cotyledons are spirally rolled so that in a transverse section they are cut through twice, Spirolobece (diagram Q || ||) ; or, finally, they may be doubly folded, and be seen four times in a section, Diplocolobem (diagram Q II II II ID- The seeds contain much fatty oil. Sub-order 1. SILIQUOS.E. Fruit a siliqua, much longer than it is broad. GROUP V.— ANGIOSPERM*: ; DICOTYLEDONES. 537 Tribe 1. Arabidece. Q=. Cheiranthus Cheiri, the Wall-flower, and Matthiola annua and incana, the Stocks, are cultivated as garden-plants. Nasturtium qfficinale is the Water-cress. Barbarea vulgaris is the Yellow Rocket. Cardamine (incl. Dentaria) also belongs to this tribe. Tribe 2. SisymbrtecK. Q \\ . Sisymbrium officinale, the Hedge-Mustard, is common on rubbish heaps ; and Erysimum, the Treacle-Mustard, on walls, etc. Hesperis is the Dame's Violet. Tribe 3. Brassicecn. Q^> The species and varieties of Brassica are much cultivated. Brassica oleracea is the Cabbage, with the following varieties ; acephala, Scotch kale, Cow-cabbage or Borecole ; bullata, the Savoy -cabbage 5 capitata, the red and white Cabbage; caulorapa, with thi1 stem swollen at base, is the Kohl-rabi; Botrytis, with connate fleshy peduncles and abortive flowers, is the Broccoli (asparagoides) and the Cauliflower (cauliflora) ; gemmifera^ with numerous lateral leaf-buds, known as Brussels-sprouts. Brassica campestris is the wild Navew; it includes the following sub-species; Itapa, the wild Turnip, with bright green hispid leaves and flat corymbs of flowers, among the cultivated varieties of which is the var. depressa, the Turnip : Napus, the wild Rape, with glabrous glaucous leaves and long racemes of flowers, several varie- ties of which are cultivated for their oily seeds, and one (var. esculenta, the Teltow Turnip) for its fleshy root : Napobrassica, the Turnip-cabbage, including Rutabaga, the Swedish Turnip. B. campestris oleifera is the true Colza or Coleseed, from the seeds of which colza-oil is obtained. Brassica (Sinapis) nigra and alba are the black and white Mustard. Brn**i<-i' Sinapis (Sinapis arvensis) is the Charlock or Corn-Mustard. To this tribe belongs also the genus Diplotaxis. Sub-order 2. SILICULOS.E. Fruit a silicula. A. LatiseptcE. The dessepiment is. in the longest diameter of the silicula. Tribe 4. Alyssinece. Q =. Cochlearia officinalis is the Scurvy-grass ; C. Armoracia, the Horse-radish, has a thickened root. Alyssum cali/cinum and Draba (Erophila) vernat the Whitlow-grass (Fig, 348 E), are common weeds : Lunaria biennis is Honesty, Tribe 5. Camelinece. Q || . To this tribe belong Camelina (Gold-of- pleasure), and Subularia, the Awl- wort, an aquatic plant. B. Anguntiseptce. The dissepiment is in the shortest diameter of the silicula. Tribe 6. Lepidinece. Q || . Capsella Bursa Pastoris, the Shepherd's Purse, is common, as also various species of Senebiera and Lepidium (Cresses). Tribe 7. T/ilaspidece. Q =. Various species of Thlaspi, the Penny- Cress, are common. To this tribe belong also the British genera Iberi* (Candytuft), Teesdalia, and Hutchinsia. Sub-order 3. NUCUMENTACEVE. Silicula indehiscent, few-seeded. Tribe 8. Isatidece. laatis tinctoria, the Woad, has compressed, pendul- ous, unilocular, one-seeded fruits (Fig. 348 F) : the leaves yield a blue dye. Sub-order 4. LOMENTACE^:. Fruit a siliqua or silicula, constricted into one-seeded segments (lomentaceous) (Fig. 348 G). 538 PART IV. — CLASSIFICATION. Tribe 9. CakilinecK. Silicula two-jointed. This tribe contains the genera Cakile, the Sea-Rocket, and Crambe, the Sea-Kale. Tribe 10. RaphanetK. Silicula more or less moniliform. Raphanm sati- vus is the Radish ; It. Baphanistrum, the wild Radish or White Charlock, is a common weed. Order 4. CISTACE.E. Flowers usually actinomorphic and penta- merous : the two external of the five sepals are generally smaller, aud sometimes they are absent : stamens numerous, in consequence of multiplication : carpels 3-10, forming a uni- or multilocular ovary ; ovules orthotropous ; seed with endosperm. Trees or shrubs with generally opposite stipulate leaves. Cistus has 5-10 carpels forming a chambered or completely mutilocular ovary. Cistus ladaniferus, creticue, ^.nd other species, grow in the south of Europe : a balsam is derived from them. Helianthemum has a unilocular trimerous ovary : Helian- themum vulgare, the Bock Hose, is an under -shrub which grows wild on dry soils. Order 5. VIOLACELE. Floral formula 7i5, Co, A5, G{~ : flowers always borne laterally : ovules anatropous : fruit a Iccu- licidal capsule (Fig. 349 (7) : seed with endo- sperm. The indigenous species have irregular dorsiventral flowers ; the anterior inferior petal is prolonged into a hollow spur (Fig. 349 A cs) in which the nectar secreted by the spur-like appendages of the two lower stamens collects (Fig. 349 Afs). The sepals are produced at the base (Fig. 349 A Is}. Viola is the Violet, Pansy, or Heart's-ease : — many species, as V. odorata, the Sweet Violet, have only an underground stem which bears cataplryll- ary leaves, and which throws up petiolate foliage-leaves, and bracteolate peduncles each bearing a single flower : V. odorata has runners, but hirta and collina have none : — in others, as V. canina, the Dog-Violet, the main stem is above ground and bears the foliage-leaves : — in V. mirabilis these two forms are so combined that, in the spring, flowers are developed from the rhizome which have large blue petals but are always sterile ; it is not Fis. 349 — Viola tricolor. A Longitudinal section of flower: v bracteole on the peduncle; I sepals; Is ap- pendage; c petals; cs spur of the lower petals ; j't glandular appendage of the lower stamens ; a an- thers (after Sachs). B Ripe fruit ; fc calyx. C After Oehiscence; p parietal placenta; steeds. (Mag.) GROUP V. — ANGIOSPERM.E ; DICOTYLEDOXES. 539 till later that inconspicuous (cleistogamous, p. 410) flowers with minute petals appear on the leafy stem, and these only ace fertile : — in V. tricolor and its allies the stipules are leafy and pinatifid. Cohort IV. Guttiferales. Flowers usually cyclic, generally actinomorphic, and pentamerous : sepals usually free, with imbricate aestivation : stamens usually indefinite : gynseceum syncarpous, ovary uni- or multi-locular : seed exalbuminous. Order 1. HYPERICACE^E. Formula usually Ko, (75, .40 + 5oo. G1- ; or ^40+ 3 oo, 6?'^. Sepals sometimes united at the base : sta- mens usually indefinite and polyadelphous ; when in five bundles, the bundles are super- posed on the petals ; this position of the stamens is generally attributed to the suppression of an outer whorl of stamens which is indicated by staminodes in species of all the genera : ovary uni- or multi-locular, or many-chambered ; capsule septicidal ; ovules numerous, anatro- . , , ., W i_ FlG- 350.— TiBgram of pous ; placentae parietal or axile. Herbs or aypericum caiydnum. under-shrubs with decussate entire leaves, which are dotted over with translucent oil-glands ; exstipulate. The following are examples of the different relative numbers of staminal bundles and of carpels : — Staminal bundles 5, carpels 5: Hypericum calycinum. Staminal bundles 3, carpels 8 : H. Humifusum, hirsutum, montanum, per- foratum, undulatum, barbatum. Staminal bundles 5, carpels 3: H. Androscemum, hircinum, elalum. Staminal bundles 3, carpels 5 : H. peplidifolium. All these species, except the last (St. John's Worts, or Tutsans), occur wild in Britain. Cohort V. Malvales. Flowers cyclic, generally pentamerous and actinomorphic : calyx often gamosepalous, with valvate aesti- vation : corolla with usually contorted aestivation : stamens typi- cally in two whorls, frequently obdiplostemonous (p. 452), sometimes branched, and often connate : carpels usually five and then anti- petalous, often forming a multilocular ovary: seed usually with endosperm- Order 1. TILIACE.E. Sepals usually free : stamens 10 or indefi- nite, sometimes polyadelphous ; in the indigenous species the stami- nal whorl opposite to the sepals is suppressed, and there are 5 antipetalous staminal bundles ; anthers 4-locular, opening by pores or valves : gynEeceum usually completely syncarpous ; style 1 ; 540 PART IV.— CLASSIFICATION. ovary usually 5-locular, each loculus containing two ovules ; but the fruit is generally only one-seeded. Mostly trees or shrubs : leaves alternate, stipulate. The only indigenous genus is Tilia, the Lime-tree. It has oblique leaves with deciduous stipules; the annual shoots have not a terminal bud. The inflorescence is cymose, few-flowered : the peduncle is adnate to the leafy bracteole ; this is brought about in the following manner : in the axil of the leaves there is usually a bud, together with an inflorescence (Fig. 351) : the large bracteole ( 7t) and a bud-scale, which is opposite to it, are the first two leaves of the axillary shoot which is terminated by the infloresence, the peduncle of which is adnate to the large bracteole for some distance : the bud is a winter-bud developed in the axil of the above-mentioned bud-scale. The inflorescence itself terminates in a flower ; other flowers are borne in the axil of two upper bracteoles which soon fall off, and other flowers again may be developed in the axils of their bracteoles, and so on. T. platy- phyllos, the large-leafed Lime, has a few-flowered inflorescence, and leaves which are bright green and downy on the under surface: T. cordata has an inflorescence which consists of a large number of flowers, and has small leaves which are bluish-green and pubescent with red hairs on the under surface. T. vulyaris is the com- mon Lime. Corchorus, in the East Indies, yields Jute, which consists of the bast-fibres. Order 2. MALVACEAE. Calyx usually gamosepalous, frequently invested by an epicalyx (p. 443) ; the corolla is adnate at the base to the androecium : the typically obdiplostemonous androesium is a long tube (Fig. 352 A} consisting of five monadelphous usually branched stamens which are opposite to the petals, each branch bearing a bilocular anther ; there is sometimes an inner series of staminodes opposite to the sepals : carpels 5- oo ; styles many, connate ; the gynseseum is sometimes Fio. 351. — Inflorescence of the Lime : n branch ; It petiole subtending an in- florescence and a bud. Attached to the peduncle is the large bracteole (h) : fe calyx ; c corolla ; s stamens ; / ovary ; fc?i flower-bud (nat. tize). GROUP V. — ANGIOSPERSLE ; DICOTYLEDONES. 541 almost apocarpous (Malopeae) ; usually syncarpous with a multi- locular ovary, splitting into cocci (Fig. 352 C D), with usually one ovule in each coccus (p. 473), or a loculicidal capsule (Hibiscese). Under-shrubs or herbs : leaves stipulate and generally palmately veined. Malva, the Mallow, has an epicalyx of three bracteoles, Hibiscus has one of many bracteoles, and Althaea has one of 6-9 brocteoles : Althcea rosea is the Hollyhock, and A. officinalis is the Marsh-mallow: several species of Malva are indigenous, M. sylvestris, rotundifolia, and moschata : Gossij- n FIG. 352. — A Flower of Malva Alcea (nat. size) : fc calyx ; c corolla ; s connate stamens, with the anthers (a) ; 71 stigmata. B Fruit of Althcea rosea enclosed iu {fc) the calyx : ak epical3-x. C The same after the removal of the calyx. D A single coccus of the same in longitudinal section : » seed ; w radicle ; st cotyledon of the embryo (mag.). plum herbaceum (with the vars. religiosum and hirmtum) and G. arboreum in Egypt and the East Indies, and G. barbadtnse (with var. peruvianuni) in America, yield Cotton, which consists of the long hairs on the testa of the SERIES II.