Part I

isogamous plants, in those cases in which they are set free from the gametangium and are free-swimming, are well 17.] CHAPTER II. SPECIAL MORPHOLOGY OF THE MEMBERS. 81 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 110 defined form nor are they ciliated. The gametes of heterogamous 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 Algae (e.g. Vaucheria, Volvox, Sphaeroplea, (Edogonium, Chara, Fucus), in the Bryophyta, and in the Pteridophyta. In the lower forms the sperm atozoid is more or less rounded or pearshaped, somewhat resembling a planogamete of the isogamous forms: but in the higher it is club-shaped or filamentous, thicker at the posterior end, pointed at the anterior end where the two or more cilia are borne, and more or less spirally coiled. An exception to this form of free male gamete is afforded by the Red Algae, where the gametes are small rounded or oval bodies destitute of cilia, and are distinguished as spermatia ; when first set free they have no cell- wall, but they develope one before they come into contact with the female organ. Very similar to these are the spermatia of certain Ascomycetous Fungi, which have, however, a cell- wall from the very first ; but there is some doubt as to the sexual nature of these cells. When, owing to the proximity of the male and female organs at the time of fertilisation, the male gamete has 110 considerable dis- tance to traverse (e.g. Peronosporeae, Phanerogams), it is not differentiated as a spermatozoid, but it 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 fertilisation : but in Fucus and its allies, the oosphere is extruded from the female organ before fertilisation. It is, generally speaking, spherical in form, as its name denotes. 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 con- siderable number (e.g. Botrydium, Ulothrix) ; but in Ectocarpus and some other Phaeosporeae, 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 v. s. B. G 82 PART I. THE MORPHOLOGY OF PLANTS. 17. [ lower it is the rule that the male gametes are produced several together from one mother-cell. The female gametes are de- veloped singly in the mother-cell, except in the Saprolegnieee among Fungi, and in some genera of Fucaceae (Pelvetia, Ozothal- lia or Ascophyllum, Fucus), in whiich from two to eight (Fucaceae) or up to twenty (Saprolegniese) oospheres are produced from one mother- cell. With regard to the development of the gametes, it is sometimes the case that the whole of the protoplasm of the mother-cell is used in their formation. Thus, the gamete of Spirogyra is formed by the rejuvenescence of the whole protoplasm of a cell of the filament ; and the oospheres of Fucus are formed by the division of the whole of the protoplasm of the mother-cell. But in most cases a portion of the protoplasm, and more particularly of the nuclear substance, is excluded from taking part in the formation of the gametes : a portion of it is either directly cut off by division, or is simply not used in the development of the gamete. Thus, in the Muscineae, Pteridophyta, and Gymnosperms, the development of the oosphere begins with the division of the nucleus of the mother-cell (central cell) into two ; this is followed by the division of the mother-cell into two unequal parts, no cell-wall being however formed ; the larger portion is the oosphere ; the smaller, which eventually decomposes, is termed the ventral canal-cell, on account of its position in the female organ. In cer- tain Algse the development of the oosphere is attended by (e.g. Vaucheria, CEdogonium, Coleochsete) the discharge of a mass of protoplasm from the female organ prior to fertilisation, which probably corresponds to the ventral canal-cell of the higher plants. With regard to the development of the spermatozoid, it ap- pears, in those cases in which it has been most fully investigated (Bryophyta, Pteridophyta), that only a portion of the cytoplasm and nuclear substance of the mother-cell is used in its formation ; the residue is usually discharged with the spermatozoid, as a vesi- cle which adheres to it, until thrown off by its active movements. (For details, see Part II., Cell- Format ion, p. 116.) The masses of protoplasm which are derived from the mother- cells, but are excluded in any of the above ways from entering into the formation of the gametes, are termed, generally, polar bodies. Their physiological significance is considered in Part IV. 17.] CHAPTER IT. SPECIAL MORPHOLOGY OF THE MEMBERS. 