Alternation of Generations in Plants

genus, Cutieria, the asexual form is very different from the sexual in appearance, and has long been regarded as a distinct genus under the name Aglaozonia. With regard to the other genera of the group, it is probable that in those forms (e.g. many EctocHrpaceae) in which the same individual bears at one time sexual, and at another, asexual organs, there is no alternation of generations ; and further, that in those forms (e.g. many Sphacelarieae) in which the asexual and the sexual organs are never borne by the same individual, there is 262 PART III. THE CLASSIFICATION OF PLANTS. an alternation of generations, the asexual form being the sporophyte. It is also probable that in the case of those forms of which only individuals bearing either asexual (e.g. Desmarestia, Laminaria) or sexual (e.g. Scytosiphon, Phyllitis, Colpomenia) organs are known, the life-history may present an alter- nation of generations. The Phaeosporeae are almost exclusively marine, the only fresh-water forms being the genus Pleurocladia (Ectocarpacese) and two species of the genus Lithoderma. The size of the plants included in this series varies widely, from microscopic Ectocarpaceee to gigantic tree-like Laminariaceae, such as Macro- cystis, Nereocystis, etc., which may attain a length of several hundred feet. In some of theLaminarias, which have cylindrical stalk-like region in their thalloid shoot (see Fig. 159), 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^IOGAM^B. The orders comprised in this group are characterised by the oogamous sexual process, by the absence of gonidia, and by the non- motile spores ; the orders may be conveniently described separately, as they are well defined. Order 8. Tilopteridaceae. Body filamentous, differentiated into shoot and root ; the shoot is, in its younger parts, monosiphonous ; in its older parts polysiphonous, but without cortex ; growth in length by means of intercalary growing-points ; lateral members, some with unlimited growth (branches), some with limited growth ; the root consists of a single row of cells, and its growth is intercalary, without any special growing-point ; marine. The asexual and sexual reproductive organs are borne on distinct individuals. The asexual organs are terminal, or less commonly intercalary, unicellular sporangia ; the sexual organs are unicellular oogonia, and multicellular an- theridia (resembling the gametangia of the Phseosporeae), and are intercalary. The sporangium gives rise to a single non-motile spore, which, at the time it is set free, has a delicate cell-wall, and contains four nuclei. The oogonium (which somewhat resembles the sporangium in form) gives rise to a single oosphere, which, when it leaves the oogonium, has no cell-wall. Each cell of the antheridium gives rise to a single spermatozoid, oval in form, with two cilia. The sexual process and the germination of the oospore have not been observed. The spore, on germination, undergoes repeated division, forming a solid multicellular body, from which a long root grows out ; from the multicellular embryo spring the adult shoots. The life-history of the TilopteridaceaB probably presents an alternation of generations, the form bearing the asexual organs being the sporophyte: for instance, the forms known as Haplospora and Scaphospora are probably the asexual and sexual generations, respectively, of the same plant, a Tilopteris. Order 9. Dictyotaceae. Body flattened, ribbon or fan-shaped, sometimes dichotomously branched, consisting of several layers of parenchymatous cells, with a well-marked midrib in Dictyopteris, differentiated into root and shoot ; growth of the shoot takes place in Dictyota by means of a single apical cell (see Fig. 140), in the other forms by a marginal series of merismatic cells. Both asexual and sexual organs are known. The asexual organs are unicellular GROUP I. THALLOPHYTA : ALGM : PILEOPHYCEjE. 