— DISCIFLOR.E. Flowers typically encyclic and generally pentamerous, often obdiplostemonous : sepals free and coherent : petals in a single whorl : stamens usually definite, and hypogynous : a disc is usually present : gynseceum generally syncarpous. Cohort I. Geraniales. Flowers usually pentamerous through- out; formula Kb, Co \ -45 + 5, G-} ; generally obdiplostemonous; the carpels are opposite to the petals : ovary usually 5-locular, with 542 PART IV. — CLASSIFICATION. 1 or 2 suspended ovules ; the micropyle is directed inwards : disc various or wanting. Order 1. GEBANIACE^E. Disc usually represented by a gland at the base of and outside each of the antisepalous stamens : flowers usually actinomorphic : stamens connate at the base : the carpels are prolonged into a carpophore (Fig. 353 A a) ; two ovules in each loculus ; the fruit is septicidal from below upwards, the awns of the separating carpels (cocci, see p. 473) rolling up (Fig. 353 B}. Seed devoid of endosperm. Herbs ; leaves simple, stipulate. Geranium has 10 stamens : in most species the seed is expelled on the rolling up of the awn: Geranium pratense, sylvati- cum, sanguineum, columbinum, and other species, the Crane's-bills, are wild in Eng- land ; G. Robertianum, Herb-Robert, is universally distributed. Erodium, the Stork's-bill, has the 5 stamens which are opposite to the petals transformed into staminodes ; E. cicutarium is common in waste places. Pelargonium, in many varieties, is a well-known garden-plant ; the flowers are irregular and dorsiventral ; the disc is absent, but the posterior sepal is provided with a glandular spur which adheres to the pedicel. The cocci of Ero- dium and Pelargonium are indehiscent, and are forced into the ground by the movement of the hygroscopic awn. Order 2. LINACEJE. Disc generally a whorl of 10 small extra- staminal glands : formula K5, (75, ( | A f 5 + 5), G- : flowers acti- nomorphic, rarely all the whorls are tetramerous : stamens mona- delphous at the base ; the whorl of stamens opposite to the petals is replaced by staminodes . each loculus of the ovary contains two ovules, and is often divided into two by a more or less complete false dissepiment : seed usually contains endosperm : capsule septi- cidal. Herbs or shrubs ; leaves simple, entire, with or without stipules. Linum usitatissimum is the Flax : the strong bast-fibres are used in weaving linen ; the seeds contain oil ; the walls of the outer cells of the testa are mucilaginous. There are several British species of Linum. Radio! a, the other British genus, has tetramerous flowers. FIG. 353.— Fruit of Geranium. A Before, R after splitting into cocci ; s pedicel ; / loculi of the ovary ; a iu B the awn; n stigma ; a and b carpophore (magr.). GROUP V.— AXGIOSPER&LE J DICOTYLEDON KS. 543 Order 3. OXALIDACE.E. Disc present as small glands at the base of the antipetalous stamens, or of all of them : flowers actino- morphic; formula Ko, Co, ( | 45 + 5), G^ ; the antipetalous stamens are sometimes staminodial ; those which are opposite to the sepals are the longest : ovules numerous ; fruit a capsule, or more rarely a berry ; seed containing endosperm. Herbs, with compound (ternate), generally exstipulate leaves. Oxalis Acetosella, the Wood-S3rrel, is frequent in woods; it contains much potassium oxalate. The tuberous roots or underground stems of some American species, as 0. esculenfa, crenata, and Depjyei, contain much mucilage, and are used as food. Some species (e.g. O. gracilis) show tri- morphic heterostylism (p. 411) : others (e.g. O. Acetosella), have cleistoga- mous flowers (p. 410). The leaves of Oxalis and Averrhoa show sleep- movements : those of Biophytum are sensitive to touch. Order 4. BALSAMIXACE.E. Disc 0: flowers irregular, dorsiven- tral ; formula Kb, (75, | 40 + 5, G(- : the posterior sepal is spurred, and the two anterior are small or absent : the anterior petal is large : ovary 5-locular ; ovules numerous ; the fruit is loculicidally septifragal, the valves separate elastically and roll upwards, so that the seeds are projected to some distance ; seed without endosperm. Herbs, with simple exstipulate leaves. Impatiens Noli-me-tangere, the yellow Wild Balsam, occurs in damp and shady spots ; the ripe fruit flies open with violence at a touch. Impatient Balsamina, an Indian species, is cultivated. Order 5. RuTACEjE. Disc usually annular : flowers usually actinomorphic : gynseceum sometimes partially apocarpous, but the styles are usually connate : seed with or without endosperm. There are numerous oil-glands on the leaves and stems. Sub-order. E.CTE.E. The placentse project into the loculi of the ovary; each bears 3 or more ovules : fruit a loculicidal capsule : seed with endosperm. Ruta grateolens, the Hue, has pentamerous terminal flowers, and tetramerous lateral flowers. Dictamnus Fraxin- ella has an irregular dorsiventral flower. Sub-order. AURANTIE^:. Gynseceum syncar- pous : calyx gamosepalous : fruit a berry (p. 476) : seed without endosperm. The genus Citrus has an indefinite number of bundles of connate stamens (polyadelphous) (Fig. F|G- 351.-Diapram of 355 A), all belonging apparently to the aiitise- palous inner whorl : the carpels are usually more numerous than the petals, and during ripening they become filled with a succulent tissue de- 544 PART IV.— CLASSIFICATION. derived from their walls ; the various parts of the flower and the fruit (p. 97) contain much ethereal oil : the leaf, which is typically pinnate, is reduced to its terminal leaflet which is articulated to the winged petiole (Fig. 23 (?); the leaf is sometimes spinous. Fio. 355.— Flower and aoral diagram of Citrus. A Open flower ; c corolla; s the partially connate stamens ; n the stigma. B Bud ; fc calyx; c corolla; d oil-glands. Citrus medico is the Citron; C. medica var. Limonum, is the Lemon; C. medico var. Limetta, is the Lime; Citrus Aura nti um var. Bigaradia (or C. vulyaris) is the Bitter or Seville Orange, and C. Aurantium sinense is the Sweet Orange; Citrus nobilisis the Mandarin Orange ; and Citrus decumaua is the Shaddock: all originally derived from tropical Asia- Cohort II. Sapindales. Flowers typically pentamerous and obdiplostemonous but with reduction in the andrcecium, actino- morphic or zygomorphic, sometimes unisexual : gynseoeuin oligomerous, usually syncarpous. Mostly trees. Order 1. SAPIXDACE.E. Flowers usually irregular, obliquel}* zygomorphic or asymmetric, in that the two petals of one side are larger and of somewhat different form to the three others ; of these, one, which lies in the plane of symmetry, is sometimes wanting FIG. 350.— Floral dia- gram of JEsculus': but the missing stamens should be represented us antisepalous. FIG. 357.— Fruit of A. platanoides, dividing into tv mericarps in ; s pedicel ; ;! wings (nat. size). GROUP V.— ANGIOSPERJOE ; DICOTYLEDONES. 545 two or three of the antisepalous stamens are usually suppressed, so that the number is eight or seven ; they are inserted within the disc : the ovary is trilocular ; ovules two "in each loculus : seed without endosperm. jEsculus has opposite, palmately compound, exstipulate leaves ; the flowers are in terminal scorpioid racemes ; the fruit has a loculicidal de- hiscence : ;E. Hippocastanum is the Horse-Chestnut, derived from Asia ; X. carnea, JE. Pavia, and other species are frequently cultivated. A great number of genera and species grow in warm climates; they have generally scattered pinnate leaves : often climbers with branch-tendrils. The fleshy fruit of Sapindus Saponaria makes a lather with water like soap. Order 2. ACERACE^E. Flowers regular: stamens commonly eight, in consequence of the suppression of the two median ones, variously inserted : disc annular, rarely absent, extrastaminal or intrastaminal : ovary bilocular ; ovules two in each loculus ; when ripe the fruit splits into two one-seeded winged mericarps (samaras, p. 473, Fig. 357) : leaves opposite, palmately lobed, sometimes com- pound, exstipulate : flowers in terminal racemes, sometimes in corymbs with an apical flower : seed without endosperm. The principal species of Acer, the Maple, are A. Paeudoplatanus, the Sycamore, having leaves with crenate margins, flowers in elongated pen- dulous racemes, blooming after the unfolding of the leaves, and parallel- winged fruits ; A. platanoides, having leaves with serrate margins, flowers in short erect racemes blooming before the unfolding of the leaves, and fruits with widely diverging wings (even more than in Fig. 357) ; A. cam- pest re, the common Maple, which is sometimes shrubby, with a trilobate leaf, short erect racemes of flowers which bloom after the unfolding of the leaves, and fruits with wings which are diametrically opposite. Some North American species are often cultivated, such as A. rubrum, with five stamens opposite to the sepals, and a rudimentary disc; A. dasycarpum, with the same number and position of the stamens, without any corolla, and having dioecious flowers; A. Negundo, with compound 3-5 foliolate leaves, and dioecious flowers like those of the preceding species. Sugar is prepared from the sap of A. saccharinum and dasycarpum especially. Order 3. POLYGALACE^E. Flowers irregular, dorsiventral ; the two lateral sepals conspicuously large and known as " wings " (Fig. 358 A k') : petals three, the two lateral being absent ; the anterior petal is very large and carinate : stamens usually eight, forming a tube open posteriorly, to which the corolla, or at least the anterior petal, is adnate (Fig. 358 B} : disc rudimentary : carpels two, median, forming a bilocular ovary, each loculus containing a single suspended ovule : fruit usually a capsule. The flower somewhat 546 PART IV. — CLASSIFICATION. resembles that of the Papilionese, but it must be borne in mind that here the two " alee " or wings belong to the calyx. FIG. 358.— Flower of Polygola gfandiflora. A Seen from outside after the removal of the wing-sepal fc. B Longitudinal section : fc calyx : fc' wing ; c corolla ; s tube of stamens. (After Sachs.) The flower of the Polygalacese resembles that of the Aceraceae in the suppression of two stamens in the plane of the two carpels. Polyyala vulgar is, amara, and others, the Milkworts, are herbs, woody at the base, occurring in woods and meadows. Cohort III. Celastrales. Flowers regular, frequently actino- morphic, 4-5-merous ; only one whorl of stamens, which either alternates with or is opposite to the petals, is usually present : disc usually within, sometimes external to, the androscium : ovules usually erect : the seed nearly always contains endosperm. Trees or shrubs. Order 1. CELASTRACE^:. Formula, Ah, Cn, ^4n, G (n) or less, n = 4 or 5: sepals imbricate: stamens and carpels inserted on a flattened disc : stamens alternate with the petals : usually two ovules in each loculus of the ovary : leaves scattered, entire, stipulate. In the genus Euonymus, the Spindle-tree, the loculicidal capsule contains seeds in- vested by an orange-coloured arillode (p. 416) ; E. turopwa occurs both cultivated and wild. Order 2. RHAMNACE^E. Formula, Kn, Cn, | An, G-~ ; n = 4 or 5 : calyx usually gamosepalous, valvate ; petals usually small and often hood-shaped (Fig. 359 c), enclosing the stamens which are opposite to them : flowers sometimes diclinous : usually a single ovule in each loculus of the ovary which is in- Fio. 359. — Flower of Ehnminis Frangula (mag.) : fc sepals con- nate at the base iuto a tube (d) ; c hood-shaped petals enclosing the stamens (a) . GROUP V. — AXGIOSPERM^E ; DICOTYLEDONES. 