83 (6) The Gametangia. The general morphology of the gametangia 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 differentiated 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 archegonium, in which one or more oospheres are differentiated. To the latter category belong the male organ termed pollinodlum {e.g. in Peronosporeas and some Ascomycetes), and the female organs termed procarp (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. Paudorina, Ulothrix, Conjugate), but in some heterogamous plants {e.g. Sphseroplea) in which the gametes are perfectly differentiated into spermatozoids and oospheres. In plants of higher organisation there are specialised gametangia. In the simpler forms of these the male and female gametangia are externally similar, as in the Volvocinene, Ectocarpus, and Cutleria, among the Algse, and in the Zygomycetes 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 multilocular, in some Phseosporese {e.g. Ectocarpus, Giraudia, Scytosiphon, Cutleria). The differentiated gametangia are of various structure. The antheridium is unicellular in most of the lower plants (Green 84 PART 1. THE MORPHOLOGY OF PLANTS. [ 17. Algoe, except Characeae ; Fucaceae), as also generally in Phanero- gams. In all the other cases it is mnlticellular, and of simple structure, except in the Characeae, where the structure is ex- tremely complex. In some cases (Rhizocarps) the antheridium consists almost entirely of the mother-cells of the spermatozoids ; in most cases the mother-cells are surrounded by a parietal layer of cells. The pollinodium is generally unicellular. The oogonium is either unicellular, as is generally the case, or it is a coenocyte (e.g. Peronosporeae, Vaucheria). The archegonium is generally multicellular, consisting of a cellular wall investing the oosphere, usually prolonged into a tubular neck ; but in some of the higher plants, Welwitschia among the Gymnosperms, and all Angiosperms, the archegonium is reduced to a single cell, the oosphere. The archicarps and procarps are unicellular in some cases, multicellular in others ; in most cases the organ is pro* longed into a filament, the trichogyne, by means of which fertili- sation is effected. The oogonia (except those of Peronosporeae, Saprolegniae, and Characeae) and the archegoiiia, 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 III. in connexion with the plants to which the various organs belong. (c) The Gametophore. In some plants in which the gametophyte is well-developed, the sexual organs are not borne indifferently on any part of the body, but are confined to particular regions or branches of the thallus or of the shoot, which may be distinguished as gametophore^ ; as in some Algae (e.g. Himanthalia lorea), Liver- worts (e.g. Marchantia), Mosses (e.g. Sphagnum), and some Ferns. In Himanthalia lorea (Fig. 44) the vegetative portion of the body is conical in form and is attached at its pointed end, and from the middle of the cup-like upper surface there grows a long branched gametophore. In Marchantia, and some other Liverworts, some of the flattened prostrate branches of the thallus develope termi- nally into an erect cylindrical gametophore, bearing male or female organs (antheridia or archegonia) on a receptacle at its summit, in Sphagnum, also, the antheridia and archegonia are borne re- spectively on distinct and specialised branches. Such unisexual gametophores may be specially designated antherid Chores or archegoniopJiores. In some Ferns which have an altogether fila- mentous gametophyte (e.g. Trichomanes), the female organs (ar- chegonia) are borne on short multicellular lateral branches form- 17.] CHAPTER II. SPECIAL MORPHOLOGY OF THE MEMBERS. 85 ing cushion-like gametophores, or, more precisely, archegoniophores. When, as in most Ferns, the adult gametophyte is a flattened dorsiventral cellular expansion, the archegonia are borne on a cellular cushion projecting on the ventral surface, which constitutes the archegoniophore. This kind of archegoniophore reaches its highest development in the Fern Gymnogramme leptophylla, where the cushion grows out into a tuberous subterranean archegoniophore. When the shoot is differentiated into stem and leaf, the sexual organs are in some cases borne on more or less specialised leaves which may be termed ytymetophylls, as in some Algae, such as the Cha- raceae (both antheridia and oogonia), the higher Sphacelariese (gametangia of Cladostephus) and the Rhodomeleae (e.g. antheridia of Polysipho- nia). The sexual organs may either be scattered over the surface of the mem- ber bearing them, or they may be developed in groups (sori). Occasion- ally a sorus is enclosed in a receptacle ; such re- ceptacles are the concep- tacle of the Fucaceae and of the Corallineae (Flori- Fig. 44. Himanfhal ia lorea (mu^h reduced), o vegetative part of body ; b branched gametophore. deae) ; the spermogonium of the