547 vested by a disc : leaves usually scattered, entire, stipulate : fruit a drupe or a capsule. Rhamnns catharttca, the Buckthorn, has opposite leaves and thorny twigs : the berries of R. infectoria, in Southern Europe, yield a green or yellow dye : R. Frangula has scattered leaves ; its wood produces a par- ticularly light charcoal. Order 3. AMPELIDACEJS. Formula same as in Rhamnacese : sepals small ; the corolla is often thrown off before it opens (Fig. 360 A c) : a glandular disc between the androecium and the gynse- ceum : ovules one or two in each loculus : fruit baccate. Climbing plants, with stem-tendrils ; leaves palmate, exstipulate or stipulate. Vitis vinifera, the Grape-Vine, probably derived from the East, is culti- vated in endless varieties ; other species, such as V. vulpina and Labrnsca, as also Ampelopsis hederacea, the Virginian Creeper, are also frequently cultivated. The tendrils of the Vine (Fig. 15 A) are branches bearing scaly leaves in the axils of which other , branches arise : their peculiar position op- posite to the foliage- leaves may be ex- plained as follows : the ordinary shoots are sympodia, and each tendril is the terminal , , FIG. 360.— Flower of Vitis vinifera, and diagram. A At segment of a member the moment of opening B Open. fc ca]yx. c corolla. d of the sympodmm ; the glands . , 8tamens ; /ovary ; n stigma (slightly mag.), following member is a shoot springing from the axil of the foliage-leaf which is opposite to the tendril. Every third leaf has no tendril opposite to it, that is to say, the members of the sympodium alternately bear one or two leaves. The inflorescences occupy the same positions as the tendrils. Each leaf has also a bud in its axil, which either remains undeveloped or gives rise to a dwarf-shoot: from the axil of the cataphyllary leaf of the dwarf-shoot an ordinary shoot is developed. In some species of Ampelopsis (e.g. A. Veitchii and Roylei) the tendrils attach themselves to flat surfaces by means of discoid suckers developed at their tips. SEEIES III. CALYCIFLOR-E. Flowers epigynous or perigynous : calyx usually gamosepalous : stamens definite or indefinite : gynseceum syncarpous or apocar- pous. Cohort I. Um be Hales. Flowers regular, sometimes actino- morphic, epigynous, with generally a single whorl of stamens 548 PART IV. — CLASSIFICATION. opposite to the sepals : calyx inconspicuous : ovary bilocular, with one ovule in each loculus : a disc between the stamens and the styles : inflorescence usually umbellate : seed containing endosperm : leaves exstipulate. Order 1. UMBELLIFERJE. Flowers generally regular, but with oligomery in the gynseceum ; formula, K6, (75, Ab, G& : the calyx is generally very small, often hardly visible, though sometimes well developed (e.g. Eryngium, Astrantia) : the corolla consists of five rather small white or yellow petals ; occasionally the outermost petals of the flowers at the circumference of the umbel are larger than the others, and the umbel is then termed radiant : stamens five ; ovary inferior, bilocular : the base of rr FIG. 361. — A Flower of Foeniculum (mag.) : / ovary; c corolla; s stamens; d disc. B Fruit of Heracleum : p pedicel ; g style ; r r r ridges (costa?) : rr marginal ridges ; o oil- ducts (vittae) (mag.). C Transverse section of mericarp of Carwro Cantt (Orthosperine*) : w surface that comes into contact with the other mericarp; o vittse; e endosperm. D Transverse section of mericarp of Conium (Campylospermece). S Fruit of Coriandrum, (Ccelospermece) : k margin of the surface along which the two mericarps are in contact ; r ridges; n secondary ridges : F section of a mericarp. (Mag.) the two styles is fleshy and thickened, forming an epigyuous disc (Fig. 361 A d) ; one suspended ovule in each loculus of the ovary (Fig. 284 E) : the fruit, when ripe, splits into two meri- carps, each loculus of the ovary being permanently closed by a median septum (Fig. 362 B a; see p. 473). The structure of the pericarp is an important characteristic for the classification of the family. The fruit is commonly either oval in form, or compressed (Fig. 361 B\ or nearly spherical (Fig. 361 E] : its surface generally bears longitudinal ridges (costce or juga primarid) enclosing vas- GROUP V. — ANGIOSPERMJE ; DICOTYLEDONES. 541> cular bundles, five generally on each mericarp ; of these, two run along the margins (Fig. 361 B, C, D, rr\ and the other three along the dorsal surface (Fig. 361 B, <7, Z), r). In the spaces between the ridges which form furrows, lie oil-ducts or receptacles (vittce) (Fig. 361 B, C, o), and sometimes other secondary ridges (juga. sccundaria) (Fig. 361 E, F, n), which do not enclose vascular bundles. The mericarp when ripe is filled by the seed, which consists of the abundant endosperm (Fig. 361 C, D, F, e~) enclosing a small embryo. According to the form assumed by the endosperm, the following groups may be distinguished : the OrthospcrmecKr in which the surface of the endosperm, which is directed towards the plane of junction of the two mericarps, is flat or convex, as in Carum (Fig. 361 C) : the Campylospermece, in which the endosperm is concave to- wards the same plane, as in Conium (Fig. 361 D], and the Coelospermece, in which the whole endosperm is curved, so that it is seen to be concave towards this plane both in longitudinal and in transverse section, as in Coriander (Fig. 361 F). The flowers, with few exceptions (Hy- drocotyle, Astrantia, Eryngium, where the umbels are simple), are in compound um- bels (p. 440) ; in some few cases, as in Caucus, the umbel has a distinct terminal flower which is black in colour: an in- volucre and involucels are largely de- veloped in some species, in others they are wholly wanting. The hollow stem bears large leaves with generally well-developed sheathing bases and much divided laminae : rarely the leaves are simple, as in Hydro- cotyle and Bupleurum. The British genera are arranged as follows : — Sub-order I. ORTHOSPERMK.E. A. Umbels simple. Tribe 1. Hydrocotylece. Fruit laterally compressed. The genus Hy< In. - cotyle consists of marsh-plants with peltate leaves (Fig. 22). Tribe 2. Saniculece. Fruit nearly cylindrical. This group includes the genera Astrantia, Eryngium, and Sanicula. B. Umbels compound. Tribe 3. Amminece. Fruit without secondary ridges, laterally com- Fia. 362.— Fmit of Caram. Carui. A Ovary of the flower (/). B Ripe Fruit. The two carpels have separated go a* to form two mericarps (m). Part of the septum consti- tutes the carpophore (a). 550 PART IV. — CLASSIFICATIOX. pressed : Ammi, Bupleurum, Petroselinum, Apium, ^Egopodium, Carum (Figs. 361 C, and 362), Cicuta, Slum, Pimpinella, Trinia, Conopodium, Sison. Tribe 4. Seselinetv. Secondary ridges absent, or if present (Siler) not so prominent as the primary : fruit not compressed : ^Ethusa, Foeniculum, (Enanthe, Seseli, Meum, Ligusticum, Silaus, Crithmum, Siler. Tribe 5. AnYLOSI>ERME.£. Tribe 8. Caucalinece. Secondary ridges spinous : Caucalis (incl. Torilis). Tribe 9. Smyrniece. Fruit without secondary ridges : Anthriscus, Myrrhis, Conium (Fig. 361 Z>), Smyrnium, Physospermum. Sub-order III. C is the Pumpkin : the genus Cucumis has free stamens ; Cucumis sativa is the Cucumber, and Cucumis Melo is the Melon: Cilrullus vuifjaris is the Water Melon. The genus Bryonia has a small white corolla : the loculi of the ovary are 2-seeded, and the fruit is a succulent berry ; B. dioica is common in shrubberies and hedges. FIG. 363.— A Longitudinal section of $ flower of Cucumis: /ovary; sfc ovules ; t calyx ; C corolla; n stigma ; st' rudimentary stamens. B Longitudi- nal section of •) ; sk the ovules ; k calyx ; c corolla ; st stamens ; a an- thers ; g style (enlarged) . Order 3. MYRTACEJE. Flowers 4- or 5-merous, epigynous : sta- mens often very numerous, free, or connate in usually antipetalous bundles (Fig. 365) ; sometimes few and obdiplostemonous : ovary 1— oo-locular ; seeds 1-x in each loculus, without endosperm: placentation and fruit various : leaves usually opposite, dotted with oil-glands. Shrubs or trees. Myrtus communis is the Myrtle of Southern Europe ; the genus Eugenia includes a number of ornamental shrubs, among which is E. (Jambosa) Ceiryopkytttit, the buds and flowers of which yield the spice known as cloves (Fig. 366). Eucalyptus Globulus, from Australia, is much planted 554 PART IV.— CLASSIFICATION. in marshy districts, which it tends to dry up by its active transpiration. Berthdletia excelsn grows in tropical America; its seeds are known as Brazil nuts. Punica Granatum, the Pomegranate, grows in Southern Europe ; flowers 5-8-merous ; receptacle petaloid ; stamens indefinite ; in the ovary there are two whorls of loculi, an external superior of which the loculi are as numerous as and are opposite to the petals, and an internal inferior consisting of three loculi. Cohort IV. Resales. Flowers actinomorphic or zygomorphic, usually monoclinous and perigynous : stamens rarely fewer in number than the petals or equal to them, generally indefinite in numerous whorls : gynseceum more or less completely apocarpous : ovules anatropous, suspended or erect : seed generally without endosperm. Order 1. ROSACELE. Flowers actinomorphic, rarely zygo- morphic, perigynous : gyngeceum generally apocarpous ; carpels 1- oo ; ovules 1 or few, anatropous : fruit various ; seed generally without endosperm : leaves scattered, stipulate ; the odd sepal is posterior (see Fig. 267). Tribe 1. Roseau. Carpels numerous, attached to the base and sides of the hollow receptacle, which is narrow above (Fig. 367 C) ; each contains a, single suspended ovule ; when ripe, they are achenes enclosed in the fleshy receptacle : the sepals are frequently persistent at the top of it. Shrubs with imparipinnate leaves; the stipules are adnate to the petiole. Many species of Rosa, the Rose, are wild, such as R, arvensis, canina, and rubiyinosa (Sweet-Briar or Eglantine) ; and many others are culti- vated, as R. centifolia, damascene/,, indica, gallica, etc. Tribe 2. Spirceece. Carpels usually 5, each containing two or more suspended ovules ; they are inserted upon the floor of the flat open receptacle, and becomfe follicles ; the calyx is persistent till the fruit is ripe. Spircea Ulmaria, Meadow-sweet, and S. Filipendula, Dropwort, occur in woods, meadows, etc.; Sp. sorbifoiia, media, ulmifolia, and other species, Kerria japonica, and Rhodoty pus (with drupes), are ornamental shrubs. Tribe 3. Prunece. The single carpel, containing two suspended ovules, is inserted on the floor of the receptacle (Figs. 367 A and 368 A) ; the receptacle and the calyx fall off when the fruit is ripe: stamens usually in three whorls of 5 or 10 ; fruit a drupe (p. 475, Fig. 290) ; only one seed is usually present. Prunus is the principal genus of the tribe. In the sub-genus Amyg- dalus the fruit has a furrowed coriaceous endocarp ; Prunus Amygdalus (A. communis), the Almond-tree, and nana, are trees of Southern Europe ; P. Persica is the Peach. In the sub-genus Prunophora, the fruit has a smooth, stony endocarp ; P. communis (spinoya) is the Sloe or Blackthorn ; GROUP V, — ANGIOSPERM^E ; DICOTYLEDONES. 555 P. Armeniaca is the Apricot; P. domestica is the Wild Plum, it has an ovoid fruit and glabrous shoots : P. insititia is the Bullace, it has a globoid fruit and hirsute shoots. In the. sub-genus Cerasus, P. Cerasus, the Dwarf or Morello Cherry, has foliage-leaves at the base of its umbellate inflor- FIG. 367. — Diagrammatic longitudinal sections of Rosaceous flowers. A Prunes. B Potentillese. C Roseae. J> Pomea? : fc calyx ; c corolla; /ovaries ; n stigmata. escences ; P. Avium, the Wild Cherry or Gean, has only scales at the base of its inflorescences. In the sub-genus Laurocerasus, P. Mahaleb, the Damson, has fragrant bark; P. Padus, the Bird-Cherry, has elongated racemose inflorescences ; P. Laurocerasus, the Cherry-Laurel, has evergreen leaves which somewhat resemble those of the true Laurel ; P. lusitanica is the Portugal Laurel. Tribe 4. Poteriecn. Flowers often diclinous : corolla often absent : ovaries few, often but one, monomerous, enclosed in the cavity of the receptacle : ovules solitary, suspended: fruit, a dry receptacle bearing one or more nut-like achenes. The genus Alchemilla has tetramerous flowers destitute of a corolla the stamens (4 or fewer) alternate with the sepals ; an epicalyx is present : A. vulgaris, the Lady's Mantle, and A. arvensis, are common. In the genus Poterium, the flowers of the sub-genus Sanguisorba (P. officinale, the great Burnet), have no corolla, the four stamens are opposite the sepals, and they have no epicalyx : the flowers of the sub-genus Poterium (P. Sanyuisorba, the Salad Burnet), resemble those of the preceding, but the stamens are indefinite, and they are polygamous. The flower of Agrimonia is pentamerous ; it has a corolla and indefinite stamens : the outer surface of the receptacle is beset with bristles. Tribe 5. PotenlillecK. The ova- ries, which are numerous, are inserted upon a prolongation of the axis into the cavity of the receptacle (Tigs. 367 B and 368 B) ; each usually contains one ovule. FIG. 368.— A Flower of the Cherry: » pe- The calyx is often surrounded by duncle • c corolla: a """"ens ; 9 style, pro- , , , , -i jectinjT out of the cavity of the receptacle. an epicalyx formed by the con- B Fruit of the Blackberry. EvbutfnticMW, nate stipules of the sepals (Fig. fc calyx; /fleshy ovaries. 556 PART IV. — CLASSIFICATION. 275 C). The stamens are usually indefinite, each whorl consisting of as many or twice as many stamens as there are petals. These flowers are distinguished from those of the Ranunculacese, which they somewhat resemble, by the whorled arrangement of the stamens and by the presence of the hollow receptacle; for in Ranunculaceous flowers the stamens are arranged spirally and the sepals are quite free. Of the genus Potentilla, the fruit of which is an etserio of achenes on a dry receptacle, many species are common, such as P. anserina, the Silver- weed, reptans, Tormentilla, and others: the sub-genus Sibbaldia includes the species P. procumbens, which is found on Scottish mountains : the sub-genus Comarum includes the species P. Comarum, the Marsh Cinque- foil. Fragaria is the Strawberry ; the receptacle becomes succulent as the fruit ripens and bears the small achenes on its surface 5 F. vesca and elatior are found in woods ; F. virginiana and other North American species are cultivated. In the genus Rubus there is no epicalyx, the ovary contains two ovules, and the fruit is an etserio of drupels ; Rubu* Jd'>- lettuce, is eaten. Centranthns ruber is an ornamental plant ; only one stamen and one carpel are developed (Fig. 893, Diagram £); at the base of the tube of the corolla is a spur which is indicated in Valeriana by a protuberance. FIG. 393.— .4 Flower, B Fruit of Valerian : ovary ; t calyx ; c corolla ; a spur ; st stamens g style ; p pappus. Floral diagram*, A of Valerian; abortive carpels x x : B of Cen- trantbus. Order .2. M.B. DIPSACE.E. Flower more or less dorsiventral, sur- p P 578 PART IV. — CLASSIFICATION. rounded by an epicalyx (Fig. 394 /c') formed of connate bracteoles : calyx often plumose or bristly (Fig. 394 fc): corolla usually bila- biate : stamens only four, the posterior one being suppressed : ovary apparently dimerous, one carpel being more or less com- pletely suppressed, unilocular, with one suspended ovule: seed with endosperm: leaves decussate, exstipulate: flowers in a dense capitulum surrounded by an involucre of bracts: the outer florets are usually ligulate : the Receptacle may or may not bear scaly bracts (palese) : fruit invested by the epicalyx which is cleft longi- tudinally. Fto. 394.— Flower of Scabiosa (mag.) : / ovary; k' epiculyx (long sect.) ; fc calyx ; c corolla ; «t sta- men * ; n stigma. FIG. 395.— Floral dia- gram of Composite (tubular floret). Fis. 396.— Flower of Arnica (mag.). A Tubular floret from the centre (disc) (longitudinal sect.). B Ligulate marginal floret (ray): /ovary; p pappus; c corolla; a anthers; st stamen ; n stigma ; g style ; » ovule. Dipsacus, the Teazle, has a calyx without bristles; the capitula of Dipsacus Fullonum are used in finishing woollen cloth, for the sake of the strong hooked spines of the paleae : D. si/lvestris is common on waste ground. In the genus Scabiosa, the paleae, which are usually present, are not spinous : in the sub-genus Asterocephalus, the epicalyx (or involucel) is 8-furrowed, and its projecting limb is dry and scarious ; S. Columbaria, with a 5-lobed corolla, is common in dry pastures: in the sub-genus Succisa, the limb of the 8-furrowed epicalyx is herbaceous ; S. succisa, with a 4-lobed corolla, occurs in damp meadows: in the sub-genus Knautia, there are no paleae but the receptacle is hairy, and the epicalyx is 4- furrowed ; S. arvenais is common in fields. GROUP V. — ANGIOSPERMJS ; DICOTYLEDOXES. 579 Order 3. COMPOSITE. The flowers are always collected into many-flowered capitula (sometimes only 1 -flowered) ; different kinds of flowers ( $ , ? , or sterile) generally present in the same head: ovary dimerous, unilocular, with a basal, erect, anatropous ovule : the calyx is rarely present in the form of small leaves or scales (Fig. 397 D, p) ; more commonly it is a crown of simple or branched hairs (pappus; Figs. 396 p', 397.4, E, p}, and is not developed till after the flowering is over ; sometimes the calyx is wholly wanting : corolla tubular, either regular, and 5-toothed (Figs. 396 A, c ; 397 C, w, c), or irregular and expanded at the upper end into a lateral limb with 3 or 5 teeth (Figs. 396 B ; 397 B, ra ; 397 .4, c), when it is said to be ligulate : the stamens are short, inserted upon the corolla (Fig. 396 A, st) ; the anthers are elongated and syngenesious, forming a tube through which the style passes (Figs. 396 A, a) : this is bifid at its upper end (Fig. 396 A, n ; 397 A and (7, n) : on each of these branches the stig- matic papillae are arranged in two rows : in the wholly ? flowers the styles are usually shorter (Fig. 396 B, g~) : fruit a cypsela (p. 473), crowned by the pappus (Fig. 397 A, E, D, p) when it is present (Fig. 397 F, /) : sometimes the fruit has its upper end prolonged into a beak, and its surface is covered with ridges or spines (Fig. 397 E) : seed without endosperm. Usually herbs with scattered (more rarely decussate), exstipulate leaves, often with milky latex. The capitula are always surrounded by a number of bracts forming an involucre (Fig. 397 B, C, i). The scaly bracteoles of the individual florets (paleae) may be present or wanting (Fig. 397 C, d). The Composite are classified according to the form of the flowers and to the distribution of the different kinds of flowers in the inflorescence. Sub-order I. TDBCLIFLOBJE. The capitula either consists entirely of 5 tubular florets (by tubular flowers are meant those with a regular 5- toothed corolla); or the central florets (florets of the disc) are tubular and ? (Fig. 396 A), whereas the florets of the ray are ligulate and ? or sterile and form one or two rows (Figs. 396 B ; 397 B and C, ra). Tribe 1. Eupatoriece. Leaves mostly opposite : flowers all tubular, ? the branches of style narrow ; papillae extending to the middle. Eupatorium cannabinum, the Hemp Agrimony, is common in damp places. Tribe 2. Asteroldece. Leaves alternate : ray-florets ? or sterile, gener- ally ligulate: branches of the style hairy above, papillae extending to where the hairs begin. Many species of Aster, belonging chiefly to North 580 PART IV. — CLASSIFICATION. America, are cultivated as ornamental plants, as also Callisteplius hortensis, commonly known as the China Aster. Erigeron acre, alpinum, and cana- dense occur in England ; the last is an imported weed. Bellis perennis, the Daisy, has no pappus. Solidago virgaurea is the Golden Rod. Tribe 3. Senecionidece. Leaves alternate : ray-florets in one row, ligulate $ , rarely absent : branches of the style tufted at the tips. Senecio vulyaris, the common Groundsel, has no ray-florets. Arnica mon- tana occurs in Alpine woods. Two species of Doronicum (D. Pardalianches and plantagineuini) have become naturalized in England. Petasitesvulgaris, the Butter-bur, and Tussilago Farfara, the common Coltsfoot, are common in damp fields. FIG. 397. — Flowers of Compositae : / fruit or ovary ; fc its beak ; p pappus ; c corolla ; s stamens ; o anthers ; n stigmata. A Ligulate flower of Taraxacum, with a 5-toothed corolla- limb, $ . B Capitnlum of Achillea: ra floret of the ray, with ligulate 3-toothed corolla, ? ; in $ florets of the disc, with a 6-toothed tubular corolla ; t involucre. C Longitudinal section more highly magnified ; r receptacle ; t involucre ; d bracteoles (paleae) ; ra floret of the ray ; m florets of the disc ; n' stigmata of the ? flowers. D Fruit of Tanacetum with a scaly pappus : E of Taraxacum, with a hairy pappus ; Ji beak : F of Artemisia with- out a pappus (mag.). Tribe 4. Anthemidece. Leaves alternate: ray-florets $ , ligulate or tubu- lar : branches of style tufted at the tips : involucral bracts scarious : pappus 0, or minute. Artemisia Absinthium, Wormwood, A. vulyaris and campestris are common : Chrysanthemum Leucantkemum, the Ox-eye Daisy, is common in fields : Matricaria Chamomilla, the Wild Chamomile, has a hollow conical recep- GROUP V. — ANGIOSPERM.E ; DICOTYLEDONES. 581 tacle destitute of palese : Anthemis nobilis, the Common Chamomile, has a receptacle bearing paleae, as also A. arvensis, the Corn Chamomile: Achillea Millefolium is the Milfoil, or Yarrow : Tanacetum vulgare is the Tansy : Diotis maritima is the sea-side Cotton-weed. Tribe 5. Helianthoidece. Leaves opposite : ray-florets 0 or ligulate yellow, $ or sterile : branches of style as in Asteroidese. Bidens is common in wet places. Galinsoga is naturalized in England. Helianthus annuus is the Sunflower; oil is extracted from the seeds: the tubers of H. tuberosus, a AVest Indian species, are rich in inulin (p. 83), and serve as a vegetable (Jerusalem Artichoke). Species of Zinnia, Bud- beckia, Dahlia, and Coreopsis are cultivated. Tribe 6. Helenioidece. .Resemble the Helianthoidese, but the receptacle is without paleae. Species of Helenium, Tagetes, Gaillardia, are commonly cultivated as garden flowers. Tribe 7. Inuloidece. Leaves alternate : ray-florets frequently ligulate, ? , yellow : anthers appendiculate at base. In Inula (7. ffelenium, the Elecampane), Pulicaria (P. dyssnterica, the Fleabane), and others, the ray-florets are ligulate ; whereas, in other genera, Gnaphalium (the Cudweed), Filago, Antennaria, the ray-florets are fili- form ; Antennaria is dioecious. Tribe 8. Cynarece. Flowers all tubular, the outer ones sometimes $ or sterile: style thickened below the branches: anthers often appendiculate at base : leaves generally armed with spines, alternate. Arctium Lappa (A. majus), the Burdock, is common by roadsides ; the leaves of the involucre are hooked and spinous. Carduus nutans and crispus are the common (true) Thistles; Cnicus lanceolatus, palustris, pratensis (Plume-Thistles), are common in damp districts. Carlina vulgaris is the Carline-Thistle ; the inner leaves of the involucre, which are white, fold over the flower-head under the influence of moisture, but in drought spread widely open. Onopordon Acanthium is the Scottish or Cotton Thistle. Cen- taurea Scabiosa and nigra, the Knapweeds, are common everywhere: C. Cyanus is the Corn-flower or Bluebottle, occurring in wheat-fields. Cynara Scolymus is the Globe-Artichoke ; the flower-buds are eaten as a vegetable. Carttiamus tinctorict, the Safflower, is used in dyeing. In Echinops, the Globe-Thistle, numerous one-flowered capitula are collected into one large spherical head. Saussurea and Serratula are the Saw-worts. Tribe 9. Calendulece. Kay-florets ? and usually ligulate : disc-florets usually sterile. Calendula officinalis, the Pot-Marigold, is a familiar garden plant. Sub-order II. LABIATIFLOR.E. The $ disc-florets have a regular or a bilabiate corolla; the ray-florets have usually a bilabiate corolla. Tribe 10. Mulisiece. This tribe includes all the Composite with a bila- biate corolla : they come mostly from South America. Mutisia is one of the few climbing genera : it climbs by means of leaf-tendrils. Sub-order III. LIGULIFLOR.E. All the florets are $ ; limb of the corolla 5-toothed and ligulate (Fig. 397 A). Tribe 11. Cichoriece. Mostly herbs, all containing latex in laticiferous vessels (Fig. 80, p. 99). 