Ascomycetous and ^Ecidiomycetous Fungi ; the receptacle of many Hepaticae. In some Hepaticae (e.g. foliose Jungermanniese), and in all Mosses, the sorus of sexual organs is surrounded by leaves, which may differ more or less from the ordinary foliage leaves. The investment thus formed is termed a perichcetium, and the leaves are termed perichcetial leaves. Since, as has been pointed out, the gametangia correspond to the sporangia, the gametophore corresponds also to the sporophore ; they are both portions of the shoot, or of the thallus, which are 86 PART I. THE MORPHOLOGY OF PLANTS. [ 17. specially adapted to bear the reproductive organs, in the one case sexual, in the other, asexual. And just as an axis of the sporo- phore bearing one or more sporangia (whether directly or on sponphylls) is termed a flower, and the sporophore itself an inflorescence ; so these terms may be applied to the gametophore, though the physiological differences between the sporangia and gametangia must be borne in mind. It has, in fact, long been customary with reference to Mosses, to speak of the sori of sexual organs, with their perichaetia, as " flowers." (d.) The 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 dinecious. The following are instances of monoecious gametophytes : Algce ; Volvoac Globator, Khynconema (Zygnemeae), Vaucheria, Sphseroplea, Coleoehaete, some species of Chara and Nitella, Fucus platycarpus, Hali- drys, Cystoseira, Pycnophycus. Fungi ; monoecism is the rule. Muscinete ; essentially monoecious. Pteridophyta ; homosporous forms generally monoecious, except Equisetum. The following are instances of dioecious gametophytes : Alga; Volvox minor, Eudorina, Conjugatae generally, Cutleria, most species of Fucus, Ozothallia (Ascophyllum), most Eed Algae. Pteridophyta; Equisetum, and all heterosporous forms (Hydropterideae Isoeteae, Selaginelleae). The Phanerogams have not been included in the preceding list, for the conditions which obtain among them in this respect are peculiar, and demand separate consideration. Inasmuch as the Phanerogams are heterosporous, they are essentially dioecious, since each kind of spore produces its corresponding male or female gametophyte. Bat in consequence of the fact (see p. 74) that the macrospore is not set free, but remains attached to the sporophyte, and germinates in that position, the female gametophyte is attached to the sporophyte. On this account, and on account of the rudimentary development of the male gametophyte (pollentube), the spores, that is the pollen- grain and the embryo-sac, have come to be inaccurately regarded as sexual reproductive cells, and the stamens and carpels, which are really sporophylls, as sexual reproductive organs. Hence a Phanerogam is said to be monoecious when the same individual bears both stamens and carpels, dioecious when they are borne by distinct individuals. 18.] CHAPTER II. SPECIAL MORPHOLOGY OF THE MEMBERS. 87 Monoecism, in this sense, is the general rule in Phanerogams ; but dicecism obtains in the Cycadaceas, the Taxeae, the Araucarieae, some Juniperinas, and Ephedra, among Gymnosperms ; and in various Angiosperms, such as Naias, the Pandanese, some Palms, Cannabinea3, Salicineae, Aucuba, etc. When in monoecious plants the male and female organs are both present in the same sorus, as in some Algae (e.g. Fucus platycarpus, Halidrys, and other monoecious Fucaceoe) 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 Hepaticae 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 in accordance with the foregoing explanation; diclinism is the rule in the Gymnosperms, and it occurs also in various Angiosperms. 18. Apospory and Apogamy. Although it is the rule that the gametophyte springs from the spore produced asexually by the sporophyte, and that the sporophyte springs from the spore produced sexually by the gametophyte, yet the transition from the one generation to the other may be effected otherwise. In some cases vegetative reproduction is substituted for reproduction by an asexually-produced spore : this process is termed Apospory. Thus in some Ferns (e.g. Athyrium Filix foemina) the gametophyte (prothallium) is produced directly by budding from the leaves of the sporophyte. In other cases, either vegetative reproduction, or reproduction by asexually-produced spores, is substituted for reproduction by sexually-produced spores : this process is termed Apogamy, distinguished in the one case as vegetative apogamy, and in the other as parthenogenetic apogamy or parthenogenesis. Examples of vegetative apogamy are afforded by some Ferns (e.g. Pteris cretica) where the sporophyte is developed as a bud upon the gametophyte ; and also in certain Ascomycetous Fungi where the sporophyte (ascocarp) is directly developed upon the mycelium ; in neither case is there any development of sexual organs. Examples of parthenogenetic apogamy are afforded by the Saprolegnieae. where, although oogonia are developed, there is no fertilisation, but the cells (corresponding to oospheres) contained in the oogonia germinate as though they were oospores; and also by certain Ascomycetes where the archicarp developes without fertilisation into an 88 PART I. THE MORPHOLOGY" OF PLANTS. [ 19, 20. ascocarp. In either case the result, is that a sporophyte is de- veloped from that which either is, or represents, a gametophyte, without the intervention of a sexual process. 