582 PART IV.— CLASSIFICATION. Taraxacum officinale, the Dandelion, is the commonest of wild flowers. Lactuca sativn is the Lettuce : L. Scariola, virosa, and others are common in waste places. Scorzonera hispanica is eaten as a vegetable. Trayopoyon porrifolius is the Salsafy; T. pratensis, the Goafs-beard, is common. Cicho- rium Intybus, the Chicory, is found by roadsides; the roasted roots are mixed with Coffee : C. Endivia (Endive) is a vegetable. To this tribe be- long also the British genera Hypochseris (Cat's-ear), Arnoseris (Lamb's Succory), Leontodon (Hawkbit), Hieracium (Hawkweed), Sonchus (Sow- Thistle), Crepis (Hawk's-beard), Lapsana (Nipplewort), Picris. INDEX. PART I.— MORPHOLOGY, ANATOMY, AND PHYSIOLOGY. Abscission-layer, 156. Absorption, 158, 177. of gases, 178. of liquids, 178. Accumbent, 536. Achene, 473. Achlamydeous, 457. Acicular leaves, 32, 433. crystals, 81. Acids, organic, 187. Acrocarpous, 333. Acropetal development of members. 9. of tissue, 126. Acrotonous, 504. Actinomorphic symmetry, 453. Acuminate, 37. Acute, 37. Acyclic flower, 445 (Fig. 262), 527. Adhesion, 21, 462. Adnate, 460. Adventitious members, 9, 136. .Ecidiospore, 300. .Ecidium, 300 (Fig. 178). Aerial roots, 45, 109 (Fig. 87). Estivation, 43. Air-bladders, 269. Air-cavity, 108 (Fig. 88), 115. Air-chamber, 89, 321 (Fig. 199). Al», 546 (Fig. 358), 557 (Fig. 369). Albuminous seed, 414 ^Fig. 245). Alburnum, 143, 165. Aleuron, 80 (Figs. 54, 55), 201. Alkaloids, 186, 201. Alternation of generations, 2, 234. Aluminium, 189. Amentum, 442, 517. Amides, 186, 196, 199, 201. Ammonia, 190. Amoeboid stage, 283. Amphigastria, 319. Amphithecium, 136, 316, 337. Amplexicaul, 30. Amygda'lin, 187. Amy 1 in, 187. Amylolytic enzyme, 198. Amyloplastic function, 70. Amyloses, 187. Anabolism, 193. Anatomy, 63. Anatropous ovule, 399 (Fig, 237). Androscium, 443, 460. Androspore, 258. Anemophilous, 410. Angiocarpous sporophore, 303. Angustiseptal silicula, 535 (Fig. 348). Annual plants, 417. rings, 141 (Figs. 112-3). shoots. 23. Annular vessels, 74 (Fig. 44). Annulus, 302 (Fig. 182), 343 (Fig. 208), 363 (Figs, 215-7), Anterior, 448. Anthela, 442. Anther, 395, 460 (Figs. 276, 279), Antheridial cell, 406. Antheridiophore, 310 (Fig. 198). Antheridium, 60, 243, 310, 350, 369 (Figs. 139, 140, 147, 148, 150, 158, 160, 192, 193. 222), Anthocyanin, 83. Anthophore, 444, 532 (Fig. 344). Anticlinal, 102. Antipetalous, 447. Antipodal cells, 408 (Fig. 212) Antisepalous, 446. Apex, 4. Apical cell, 101, 104 (Figs. 85, 86), 132 (Fig. 106), 133, 324 (Fig. 200). Aplanogamete, 240. Apocarpous, 465 (Fig. 281), 472. Apophysis of Moss-capsule, 388. of Pmus, 435. Apostrophe, 172 (Fig. 124). Apothecium, 294 (Fig. 177). Aqueous tissue, 115. 584 IXDEX, PART I. Archegoniophore, 310 (Fig. 198). Archegonium, 60, 235, 311, 350, 409 (Figs. 194, 195, 223, 241, 252). Archesporium, 53, 136 (Figs. 197, 206, 210, 216). Archicarp, 60, 237, 278. 293 (Fig. 172). Aril (or arillus), 415 (Figs. 246, 258). Arillode, 415, 546. Arista, 488. Arrangement of lateral members, 10. Ascidium, 41 (Fig. 28), 175. Ascocarp, 62, 277, 280, 293 (Figs. 171, 173, 175). Ascogenous hyphse, 293 (Fig. 175). Ascogonium, 278, 291, 296 (Fig. 175). Ascospore, 290, 294 (Figs. 173, 175, 176). Ascus, 278, 293 (Figs. 171, 175, 176). Asexual formation of spores, 50, 230. reproductive cells, 2, 50. reproductive organs, 2, 51. Ash, 188. Asparagin, 186, 199. Assimilation, 158, 193. Asymmetry, 456 (Fig. 273). Auriculate", 31 (Fig. 20), 326 (Fig. 202). Autumn-wood, 142 (Figs. 112-3). Auxospore, 265. Awn, 487 (Fig. 301). Axial placentation, 468. Axil, 11. Axillary branching, 11. Axile placentation, 468 (Fig. 284). Axis, 4, 18. Bacca, 475. Bacterioids, 191. Balsam, 97. Bark, 153 (Fig. 119). Basal wall, 314, 346. Base, 4. Basidiospore, 303 (Figs. 183, 184). Basidium, 303 (Figs. 183, 184). Basifixed anther, 460. Basitonous, 504. Bast, 121 (Figs. 100-5), 130, 143. hard. 143. soft, '143. Bast-fibres, 130, 143 (Fig. 111). Berry, 475. Biennial plants, 418. Bifurcation, 19. Bijugate, 35. Bilabiate, 455. Bilateral Symmetay, 6. Bilocular anther, 4^63. Bipinnate, 35. Biseriate perianth, 457. Bisexual, 61. Biternate, 37. Blade of leaf, 29, 32. Bleeding, 182. Bloom on plants, 107. Body, I, 63. septate or unseptate, 63. Bordered pits, 74 (Figs. 48, 49). Bostrychoid dichotomy, 19 (Fig. 9). Bostryx, 441. Bract, 57, 442. function of, 175. Bracteole, 57, 443. Branches, development of, 9, 132. Branching, 3. axillar3r, 11. dichotomous, 18 (Fig. 9). cymose, 19 (Fig. 11) racemose, 19. of leaf, 34. of root, 45 (Fig. 30), 133 (Fig. 107). of shoot, 12, 132. Branch-systems, 18 (Figs. 9-11). Bromine," 189. Bud, 11. adventitious, 136. Budding, 229. Bud-scales, 42. Bulb, 25 (Fig. 13), 50. Bulbil, 25, 50, 332. Bundle, vascular, 121. bicol lateral, 123. cauline. 122. closed, 127. collateral, 123. common, 122. concentric, 123, 125 (Fig. 101). conjoint, 121. medullary, 123. open, 127. phloem-, 125. xylem-, 125. longitudinal course of, 122 I (Fig- 99). structure of, 129. termination of, 130. Bursicula, 504. Calcium, 188, 192. carbonate, 78 (Fig. 52). 81. 202. oxalate, 78 (Fig. 51), 81 (Figs. 57, 58), 202. Callus, 137, 155 (Fig. 121). of sieve-tubes, 95 (Figs. 75, 76). Calyculus (epicalyx), 445. MORPHOLOGY, ANATOMY, AXD PHYSIOLOGY. 585 Calyptro, 62, 313 (Figs. 196. 206, 211). Calyx, 58, 443. Cambium, 127, 130 (Fig. 105), 137 (Figs. 108, 109). Cambium-ring, 137 (Fig. 110). Campanulate, 458. Campy lotropous ovule, 399 (Fig. 237). Cane-sugar, 187, 198. Caoutchouc, 167. Capillitium, 283 (Fig. 164). Capitate hairs, 46. Capitulate raceme, 440. Capitulum of Charoideae, 261. inflorescence, 439 (Fig. 259). Capsule of Bryophyta, 52, 314, 337 (Figs. 197, 206, 208-11). Capsule, a fruit, 475 (Figs. 288, 289). Carbohydrates, 186, 201. Carbon, 189. Carbon, dioxide, absorption of, 189. 193. evolution of, 200. Carcerule, 473, 541 (Fig. 352). Carina, 557 (Fig. 369). Carinal cavity, 383 (Fig. 230). Carpel ,56, 395, 465. Carpellary flower, 56, 395. Carpogamy, 241, 277. Carpogonium, 242, 272 (Fig. 160). Carpophore, 466 (Fig. 287). Carposporangium, 244, 273 (Figs. 160, 161). Carpospore, 274. Caruncle, 416, 526. Caryopsis, 473. Catabolism, 158, 185, 197. Cataphyllary leaves (Cataphylls), 42, 175. Catkin, 442, 517 (Figs. 327-31). Caudicle, 504. Caulicle, 377. Cauline vascular bundles, 122. Cell, 1. 63, 66 (Fig. 36). Cell-contents, 78. C^ll-division, 83 (Fig. 60). Cell-formation, 83 (Figs. 60-85). Cell-sap, 66, 82. Cell-wall, 63, 66, 72, 76. growth of. 73. Cellular structure, 64. Cellulose, 66, 72, 76, 187, 201. Chalaza, 399 (Fig. 237). Chalk-glands, 96, 202. Chambered ovary, 466 (Fig. 282). Chemical composition of plants. 185. Chemical effects of light, 161. Chemiotaxis, 220, 232. Chlamydospore, 277, 286, 304. Chlorine, 188. Chlorophyll, -corpuscle, 71, development of 161, 192. function of, 194. Chloroplastids, 69 (Figs. 88, 40, 41). functions of, 70. movements of, 172 (Fig. 124). Chlorotic, 192. Chromatin, 69. Chromatophore, 69 (Figs. 41, 42). Chromoplastid, 69 (Fig. 42). Cincinnal dichotomy, 19. Cincinnus, 441. Cilium, 72, 207 (Fig. 62). Circinate vernation, 43. Circulation of protoplasm, 204, 206. Citric acid, 187. Classification, 233. Claw, 458 (Fig. 275). Cleistogamous flowers, 410. Cleistothecium, 294 (Fig. 173). Climbing plants, 27 (Fig. 15), 212, 223. Closed vascular bundles, 127. Cobalt, 189. Coccus, 472. Coenobium, 239, 253. Coenocyte, 64, 100(Fig.81), 25a(Figs. 140-2), 252 (Fig. 143), 276. Cohesion, 21 (Fig. 12), 462. Coleoptile, 478 (Fig. 292). Coleorhiza, 479 (Fig. 293). Collateral vascular bundles, 123 (Figs. 97, 100). Collenchyma, 91 (Fig. 69), 111. Colleter, 101, 167. Colony, 238. Colouring-matters, 83, 186. Columella, 285 (Fig. 165), 817 (Figs. 197, 208). Column, 444. Combined effects of stimuli, 222. Common bundles, 122. Companion-cell, 95 (Fig. 74), 166. Compound inflorescences, 439, 441. Compound leaves, 35 (Fig. 23). Concentric arrangement of bundles, 123 (Fig. 101). Conceptacle, 269 (Fig. 157). Conditions of movement, 223. Conducting tissue of style, 467. -sheath, 184. Conduplicate vernation, 43. Cone, 55, 382, 386, 421, 433 (Figs. 228, 231, 254, 255). Conical root, !">_ Conidiophoro. -Ji'.'. Coniferin, 187. Coniin, 186. 586 INDEX, PART I. Conjoint bundle, 121. Conjugation, 58, 87 (Fig. 65), 240 255, 256 (Fig. 146), 286 (Fig. 166). Conjunctive tissue, 118, 144. Connate, 32 (Fig. 20). Connective, 460 (Fig. 276). Contorted vernation, 43. Contractile vacuoles, 206. Convolute vernation, 43. Copper, 189. Cordate, 37. Cork, 151 (Fig. 118). Corm, 25, 50 (Fig. 309). Corolla, 58, 443. Corona, 48, 458. of Characese, 262 (Fig. 150). Cortex, 111, 149, 260. Corymb, 442. Costse, 548. Cotyledon, 28, 403 (Figs. 238, 239), 414 (Fig. 245), 476 (Figs. 291-3), 509 (Figs. 320-1). Crenate, 37. Cross-fertilisation, 232. Cross-pollination, 409. Cruciate tetraspores, 271. Crumpled aestivation, 43. Crystalloids, 81 (Fig. 54). Crystals, 78 (Fig. 51), 81 (Figs. 57,58). Culm, 28. Cupule, 49, 313, 323 (Fig. 198), 337. of Phanerogams, 472, 518, 520 (Fif Cuticle, lU,. Cuticularisation of cell-wall, 76, 91. Cutin, 76. Cyathium, 526 (Fig. 341). Cyclic flower, 445. Cyclosis, 204, 205, 260. Cyme, 441. helicoid, 21 (Fig, 11), 441. scorpoid, 21 (Fig. 11), 441. Cymose branching, 19 (Fig. 11). inflorescence, 441. Cypsela, 473. Cystocarp, 62, 241, 273 (Figs. 160, 161). Cystolith, 78 (Fig. 52), 202. Cytoplasm, 68. Daily periodicity of growth, 214. Day-position, 174. Deciduous, 10. Decussate arrangement of leaves, 13. Definite inflorescences, 441. Definitive nucleus, 408. Dehiscence of anther, 396, 463. of fruits, 473. Dehiscence of sporangium, 54, 364. Dehiscent fruits, 416, 473. Dentate, 35, 37. Dermatogen, 102 (Figs. 83, 84). Development of body, 8. of branches of roots, 133. of branch-systems, 18. of leaves, 133. of secondary members, 132. of hairs, 135. of emergences, 135. of reproductive organs, 135. of adventitious members, 136. Dextrin, 187, 198. Dextrose, 198. Diadelphous, 462. Diageotropism, 171, 218. Diagonal plane of flower, 448. Diagram, floral, 447. Diaheliotropism, 171, 215. Diaphragm, 379, 393 (Fig. 233). Diastase, 198. Diastole, 206. Dichasial cyme, 442. Dichasium, 20 (Fig. 10), 441. Dichlamydeous, 457. Dichogamy, 411. Dichotomy, 9, 18 (Fig. 9), 132 (Fig. 106). Diclinous, 61, 310, 395. Dicyclic, 450. Didynamous, 462. Differentiation of tissues, 65, 101, 125, 145. Digestive sac, 135. Dimorphism, 411. Dioecious, 61, 310. Diplotegium, 475. Diplostemonous, 450. Directive effect of light, 162, 214. of gravity, 217. Disc, 470. Dissected leaves, 37. Dissemination of seed, 416. Dissepiment, 466. false or spurious, 466. Dissipation of energy, 203. Distractlle anther, 460 (Fig. 276). Distribution of water and other substances, 181. of organic plastic substances, 184. Diurnal and nocturnal positions, 173 (Figs. 125, 126). Divergence, 12. Dorsal suture, 465. Dorsifixed anther, 460 (Fig. 276). Dorsiventral arrangement, 17. Dorsiventrality, 7 (Fig. 2), 13, 454 (Figs. 271, 272). MORPHOLOGY, ANATOMY, AND PHYSIOLOGY. 587 Double flowers, 452. Drepanium, 441. Drape, 475 (Fig. 290). Drupel, 475'. Duct, 98. Duplication, 450. Duramen, 143, 165. Dwarf-males, 258 (Fig. 147). Dwarf-shoots, 23. Egg-apparatus, 408 (Fig. 242). Elater, 316, 383. Elementary constituents of the food of plants, 188. Eleutheropetalous, 458. Eleutherophyllous, 458. Eleutherosepalous, 458. Emarginate, 37. Embryo, 8. Embryo-cell, 401. Embryogeny of Bryophyta, 314, 319, 337 : of G3rmnosperms, 424 (Figs. 249-251): of Phanero- gams, 401 (Dicotyledons, Fig. 238 ; Monocotyledons, Fig. 239) : of Pteridophyta, 346, 366 (Figs. 218-220), 377, 385, 387, 393 (Figs. 233, 235). Embryonal tubes, 427 (Fig. 250). Embryo-sac, 51, 400 (Figs. 241-2), Emergences, 48, 135. Endocarp, 472 (Fig. 290). Endodermis, 111, 115, 134 (Figs. 94, 96). Endogenous development, 9, 44, 134 (Fig. 107). Endopleura, 415. Endosperm, 407 (Fig. 241), 414. Endospore, 50. Endothecium, 136, 316 (Fig. 197). Energy of growth, 209. absorption of, 194. dissipation of. 203. kinetic, 194. potential, 197. Ensiform, 32, 479. Entire, 35. Entomophilous, 410. Enzyme, 189, 197. Epibasal cell, 314, 346, 387. Epiblast, 478 (Fig. 292). Epiblema, 106. Epicalyx, 57, 175, 443, 57a Epicarp, 472 (Fig. 290). Epicotyl, 404. Epidermis, 106 (Figs. 87-90, 92). Epigean cotyledons, 405. Epigynous, 444 (Fig. 261). Epinastyr211 (Fig. 131). Epipetalous, 462. Epiphragm, 344. Epiphyllous, 462. Epiplasm, 294. Epipodium, 29, 32. Episporitim -i (Fig. 169> Zygomorphic symmetry, 4, 7, 454. Zygospore, 58, 240, 249, 254, 257, 277, 285 (Figs. 145, 146, 165, 166). PART II.- CLASSIFICATION AND NOMENCLATURE. Abies, 422 (Fig. 247), 434 ! (Fig. 255). Abietineae, 434. Acacia, 559 (Fig. 371). Acer, 545 (Fig. 357). Aceraceae, 513, 545. Acer as, 506. Acetabularia, 251 (Fig. i 141). Achillea, 581 (Fig. 397). Achlya, 289 (Fig. 169). Aconitum, 465 (Fig. 281), 528 (Fig. 342), 530. Acorus, 482 (Fig. 294). Acrocarpse (Musci), 345. Acrogynse (Hepaticse), 330. Acrostichese, 361. Actaea, 529. Adder's-tongue Fern, 355. Arliantum, 360, 366 (Fig. 219), 368 (Fig. 222), 371. Adlumia, 534. Adonis, 529. Adoxa, 576. JEcidiomycetes, 280, 298. JEcidium, 300. jEgopodium, 550. jEsculus, 545 (Fig. 356). jEthalium, 284. jEthusa, 550. Agapanthus, 498. Agaricinae, 303. Agaricus, 277, 301 (Fig. 182), 302 (Fig. 183). Agave, 508. Agavoideae, 508. Agopyrum, 491. Agrimonia, 555. Agrostideae, 490. Agrostis, 490. Aira, 490. Ajuga, 564. Ajugoidese, c63. Alaria, 265. Alchemilla, 555. Alder, 517. Aldrovanda, 561. Algse, 234, 237. Alisma, 403 (Fig. 239), 495 (Fig. 306). Alismacese, 481, 495. Alismales, 481, 494. Alkanet, 570. All-good, 523. AllioideEe, 498. Allium, 461 (Fig. 277), 498. All-seed', 532. Almond-tree, 554. Alnus, 518 (Figs. 328, 329). Aloe, 498. Alopecurus, 488, 490. Alpine Hose, 573. Alpinia, 502 (Fig. 313). Alsineae, 532. Alsophila, 361, 372. Althsea, 541 (Fig. 352). Alyssinese, 537. Alyssum, 537. Amanita, 303 (Fig. 182). Amaryllidacese, 481, 507. Amaryllidoideae, 507. Amaryllis, 507. Amelanchier, 557. Amentales, 512, 517 (Fig. 327). American Aloe, 508. Ammi, 550. Ammineae, 549. Amomales, 481, 501. Amorpha, 557. Ampelidacese, 513, 547. Ampelopsis, 547. Amygdalus, 554. Amylum Marantse, 503. Anabsena, 245 (Fig. 136). Anacharis, 500. Anacrogynae, 329. Anagallis, 572. Ananas, 501 (Fig. 311). Anaptychia, 305 (Fig. 185). Anchusa, 569 (Fig. 383). Andromeda, 573. i Andropogon, 489. i Andropogoneae, 489. I Aneimia, 358, 372. ! Anelaterese, 329. ! Anemone, 528 (Fig. 342). Anemoiiese, 527. Aneura, 327, 329, 330. Angelica, 550. Angelicese, 550. Angiopteris, 349, 355. Angiospermae, 234, 438. Angrsecum, 507. Angustiseptse, 537. Antennaria, 581. Anthemideae, 580. Aiithemis, 581. Anthericum, 498. Anthoceros, 330 (Fig. 203). Anthocerotaceae, 330. Anthoxanthuro, 489. Anthriscus, 550. Anthurium, 483. Arithyllis, 558. Antiaris, 515. Antirrhinum, 564 (Fig. 378). Antirrhoidese, 564. Apera, 490. Aphanomyces, 289. j Apiocystis, 248. i Apium, 550. Aplanes, 289. Apple-tree, 556. Apricot. 555. Aquilegia, 529 (Fig. 343). Arabideae, 537. Aracea-, 481. 482. Arachis, 558. Arales, 481, 482. Araliaceae, 513. W). Arbutoideae, 573. Arbutus, 573. Archangelica, 550. Arctium, 581. Arctostaphylos, 573. Arcyria, 284 (Fig. 164). ; Areca, 486 (Fig. 298). i Arenaria 532. CLASSIFICATION AND NOMENCLATURE. 590 Aria, 557. Awl-wort, 537. Bittersweet, 568. Arisarum, 482, 484. Azalea. 579. Blackberry, 556 (Fig. Aristolochia 412 (Fig. Azolla, 373, 380. 368). 243), 524. Black Bryony, 500. Aristolochiaceae, 512, 524. Bacillus, 281 (Fig. 163). Black Pine, J:',\ Arraeria, 572. Bacterium, 281 (Fig. 162). Black Thorn, 554 Armillaria, 303. Ballota, 563. Bladder-Fern, 372. Arnica, 580 (Fig. 396). Balm, 563. Blaeberry, 573. Arnoseris, 582. Balsaminacese, 513, 543. Blasia, 825, 330. Aroidese, 483. Bamboo, 492. Blerhnum, 359, 371. Arrhenatherum, 490. Bambusa, 486 (Fig. 299), Blue bell, 497. Arrow-grass, 495. 492. Blue-bottle, 581. Arrow-head, 495. Bambusese, 492. Blyttia, 328. Arrow-root, 502. Banana, 502. Bog-Asphodel, 498. Artemisia, 580 (Fig. 397). Baneberry, 529. Bog-bean, 570. Artichoke, 581. Bangia, 272. Bog-Orchis, 507. , Artocarpus, 515. Bangiaceae, 271. Bog-Rush, 492. Arum, 483 (Fig. 295). Banyan, 515. Bohmeria, 515. Arumlinaria, 492. Barbarea, 537. Boletus, 303. Asarabacca, 524. Barberry, 531. Borage, 570. Asarales, 512, 524. Barbula, 339, 345. Boraginacese, 513, 569. Asarum, 524 (Fig. 338). Barley, 492. Borago, 570. Ascobolus, 278, 290. Bartsia, 565. Borecole, 537. Ascolichenes, 306. Basidiolichenes, 306. Boschia, 3-JI. Ascomycetes, 280, 290. Basidiomycetes, 280, 301. Botrychium, 354 (Fig. Ascophyllum, 268. Bastard Toad-flax, r,-_>r>. 212). Ash, 570. Batatas, 567. Botrydium, 251 (Fig. Asparagoideae, 499. Batrachospermum, 272. 144). Asparagus, 499. Bauhinia, 559. Botryococcus, 248. Aspergillus, 291. Bean, 559. Botrytis, 291. Asperula, 575. Bearberry, 573. Brachypodium, 491. Asphodeloideae, 498. Bed-straw, 575. Brachythecium, 346. Asphodelus, 498. Beech, 520. Bracken, 371. Aspidieae, 372. Bee Orchis, 506. Brasenia. r>:il. Aspidium, 361 (Fig. 215), 372. Beetroot, 523. Belladonna Lily, 507. Brassica, 537 (Fig. 348). Brassiceae, 537. Asplenieae, 371. Bcllis, 580. Brazil Nut. .V.I. Asplenium, 360(Fig. 214), Bent-Grass, 490. Bread-fruit, 515. 371. Berbendaceae, 512, 531. Briza, 491. Aster, 579. Berberis, 531. Broccoli, 537. Asterales, 513, 577. Bergen ia, 560 (Fig. 372). Brodissa, 498. Asterocephalus, 578. Bertholletia, 554. Brome-Grass, 491. Asteroideae, 579. Beta, 523. Bromeliaceae, 481, 500. Astragalus, 558. Betel-Palm, 485. Bromus, 491. Astrantia, 549. Betony, 563. Brookweed, r>7± Athyrium. 371. Betula, 518. Broom, 558. Atragene, 528. Betulaceae, 512, 517. Broomrape, 566. Atrichum, 339. Biclens, 581. Broussonetia. .M">. Atriplex, 523. Bilberry, 573. Brussels-sprouts, 537. Atropa, 568 (Fig. 382). Aulacomnium, 334, 337. Bindweed, 567. Biophytum, 543. Bryineae, 338, 842. Bryonia, 551. Aurantiese, 543. Biota, 436 (Fig. 256). Bryophyta, 234, 809. Auricularia, 304. Birch, 518. Bryum. 884. Auriculariege, 303. Bird-Cherry, 555. Buck-bean, 57' ». Autobasidiomycetes,305. Autumn Crocus, 498. Bird's-foot Trefoil, 558. Bird's nest, 573. Buckthorn, 547. Buckwheat , :>-!l. Avena, 490 (Fig. 302). Bird's-nest Orchid, 507. Blllborh..-!.-. -'^. Aveneae, 490. Birthwort, 524. Bulgaria, 297. Avens, 556. Bitter or Seville Orange, Bulgarieae, 2ii<. Averrhoa,.543. 544. ' Bui lace, 555. 600 IXDEX, PAKT II. Bulrush, 484 ; 493. Bupleurum, 550. Burdock, 581. Bur-reed, 484. Butcher's Broom, 499. Butomacese, 483,, 495. Butomus, 398, 495 (Fig. 306). 496 (Fig. 307). Butter-bur, 580. Buttercup, 529. Butterfly Orchis, 506. Butterwort, 566. Cabbage, 537. Cabomba, 531. Cabombeae, 531. Caesalpinia, 559. Caesalpiniese, 559. Cakile, 538. Cakilineae, 538. Calabar Bean, 559. Calamagrostis, 490. Calamintha, 563. Calamus, 485. Calceolaria, 564. Calendulese, 581. Calendula, 581. Calla, 483. Callistephus. 580. Callithamnion, 271. Calloidese, 483. Calluna, 573. Calocera, 304. Calothamnus, 553 (Fig. 365). Caltha, 529. Calyciflorse, 513, 547. Calypogeia, 329. Calystegia, 445; 567. Camassia, 497. Camelina, 537. Camelineae, 537. Campanales, 513. 574. Campanula, 574 (Figs. 388, 389). Campanulaceae, 513, 574. Campion, 532. Campy lospermese, 550. Candytuft, 537. Canna, 503 (Fig. 314). Cannabinaceae, 512, 515. Cannabis, 516. Cannaceae, 502. Canterbury-bell, 574. Caprifoliaceae, 513, 576 (Fig. 391). Capsella, 402 (Fig. 238), 537. Capsicum, 568. Caraway, 550. Cardamine, 537. Cardamom, 502. Carduus, 581. Carex, 493 (Fig. 304). Caricoideae, 493. Carlina, 581. Carline Thistle, 581. Carnation, 532. Carob-tree, 559. Carpmus, 519 (Fig. 332). Carrot, 550. Carthamus, 581. Carum, 473 (Fig. 287), 549 (Fig. 362). Carya, 522. Caryophyllacese, 512,531. Caryophyllales, 512, 531. Cassia, 559 (Fig. 370). Castanea, 521. Castor-oil Plant, 527. Catchfly, 532. Catmint, 563. Cat's Ear, 582. Cat's-tail Grass, 490. Caucalinese, 550. Caucalis, 550. Cauliflower, 537. Cedar, 435. Cedrus, 435. Celandine, 529, 533. Celastraceae, 513, 546. Celastrales. 513, 546. Celery, 550.' Celsia, 564. Celtis, 517. Centaurea, 581. Centaury, 570. Centranthus, 577 (Fig. 393). Cephaelis, 575. Cephalanthera, 506. Cephalantherese, 506. Cerastium, 532. Cerasus, 555. Ceratodon, 316 (Fig. 197), 345. Ceratonia, 559. Ceratozamia, 432. Cercis, 456 (Fig. 272), 559. Cestreae, 569. Ceterach, 372. Cetraria, 307. Chaerophyllum, 550. Chsetocladieae, 285. Chsetomorpha, 252. Chsetopteris, 239 (Fig. 133). Chamsecj'paris, 436. Chamsedorea, 485 (Fig. 297). Chamaeorchis, 506. Chamserops, 485. Chamomile, 580. Chantransia, 272. Chara, 260 (Figs. 149, 150, 151). Characese, 246, 260. Characium 248. Charese, 263. Charlock, 537. Charoidese, 248, 260. Cheilanthes, 362, 371. Cheiranthus, 537. Chelidonium, 533 (Fig. 345). Chelone, 564. Chenopodiacese, 512. 522. Chenopodiales, 512, 522. Chenopodium, 523 (Fig. 336). Cherry Laurel, 555. Chervil, 550. Chick weed, 532. Chickweed Winter- green, 572. Chicory, 582. Chili Pepper, 569. Chiloscyphus 324. China Aster, 580. Chives, 498. Chlamy domo n a d a c e se, 249. Chlamydomonas, 249. Chlora, 570. Chlorideae, 491. Chlorochytrium, 248. Chlorococcum, 2 is. Chlorophyceae, 238, 246. Chlorospheera. 24s. Chondriopsis, 272. Chorda, 265. Chordaria, 266. Christmas Rose, 529. Chroococcacea?. 2 1 t. Chrysanthemum, 580. Chrysodium, 361. Chrysosplenium, 560. Cibotium, 372. Cicendia, 570. Cichorieae, 581. Cichorium, 582. Cicuta, 550. Cinchona, 575. Cinchonese, 575. Cinquefoil, 556. Circaa, 454 (Fig. 270), 552. Cistaceae, 512, 538. CLASSIFICATION AND NOMENCLATURE. 601 Cistus, 538. Colchicese, 498. I Cotton. 541. Citron, 544. Citrullus, 551. Colchicoideae, 498. Colchicum,474(Fig.288), Cotton-Grass, 493. Conch-Gran, 491. Citrus, 544 (Fig. 355). Cladonia, 307. Cladophora, 252 [Fig. 497 (Fig. 309). Coleanthus, 488. Coleochaete, 246, 259 Cowberry, 573. Cow-Parsley, 550. Cow-Parsnip, 550. 143). (Fig. 148). Cowslip. r>7J. Cladophoraceae, 246, 252. Coleochae.taceae, 253, 258. Cow-tree, 515. Cladophorese, 252. Coleosporium, 299. Cow Wheat, 565. Cladostephus, 264, 266 Coleus, 563. Crambe, 538. (Figs. 153, 154). Collabium, 504. Cranberrv, 573. Clary, 5(33. Collema, 306 (Fig. 187). Crane's- 1. 'ill. M2. Classification of Algae, Collemaceae, 306. Crassula, 561. 238. Colt's Foot, 580. Crassulacae, 513, 561. of Angiospermae, 476. Columbine, 529. Crat83gus, 556. of Ascomycetes, 294. of Basidiomycetes, 305. Colza, 537. Comarum, 556. Creeping Bugle, 564. Crepis, 582. of Bryophyta, 317. of Chloropliyceae, 247. Comfrey, 569. Common Basil, 563. Cress, 537. Crithmum, 550. of Coniferaj, 434. Common Bugloss, 570. Crocoidese, 508. of Dicotyledones, 512. of Filicinse, 352. of Fungi, 280. Composite, 513, 579 (Fig. 395). Conferva, 257. Crocus, 508. Cross-leaved Heath, 573. Crowfoot, 529. of Gymnospermae, 431. Confervoidese, 248, 253. Crown Imperial, 496. of Hepatic*, 320. Conifer*, 432. Crucifera?, 512, 535 (Figs. of Monocotyledones, Conium, 548 (Fig. 361). 