19. The Fruit. Although the forms of fruit occurring among plants are so various in their form and in their structure, A it is possible to include them all in a single definition. 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. To begin with instances among the lower plants, the cystocarp of the Red Algae and the ascocarp of the Ascomycetous Fungi are fruits. In these cases the effect of the fertilisation of the female organ is not merely that the female organ gives rise to sporangia (carposporangia in the one case, asci in the other) ; 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. Similarly, in the Bryophyta, and to a less extent in the Pteridophyta, the effect of the fertilisation of the oosphere is riot 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 III., 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. 86). 20. The Seed. As this is a structure which is peculiar to Phanerogams, its morphology is discussed in connection with that group (see Part III). PART IF. THE INTIMATE STRUCTURE OF PLANTS. (ANATOMY AND HISTOLOGY). 21. 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 pias- modium of the Myxomycetes ; or it may consist of a mass of protoplasm invested afc the surface by a definite membrane which is not protoplasmic (e.g Phycomycetous Fungi and Siphonaceous Algas) ; or it may consist of a mass of protoplasm segmented into portions A by non-protoplHsmic partition-walls. 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.au 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 Alga3, 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 des- cribed as completely sepfate. 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 cells, but it is more accurate to reserve this term to 90 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 21. the protoplasmic unit together with the wall (cell-wall) by which it is invested, and to term the protoplasmic unit an energid. The structure of the body or any part of it can 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, Haematococcus, 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 (energids) enclosed within a common wall. Such a body, or part of a body, may be con- veniently distinguished as a ccenocyte, 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 unsep- tate, consisting of a layer of protoplasm with many nuclei scattered through it; it eventually becomes a cellular tissue by the delimitation of the constituent energids by means of cell-walls. But even when the cell-walls are formed, they do not always enclose single energids ; in Corydalis cava, for instance, the net-work of cell-walls encloses several energids in each mesh, so that the structure of the endosperm is at first coenocytic; eventually, however, the nuclei in each ccenocyte fuse together until only one remains, and in this way the transition from coenocytic to cellular structure is effected. Again, a "laticiferous cell" of a Euphorbia (and other Phanerogams) is essentially a ccenocyte like the body of a Vaucheria or a Botrydium. On the other hand, there is such a thing as a multinucleate cell. It has been observed, for instance, that 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 (see p. 96). The distinction between a ccenocyte 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 development, whilst the latter becomes multinucleate at a quite late period ; and further, that in the ccenocyte the nuclei multiply by indirect division (see p. 97), whereas in the multinucleate cell they multiply by direct division or fragmentation. 21.] INTRODUCTORY. 91 There is another kind of structure occurring in cellular plants which has to be distinguished from both the cell and the ccenocyte : that is the syncyte. This structure is developed from alreadyformed cells by an absorption, more or less complete, of the cellwalls, which places the cavities of the adjacent cells in direct continuity. The commonest case of this occurs in the development 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