347, 348). 481. Conjugatse, 247, 253, 254. Cryptogamia. 2:il. of Musci,340. of Phseophycese, 264. Conopodium, 550, Convallaria, 499. Cryptogrammc. ::7'J. Cryptonemimi', 271. of Plianerogamia, 418. Convolvulacese, 513, 567. Cuckoo-pint, 483. of Phycomycetes, 285. Convolvulus, 458 (Fig. Cucubalus, 532. of Plants, 233. 274), 567. Cucumber, 551. of Pteridophyta, 352. Clavaria, 305. Copper Beech, 521. Coprinus, 277, 303. Oucumis, 551 (Fig. 863). Cucurbita, 397 (Fig. 235), Clavariere, 303. Coral linaceae, 272. 551 (Fig. 363). Claviceps, 277, 290, 297 Coral lorhiza, 507. Cucurbitaceae, 513, 551. (Fig. 176). Coral Boot, 507. Cudweed, 581. Cleistocarpae, 338, 343. Clematis, 528, 556. Corchorus, 540. Cordyceps, 279, 296. Cupressus, 436. Cupressinae, 4W. Closterium, 254 (Fig. Cordyline, 498. Cupressinese, 486. 144). Coreopsis, 581. Curcuma, 502. Clostridium, 282. Coriandreae, 550. Currant, 561. Clover, 558. Coriandrum, 548 (Fig. Cuscuta, 567 (Fig. 381). Club Moss, 386. 361). Cut-grass, 489. Club Rush, 493. Cork-oak, 520. Cutleria, 265. 267. Cluster Narcissus, 508. Corn Cockle, r,:(-J. Cutleneceae,2<>i. 2«i.\ Cluster Pine, 435. Corn Flag, 509. Cyanophvceae, 288, 244. Cnicus, 581. Corn-flower, 581. Cyathea/361. ;«72 Cobsea, 567. Cornish Heath, 578. Cyatheacese, 352. 872. Coccus, 281 (Fig. 162). Cochlearia, 537. Cornish Money wort, 565. Corn-salad, 577. Cyathus, 804, :«•:.. Cycadacese, 481. Cock's Foot Grass, 491. Coronilla, 558. Cycas, 482 (Fijr. 258). Coco-nut Palm, 485. Corrigiola, 533. Cyclamen. 572. Cocos, 485. Corsinia, 324. Cydonia, 556. Codium, 251. Corsinie;*'. :vj|. Cynara, 581. Coelospermese, 550. Corydalis, 534. Cynarese, 581. Cosnogonium, 306. Corvlaceae, 512, 518. Cynodon, 491. Coffea, 575. Corylus, 519 (Figs. 330, Cynoglossum. .">70. Equisetum, 380 (Fjgs. 227-230). Eranthis, 529. Eremascus, 278, 290, 293 (Fig. 171). Eremurus. 498. Ergot, 290, 297 (Fig. 17»i). Erica, 573 (Fig. 387). Ericaceae, 513, 572. Ericales, 513, 572. Ericoidese, 573. Erigeron, 580. Eriobotrya, 557. Eriophornm, 493. Erodium, 542. Eryngium, 549. Erysimum, 587. ErysipheJB, 278, 280, 295 (Fig. 173). Erythrsea. 569 (Fig. 384). Erythronium, 496. Erythrotrichia. 27h. Euastrum, 254 (Fig. 144). Eucalyptus. 553. Eugenia, 553. Euonymus, 546. Eupatoriese, 579. Eupatorium, 57! *. Euphorbia, 526 (Fig. 311). Euphorbiacea\ 512. 526. Euphorbiales, 512, 525. Euphrasia. 565. Eurhynchium, 346. Eurotium, 290, 296 (Fig. 175). Eusagus, 485. Eusporangiatse, 354. Evening Primrose. 552. Evernia, 307. Exidia, 304. Exoascus, 294. Exosporese, 284. Eyebright, 565. CLASSIFICATION' AND NOMENCLATURE. Fagaeeae. 512, 520. Fungi, 234, 275. Gnetum, 437. Fagus, 520. Furze, 558. Goafs-beard, 582. False Oat-grass, 490. Fustic, 515. Golden Hod, 580. Fatsia, 550. Feather-grass, 490. Gagea, 498. Gold-of-pleasure, 587. Goody era, 507. Fegatella, 324. Gaillardia, 581. Gooseberry, 561. Fenugreek, 558. Galactodendron, 515. Goose-foot, 523. Ferns, 359. Galanthus, 507. Gorse, 558. Fern-Royal. 372. Galegeae, 558. Gossypium, 541. Fescue-grass, 491. Galeopsis, 563. Gout- Weed, 550. Festuca, 488, 491. Galingale, 492. Graminaceae, 481, 486. Festuceae, 490. Ficus, 515 (Fig. 324). Galinsoga, 581. Galium, 575. Grape-Hyacinth, 4!i7. Grape-Yinr. 547. Field Poppy, 533. Fig, 515. Figwort, 565. Galtonia, 497. Gamopetalae, 513, 562. Garlic, 498. Graphidea?, 307. Graphis, 308 (Fig. 190). Grasses, 486. Filago, 581. Gasteromycetes, 303, 305. Grass of Parnassus, c61. Filices, 358. Gaultheria, 573. Grass- Wrack, 494. Filicinse, 234, 352, 354. Gean, 555. Grateloupia. 271. Filicinae Eusporangi- Geaster, 305. Gratiola, 564. ata?, 352, 354. Gelidiacese, 274. Great Burnet. .",:>:.. Filicime Leptosporangi- Genista, 558. <;ivcn-\v 1. K6. atae, 352, 358. Genistese, 558. Grimmia, 345. Fir, 435. Gentian, 570. Gromwell, 569. . Fistulina, 304. Gentiana, 570. Ground Ivy, 563. Flag, 508. Gentianaceae, 513, 570. Ground-nut. :,.\s. Flax, 542. Gentian ales, 513, 570. Groundsel, 580. Flax Dodder, 567. Gentianeae, 570. Guelder Eose, 576. Fleabane, 581. Georgiaceae, 343. Guttiferales, 512, 539. Floridese, 271. Geraniaceae, 513, 542. Guttulinese, 284. Flowering Hush, 496. Fly Orchis, 506. Geraniales, 513, 541. Geranium, 542 (Fig. 353). Gymnadenia, 505 (Fig. 317). Fcenicuhini.548{Fig.361). Geum, 556. Gymnadenieae, 506. Fontinalis.333(Fig.204), Gigartinaceae, 273. Gymnoascese, 294. 344 (Fig. 209). Gilia, 567. Gymnoascus, 290, 294. Fool's-Parsley, 550. Ginger, 502. Gymnogramme. 364. FI >rir*'t-me-not, 569. Ginkgo, 406 (Fig. 240), Gj'mnospermse, 234, 419 F.»7:; Leptothrix, 2s-J. Lupinus, 558. Marsh Ciiiqucfoi!, 556. Lesser Celandine, 529. Luziola, 488. Marsh-mallow, 541. Lesser Dodder, 567. Luzula, 499. Marsh-marigolil. .V_'M. Lettuce, 582. Lychnis, 532 (Fig. 344). BCanb-Munmure, •"'_':•;. Leucobryum, 342, 345. Lycium, 569. Marsi I.-;.. :;::,< Fi-. 225). Leucodon. :U. Lycopodium, 886 (Fig. Maurandia, 564. Liguliflorae, 581. ' -::K May, 556. Ligusticum, 550. Lisustrum, 571. Lycopsis, 569. Lycopus, 563. Maydese, 489. Meadow-grass. I'M. Lilac, 571. Lvgodium, 358, 372. M.-:\.|.. \v-l-iif. 528. Liliacete, 481, 496. Lyme-grass, 492. Meadow Saffron, 498. 603 IXDEX, PART II. Meadow-sweet. 554. Moon-wort, 355. : Nemalion. 272 <"Fiy. 160). Meconopsis, 533. Moor-grass, 491. Nemastoma. 271. Medicago, 558. Moracese, 512, 515. Neotinea, 506. Medlar, 556. Morchella, 297. Neottia, 506. Melampsora, 299 (Figs. Morell, 297. Neottiinae, 506. 179, 180). Morello Cherry, 555. Nepenthes, 41 (Fig. 28). Melampvrum, 565. Morus, 515. Nepeta, 563. Melanthioidese, 498. Moschatel. 576. Nepetese, 563. Melica. 487, 490. Mosses, 332. ; Nephrodium, 360 (Fig. Melic-grass, 490. Mother-wort, 563. 214), 372. Melilotus. 465 (Fig. 281), Mould, 275, 295. ; Nereocystis, 268. 558. Mountain Ash, 557. New Zealand Flax, 498. Melissa, 563. Mountain Avens, 556. ; Nicotiaiia, 458 (Fig. 274), Melissinefe.563. Mouse-tail, 529. 569. Melittis, 563. Mucor, 279, 287 (Figs, i Nigella, 529. Melobesia, 240, 271. 165, 166). Nigritella, 506. Melon, 551. Mucoracese, 285. Nipplewort, 582. Mentha, 563. Mucorinse, 285. Nitellese, 263. Menthoidese, 563. Mudwort, 564. Nostoc, 244 (Figs. 135. Menyanthese. 570. Mulberry, 515. 136). Menyanth.es, 570. ' Mullein, 564. Notorhizese, 536. Menziesia, 573. Musa, 502 (Fig. 312). Nothoscordum, 498. Mercurialis, 526. Musacese, 481, 501. 1 Nucumentacese, 537. Merismopedia, 244. Muscari, 497. j Nuphar, 531. Mertensia, 570. Musci, 234, 332. Nutmeg, 415 (Fig. 246). Mespilus, 556. Muscinese, 317. Nux Vomica, 414 (Fig. Metroxvlon, 485. Musese. 501. 245). Metzgeria, 324 (Fig. 200), Mushroom, 279, 301. Nyctalis, 304. 330. Musk, 564. Nymphsea, 530. Meum, 550. Musk Orchis, 506. Nymphaeaceae, 512, 530. Mildew, 295. Musschia, 574. NymphaeinGe, 530. Milfoil, 581. Mustard, 537. Milium, 490. Mutisia, 581. Oak, 520. Milkwort, 546. Mutisiese, 581. Oat, 490. Milla, 498. Mycorhiza, 275. Oat-grass. 490. Millet, 489. Myosotis, 569. Ochlandra, 488. Millet-grass, 490. Myosurus, 529. Ocirnoidese. 5(33. Millet-seed, 489. Myrionema, 266. Ocimum, 563. Mimosa, 559. Myristica, 415 (Fig. 246). Octaviana, 304. Mimosese, 559. Myrrhis, 550. (Edogoniaceaj. 253. 257. Mimulus, 564. Myrtaceaj, 513, 553. 1 (Edogonium, 246, 257 Mint, 563. Myrtales, 513, 552. (Fig. 147). Mistletoe, 525 (Fig. 340), Myrtle, 553. CEnanthe, 550. Moenchia, 532. Myrtus, 553. (Enothera. 552. Mohria, 361 (Fig. 216), Myxomycetes, 280, 283. Oil Palm, 48(i. 372. Old Man's Beard. 528. Molinia, 491. Naiadacese, 481, 494. Olea, 571. Monandrse, 506. Naias, 396, 494. Oleaceae, 513. 570. Monardese, 563. Nandina, 531. Olfersia, 360. Monk's-hood, 530. Napobrassica, 537. Oligoporus, 304. Monochlamydese, 512, Napus, 537. Olive, 571. 514. Monocotvledones, 234, Narcissales, 481, 507. Narcissus, 507. Onagracese. 513, 552. Oncidium, 507. 418, 476. Nardus, 491. Onion, 498. Monogramme, 370. Narthecium, 498. Onobrychis, 558. Monospora, 271. Nasturtium, 537. Onoclea, 359, 372. Monostroma, 257. | Navew, 537. Ononis, 558. Monotropa, 573. Neckera, 346. Onopordon, 581. Monotropese, 573. i Xelumbieae, 531. Oocvstis, 248. Montbretia, 509. ! Nelumbium, 531. Oomycetes, 278, 280, 287. CLASSIFICATION AND NOMENCLATURE. 007 Ophioglossaceae, 352, 354. Ophioglossum, 354. Ophrydinae, 506. Ophrys, 506. Orache, 523. Orange, 544. Orchidaceae, 481. 503. Orchidales, 481, '503. Origanum, 563. Orchis, 504 (Figs. 315, 316). Ornithogalum, 497. Orobanchaceae, 513, 566. Orobanche, 566. Orobus, 559. Orpine, 561. Orthoclada, 488. Orthoploceae, 536. Orthospermeae, 549. Orthotrichum, 339, 345. Oryza, 489. Oryzeae, 489. Oscillaria, 244 (Fig. 135). Oscillariaceae, 244. Osmunda, 359, 372 (Fig. 261), 406. Osmundacete, 352, 364, 372. Ostrya, 520. Oxalidacese, 513, 543. Oxalis, 543. Ox-eve Daisy, 580. Oxlip. 572. Oxycedrus, 436. Oxymitra, 324. Paeon ia, 530. Paeonieae, 530. Paigle, 572. Palm, 485. Palmace*, 481, 484. Palmales, 481, 484. Palmella, 247. Palmodictyon, 248. Palmophyllum, 248. Pampas Grass, 491. Pandorina. 249 (Fig. 138). Panicea?, 489. Panicoidese, 489. Panicum, 487 (Fig. 300). Pannaria, 306. Pansy. BBS. Papaver. 474, 533. Papaveracpae, 512, 5H3. Paper Mulberry, 515. Papilionese, 557. Papyrus, 492. Pariana, 488, 492. Parietales, 512, 533. Parietaria, 515. Paris, 461 (Fig. 277), 499 Phegopteris, 372. (Fig. 310). Philodendroi.l,,.. IKJ. Parmelia, 307. Parnassia, 561 (Fig. 373). Parnassieae, 560. Paronychieae, 532. Parsley, 550. Parsley Fern, 372. Parsnip, 550. Passiflorales, 513, 550. Pastinaca, 550. Patchouli, 563. Paulou-nia, 564. Pea, 559. i Peach, 554. I Pearl- wort, 532. ! Pear-tree, 556. | Pediastrum, 253. Pedicularis, 565. Pelargonium, 542. Pellia, 324, 330. Peltigera, 306, 307. Pelvetia, 268. Penicillium. 278, 291 (Fig. 170), 295. Penny-cress, 537. Pentstemon, 564. Peony, 530. Peplis, 553. Perisporiacese, 295. Perisporieae, 295. Peronospora, 279, 287. Peronosporaceae, 277, 287. Personates, 513, 564. Pertusaria, 308 (Fig. 190). Petaloidea, 481, 494. Petasites, 580. Petroselinum, 550. Petunia, 569. Peucedaneae, 550. Peucedanum, 550. Peziza,277,297(Fig.l77). Pezizaceae, 296. Phacidiese, 297. Phacotus, 249. Phjeogamse, 265, 268. Phseophycese, 238, 263. Phseosporae, 264, 265. Phalarideae, 489. Phalans, 489. Phalloidefe, 304. Phallus, 305. Phanerogamia, 234, 394. Phascum, 339. 312. Ph as. .,>!«.*, 559. Phaseolus, 559. Pheasant's Eye, 529. Pheasant's Eye Narcis- sus, 508. Phleum, 490. Phlox, 567. Ph. mix, 477 (Fig. 291). 485 (Fig. -j Phormium, 498. Photinia, 557. Phragmites, |:>1. j Phycochromacese, 244. Phycomycetes, 280, 285. Phyllantluis. :,-_'7. : Phyllobium, 21*. Phyllocladus. |:;7. | Phyllodoce, .",7:;. Phylloglossum, 386. ! Physalis, 568. Physcia, 307. Physospermum, 550. Physostigma, 559. Physureae, 507. Phytelephas, 4*0. Phyteuma, 571. Phytophthora, 287 (Figs. 167, 168), 2>*. Picea, 425 (Fig. 249), 426 (Fig. 250), 428 (Fig. 252), 435. Picris, 582. Pilacre, 304. Pilacreae, 804. Pilularia, 373, 380. Pimpernel. .".,-_'. Pimpinella, 550. Pinaster, 435. Pine, 435. Pinea, 435. Pine-apple, 501 (Fig. 311). Pinguicula, 566. Pink, 532. Pinnularia, 265 (Fig. 152). Piuoideae, 433. Piiius.-|M7,Fiir.2U!.-l-w (Fig. 21* . l-'7 Fi-. 251), 433 (Fig. 254), 435. Piptocephalideae, 285. Pisum, 559. Pithophora, •_'"._• Plagiochila. 326 (Fig. 201), 330. Plantaginacese, 513, 565. ' Plantago, 565 (Fig. 879;. j Plantain, 50-J : ; Platycerium, 860. Platvcodon. ~>7l. \ Pleospora. 27 i Pleurocarp*, 345. 608 IXDEX, PART II. Pleurocladia, 268. Primulacese, 513, 571. Raphiolepis, 557. Pleurococcacese, 248. Primulales, 513, 571. Raspberry, 556. Pleurococcus, 248 (Fig. Privet, 571. Rattle, 565. 137). Protobasidiomycetes, i Ravenala, 501. Pleiirorhizea?, 536. 304, 305. ' i Reed, 491. Plocamium, 271. Protococcacese, 248. Reed-grass, 489. Plumbaginacea?,513,572. ; Protococcoidete, 247, 248. Reed-mace, 484. Plumbago, 572. Protomvces. 301. Reindeer Moss, 307. Plume-thistle, 581. Pruned, 554 (Fig. 367). Rest-harrow, 558. Poa, 491. Prunella, 563;_ Rhamnaceae, 513, 546. Podophyllum, 531. Prunophora, 554. Rhamnus. 465 (Fig. 281), Pogostemon, 563. Prunus. 554. 546 (Fig. 359). Polemoniacese. 513, 567. Psalliota, 303. Rheum, 469 (Fig. 284). Polemoniales, 513, 567. Polemonium, 567. Pseudoneura, 327. Pseudosolanese, 564. 524 (Fig. 337). Rhinanthoidese, 564. Polybotrya, 361. Psilotacese, 353, 389. Rhinantlms, 565. Polycarpeae, 532. Psilotum, 389. Rhizocarpa?, 373. Polvcarpon, 532. Pteridese, 371. Rhizoclonium, 253. PolVgala, 546 (Fig. 358). Pteridium, 371. Rhizomorpha, 305. Pol ygalacese, 513, 545. Pteridophvta, 234, 346. Rhizopogon, 305. Polygonacese, 512, 523. Polygonatum, 499. Pteris, 360 (Fig. 214). 3(36 Rhodochiton. 564. (Fig. 218), 367 (Fig. Rhododendroidese. 573. Polvgonum, 523 (Fig. 220), 371. Rhododendron, 573. 337). Puccinia, 279, 298 (Fig. Rhodomelacese, 271. Polypetalse, 512, 527. 178). Rhodophvcea?, 238, 271. Polypodiacese, 352, 371. Puffball, 301, 305. Rhodotypus, 554. Polypodieae, 372. Pulicaria, 581. Rhodymeniacese, 273. Polypodium. 358, 360 ; Pulmonaria, 570. Rhubarb, 524. (Fig. 214), 369 (Fig. Pumpkin, 551. | Rhytisma, 297. 223), 372. Punica, 554. Ribes, 561 (Fig. 374). Polyporese, 303. Purple Beech, 521. Ribesieae, 561. Polyporus, 303. Pvcnophj-cus, 268. Ribwort, 566. Polvsiphonia, 271. Pylaiella, 266. Riccia, 324. Polytrichum, 339, 344 PjTrenomvcetes, 295. Ricciete, 322, 324. (Fig. 211). Pyrola, 572 (Fig. 387). Rice-plant, 489. Pomea?, 556 (Fig. 367). Pyrolacese, 513, 573. Richardia. 442, 483. Pomegranate, 554. Pyronema, 278, 293 (Fig. Ricinus, 462 (Fig. 278), Pond weed, 494. 172). 526. Pooidefe, 489. Pyrus, 556. Riella, 324. 329. Poplar, 522 (Fig. 335). Pythium, 287, 288. Riellese, 329. Poppv, 533. Rivulariacese, 244. Populus, 522. Quaking-grass. 491. Robin i a, 558. Portugal Laurel, 555. Quercus, 520 (Fig. 333). Roccella, 307. Potamogeton, 49-1. Quince, 556. Rock-rose, 538. Potato Plant, 568. Romulea, 508. Potentilla, 459 (Fig. 275), Racodium, 306. Rosa, 451 (Fig. 267), 554. 556. Radiola, 542. Rosacese, 513, 554. Potentilleee,. 555 (Fig. Radish, 538. Rosales. 513. .V>1. 367). Rarlula, 324, 330. Rose. :»:> I. Poteriese, 555. Ramalina, 307. Rosea;, 555 (Fig. 367). Poterium, 555. Rampion, 574. Rosemary, 563. Pothoideae, 483. Ranales, 512, 527. Rosmarinus, 563. Pothos, 483. Ranunculaceee, 512. .V27. Rowan-tree, 557. Pot-Marigold, 581. Ranunculus, 528 (Fig. Rubia, 575 (Fig. 390). Pottia, 344. 342), 529. Rubiacese, 513. 574. Preissia, 322. Rapa, 537. Bubiales. 513. 574. Prickly Samphire, 550. Rape, 537. Rubus. 555 (Fig. 368), Primrose. r>72. Raphanese, 538. 556. Primula, 453 (Fig. 269), Raphanus,' 535 (Fig. 348), Rudbeckia, 581. 571 (Fig. 386). 538. Rue, 543. CLASSIFICATION AND NOMENCLATURE. 609 Rumex, 523 (Fig. 337). Ruppia, 405, 494. Ruscus, 499. Rush, 499. Rust, 275, 298, 299. Ruta, 543. Rutacese, 513, 543. Rutese, 543. Rye, 491. Rye-grass, 491. Sabina, 436. Saccharoniyces, 295 (Fig. 174). Saccharomycetes, 295. Saccharum, 489. Safflower, 581. Saffron Crocus, 508. Sage, 563. Sagina, 532. Sagittaria, 495. Sago, 485. Sainfoin, 558. Salad Burnet, 555. Salicace*, 51± r,22. Salicornia, 523. Salisburia, 437. Salix, 522 (Fig. 335). Sallow, 522. Salsafy, 582. Salsola, 523. Salt-wort. r,-j:-5. Salvia, 461 (Fig. 276), . 562. Salvinia, 347, 374 (Fig. 224), 379 (Fig.- 226). Salviniacese, 352, 373, 380. Sambucese, 576. Sambucus, 458 (Fig. 274), 463 (Fig. 279), 576. Sainolus. :>72. Samphire, 550. Sanaa! -wood, 525. Sand- wort, 532. Sanguisorba, 555. Sanicula, 549. Saniculese. M!i. Santalacese, 512, 524. Sautalales, 512, 524. Sautalum. 52.\ Sapindacese, 544. Sapindales, 513, 544. Sapindus, 545. Saponaria, 532. Saprolegnia, 289. Saprolegniacese, 278, 289. Sarciua, 281. Sargassuml 269. Sarothamnus, 558. M.B. Sarsaparilla, 499. Satureia, 563. Satureineae, 563. Saussurea, 581. Savoy-cabbage, 537. Saw-wort, r,si. Saxifraga, 465 (Fig. 281), 560. I Saxifragacese, 513, 560. Saxifragales, 513, 559. I Saxifrage*, 560. Scabiosa, 578 (Fig. 394). I Scandiceae, 550. Scandix, 550. Scapania, 327. 330. Scarborough Lily, •">"<. Scarlet-runner, 559. Scenedesmus, 248. Scheuchzeria, 495. Schistostega, 344. Schizsea, 359, 365, 372. _ Schizomycetes, 2 Hi. 2<7± Sea-milkwort. r.72. Sea-purslane, 532. Sea-rocket, 538. Secale, 491. Sedge, 493. Sedum, 561 (Fisr. 876 St-laginella, 390 (Fig. §82 . 391 (Fig. 233), 2 Fig. 234). Selagineilaoen, 353, 889 Sempervivum, 561. Senebiera. .",:',7. I Seneci... j Senecionideae, 580. i Sensitive Plant, 559. ' Serapiadea;, ^ • i. Serratula. .\si. Service-tree, 557. I Seseli, 550. Seseliueae, 550. Sesleria. T.'l. Shaddock. .Ml. Shalot, 498. Shwp's-l.ii. :.7I. Shi-plifi-d's Purs.-, 537. Sherardia. '<> •>. Shield-Fern, 372. Sibbaldia. .V.«i. Sibthorpia, 565. Silaus. .Vid. 8ilene,682. Sileneae, 532. Siler, 550. Siliculosit-. .">:;, . Siliquosse, 536. Silver Fir, 435. Silver-weed, 556. Siniethis, 498. Sinapis, 537. Siphonacese, 250. Siphonoideae, 246, 248 250. Sirogonium, 255. Sison, 550. Sisymbriese, Tvi^ Sisymbrium, 537. Sium, 550. Skullcap, 563. Sloe, 554. Small Reed, 490. Sin ilaooidew, 499. Smilax, 499. Smut, 275, 298, 300. Smyrniese, 550. i Smyrnium, 550. Snak.-V ll.-a.l. l!>7. Snapdragon, 564. Snowlx-rry. ")7.. Snowdrop, 507. S.ai-\v..rt. M--'. Solanacese, 51: Solanat Solanin. Soldanella, 571. K R 610 IXDEXf PAKT II. Solidago, 580. Staurastrum, 254. Solomon's Seal, 499. , Stegocarpse, 338, 343. Sonchus, 582. Stellaria, 532. Sorbus, 557. Stellatee, 575. Sordaria, 278. Sticta, 307 (Fig. 188). Sow-bread. 572. \ Stinging Kettle, 514. Sow-thistle, 582. Stipa, 487, 490. Spadiciflorse, 481, 482. Stitch-wort, 532. Spanish Chestnut, ,521. St. Dabeoc'sJSeath, 573. Sparaxis, 509. i St. John's Wort, 539. Sparganium, 484. Stocks, 537. Spartina, 491. Stone Pine, 435. Spearwort, 529. Specularia, 574. Stonecrop, 561. Stork's-bill, 542. Speedwell, 565. Strap-wort, 533. Spelt, 492. Stratioteee, 500. Spergula, 532. Stratiotes, 500. Spergularia, 532. Strawberry, 556. Spermaphyta, 234, 394. Strelitzia, 501. Spermothamnion, 274 Sturm ia, 507. (Fig. 161). Stypocaulon, 239 (Fig. Sphacelaria, 263, 264. 133). Sphserella, 249. Suaada, 523. Sphaeriacese, 296. Subularia, 537. Sphaerocarpus, 327, 329. Succisa, 578. Sphseroplea, 247, 252. Sugar-cane, 489. Sphseroplese, 252. Summer Savory, 563. Sphserotheca, 293. Summer Snowflake, 507. Sphagnacese, 312, 316, Sundew, 561. 340. Sunflower, 581. Sphagnum, 332, 341 Sweet Basil, 5'63. (Figs. 205, 206). Sweet Briar, 554. Spider Orchis, 506. Sweet Flag, 483. Spinach, 523. Sweet Orange, 544. • Spinacia, 523. Sweet Potato, 567. Spindle-tree, 546. Sycamore, 545. Spiraea, 554. Symphoricarpus, 577. Spirseese, 554. Symphyogyna. 327. Spiranthese, 506. Symphytum, 569. Spiranthes, 506. Syringa, 571. Spirillum, 281 (Fig. 162). Spirochsete, 281. Tagetes, 581, Spirogyra, 246, 255 Tamarindus, 559. (Fig. 145). Tamus, 500. Spirolobeae, 536. Tanacetum, 581 (Fig. Splachnum, 338. 397). Spring Snowflake, 507. Tansy, 581. Spruce Fir, 435. Tapioca, 527. Spurge, 527. Taraxacum, 582 (Fig. Spurrey, 532. 397). Squamariacese, 272. Tassel Pondweed, 494. Squinancy-wort, 575. Taxes-, 437. Stachydeae. 563. Taxoideae, 433, 436. Stachys, 563. Taxus, 396, 437{Fig. 258). Stangeria, 432. Teazle, 578. Star-Anise, 530. Teesdalia, 537. Star Daffodil, 508. Star-fruit, 495. Telegraph-plant, 558. Tetraphis, 337, 345. Star of Bethlehem. 498. Tetraspora, 248. Statice, 572. ! Teucrium, 564. : Thalamiflorge, 512, 527. Thalictrum, 528. Thallophyta, 234. 237. Thelidium, 306. Thesium 525 (Fig. 339). T-histle, 581. Thlaspi, 537 (Fig. 348). Thlaspidete, 537. Thorn-apple, 569. Thrift. 572. ' i Thuidium, 346. Thuja,' 436 (Fig. 256). Thyme, 563. Thymus, 568. Tiger-lily. 496. Tilia, 461 (Fig. 276), 540 (Fig. 351). Tiliaceae, 512, 539. . Tilletia, 300 (Fig. 181). Timothy-grass. 490. i Tmesipteris, 389. ! Toad-flax, 564. Toadstool, 301. ' Tobacco-plant, 569. Todea, 372.' Tofieldia, 498.. Tomato, 568. Tprdylium, 550. Torilis, 550. Tragopogon, 582. •Trapa, 552. Treacle mustard, 537. Tree-fern, 358, 372. | Tremella, 303 (Fig. 184). Tremellinese. 303. Trentepohlia, 306. Trichomanes, 360 (Fig. 214), 371. Trientalis, 572. Trifoliese, 558. Trifolium, 558. Triglochin,495 (Fig. 305) Trigonella, 558! Trinia, 550. Triticum, 478 (Fig. 292), 491. Tritonia, 509. Trollius, 529. Tropseolum, 457 (Fig. 273). Trumpet Lily, 483. Tubuliflorse, 579. Tuburcinia, 301. Tulip, 496. Tulipa, 496. Tulipese, 496. Tulip-tree, 530. Tulostoma, 304. Turk's Cap Lily, 496. Turmeric, 502. CLASSIFICATION AND NOMENCLATURE. 611 Turnip, 537: Venus' Looking-glass, Wild Garlic, 498. Tussilago, 580. 574. Wild Oats, 490. Tutsans, 539. Veratrum, 498. Wild Parsnip, 550. Twayblade, 506. .Verbascum, 564. ' Wild Plum. r,:,.-,. Typha, 484. Typhacete, 481, 484. . Typhula, 277, 304. Vernal grass, 489. Veronica, 564 (Figs. 377, 378). Wild Rosemary, 573. Wild Siijro. 563. Vetch, 559. Willow'Herb, 562. Viburmimv576. Winter Aconite, 529. Ulex, 558. . Vicia, 456 (Fig. 272), 510 Winter Cherr.x Ulmaceae, 512, 516. -(Fig. 320), 559. Winter-green. ."•.:-!. Ulmus, 516 (Fig. 326). Ulothricaceae, 253, 256. •Vicieae, 558.' Victoria, 531. Wistaria, 559. Woad, 537. Ulothrix, 246, 256 (Fig. Villarsia. 570. Wolffia, l".\ 146). Viola, 538 (Fig. 349). ' W..lfs-bane, 530. Ulva, 246, 257. Violacese, 512, 538. Wood Germander, 564. Ulvaceae, 253, 257. Violet, 538. Woodruff, 575. Umbel lales, 513, 547. Viper's Bugloss, 569. Woodsia, 372. Umbelliferse, 513, 548. Virginian Creeper, 547. Wood-sorrel, 5l:s. Uncinula, 295 (Fig. 173). Viscum,525(Fig. 340). Woody Nightshade, 568 Uredinese, 299 (Fig. 179). Vitis, 547 (Fig. 360). Wormwood, 580. Uredo, 299. Vittaria, 370. Wound wort, 558. Urocystis, 301. Volvocaceae, 249. WychElm, 517. Urospora,- 253. Volvocoidese, 247, 249. Urtica" 514 (Figs. 322, Volvox, 246, 249 (Fig. Xiphion, 508. 323). . • 139). Urticacese, 512, 514. Yam, 500. Urticales, 512, 514. Yarrow, 581. Usnea, 307 (Figs. 186, Wall-flower, 537. Yeast, 295 (Fig. 174). 189). Wall-Pellitory, 515. Yellow Flag, 509. Ustilaginese, 298, 300. ' Ustilago, 300 (Fig. 181). Wall-Rue; 371. Walnut, 522. Yellow Loosestrife, 572. Yellow Monkey-flower , Utricularia, 566 (Fig. Water-Chestnut, 552. 564. 380). Water-cress, 537. Yellow Rocket, 537. Uvularia, 498. Water Crowfoot, 529. Yellow Welsh Poppy, Water-Hemlock, 550. 533. Water-Lily, 531. Yew, 437 (Fig. 258). Vacciniaceae, 513, 573. Water Melon, 551. Yucca, 498. Vaccinium,461(Fig.277), Water Plantain, 495. 572 (Fig. 387). Valerian, 577 (Fig. 393). Water-Purslane, 553. Water-Soldier, 500. Zamia, 432 (Fig. 253). Zanardinia, ~2>^. Valeriana, 577. Water- Violet, 572. Zannichellia, 494. Valerianacese, 513, 577. Weeping Willow, 522. Zantedeschia. 483. Valerianella, 577. Valisneria, 500. Weigelia, 577. Welwitschia, 437. Zea, 479 (Fig. 293), 489. Zingiber, 502. Valisneriese, 500. Wheat, 487 (Fig. 301), Zingiberacea?, 481, 502. Vallota, 507. 490 (Fig. 302). Ziiigiberese, 502. Vanda, 507. Whin, 558. Zinnia, 581. Vanilla, 507. White Beam, 557. Zostera, 494. Vascular Cryptogams, Vaucheria,250(Fig. 140). White Thorn, 556. Whitlow-grass, 537. Whortleberry, 573. Zygnema, 255 (Fig. 144). Zygnemese, 254. Zvgogonium, 256. Venus' Fly-trap, 561. Wild Balsam, 543. Zygomycetes, 280, 285. :r, The Selwood Printing Works, Frome, and London. & % 1* University of California SOUTHERN REGIONAL LIBRARY FACILITY 405 Hilgard Avenue, Los Angeles, CA 90024-1388 Return this material to the library from which it was borrowed. v - i • IPR15-96 ^ c^ 2 ef I S 4-OF-CAIIFO% I i , 3 1158012280912 _^5ME I'NIVERS^ ^IOS ANGELA ,5Jt\EUNIYERJ/A ^<~rl i I 3 f ^WIMVHOiJ. ^KB/WOIE^ ^ ! f^S 16£f I.