263 sporangia, borne on distinct individuals ; each sporangium sets free four spores (tetraspores), which are destitute of a cell- wall and of cilia. The sexual organs are unicellular oogonia and multicellular antheridia ; the oogonia are generally in groups (sori), and each gives rise to a single oosphe re, which is set free as a naked unciliated cell ; the antheridia are always in groups (sori), and give rise to a large number of small, apparently non-motile, spermatozoids, which have no chromatophores. The process of fertilisation has not been FIG. 188. Fucus vesiculosus, about half nat. size : b air-bladders; / fertile branch. observed. The male and female organs are either borne on the same plant (e.g. Padina), or on distinct plants (Dictyota, Taonia). The tetraspores and the oospores germinate alike. The spore divides into two cells, one of which grows out into the filamentous primary root, the other grows out directly into the shoot in Dictyota and Zonaria ; but in Taonia, Padina, and Dictyopteris, the development is heteroblastic, since the latter cell gives rise to a rounded multicellular embryo, from which the adult shoot grows out as a branch. In the embryonic shoot of Taonia and Dictyopteris there is a single apical cell, but eventually it gives place to a number of initial cells. 264 PART III. THE CLASSIFICATION OF PLANTS. It is extremely probable that the life-history of these plants presents a definite alternation of generations, the asexual form being the sporophyte. If this be so, the affinity to the Bhodophycese \vhich is suggested by the similarity of four spores developed in the sporangium of this group to the " tetraspores" of FIG. 189. Section of a female conceptacle, with surrounding tissue, of Fucus vesiculosus, (x 50: after Thuret.) FIG. 190. Fucus vesiculosus. a Paraphysis, from male conceptacle, bearing antheridia ; 6 an oogonium with paraphyses ; c process of fertilisation, the extruded oosphere surrounded by spermatozoids; d developing embryo, (x 160 : after Thuret.) GROUP I. THALLOPHYTA : ALG^ : PHJCOPHYCEvE. 265 G FIG. 191. A Eight oospores extruded from the oogonium, surrounded by the inner layer of the cell-wall : B contents of an antheridium set free, surrounded by a layer of the cell- wall : C antheridium : I) section of contents of an oogonium showing the divisions accom- panying the development of the oospheres : -E oospheres set free in consequence of the rupture of the layer of cell-wall by which they were surrounded when first extruded : F and H oospheres, with spermatozoids : G spermatozoids. A-F Fucus platy carpus; GH, F. vesiculosus. (C and G x 540; other figs, x 240: after Strasbnrger.) 266 PART III. THE CLASSIFICATION OF PLANTS. the Khodophyceas, becomes altogether illusory ; for in the Dictyotaceae these spores are developed on the sporophyte, whilst the " " tetraspores of the Khodophyceae are gonidia developed by the gametophyte. All marine. Order 10. Fucaceae. Body differentiated 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 gonidia or of spores, and therefore no alternation of generations. Sexual organs, unicellular antheridia and oogonia ; spermatozoids, ciliated, formed several together in the antheridium ; oospheres, set free but not ciliated ; one (Pycnophycus, Himanthalia, Halidrys, Sargassum, Cystoseira), two (Pelvetia), 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 ex- ternal 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. In some genera (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 Sar- gassum the air-bladders are borne on special branches. The sexual organs are in all cases borne in depressions of the surface known as con- ceptacles (Fig. 189.) The conceptacles are commonly confined to special portions of the thallus ; either to the tips of the branches (e.g. Fucus, Cystoseira) or to special branches, the gametophores (e.g. Himanthalia, Ascophyl- lum, Sargassum). From the inner surface of the concep- tacle there arise a number of hairs (paraphyses) among which the sexual organs are borne. The oogonia (Fig. 190) FIG. 192. Himanthalia lorea (much reduced), a Vegetative part of body ; b branched gametophore. are nearly spherical, and are borne on a short stalk con- sisting of a single cell ; the antheridia (Fig. 190) are the lateral branches of some of the hairs. The plants may be monoecious (e.g. Fucus platycarpus, Halidrys, Pelvetia, Cystoseira, GROUP I. THALLOPHYTA : ALG^ : EHODOPHYCE^E. 267 Sargassum), or dioecious (e.g. Himanthalia, Ascophyllum, Fucus vesiculosus and serratus) ; in the former case each conceptacle contains both antheridia and oogonia. 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 190 d.) Sub-Class IV. RHODOPHYCEJ] or Red Algae. Multicellular plants ; body, generally differentiated into shoot and root ; shoot, sometimes differentiated into stem and leaf ; flattened or fila- mentous ; when filamentous, consisting of a single longitudinal row of cells (monosiphonous) or of several rows (polysiphonous} 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 alternately right and left ; the flattened forms grow by means of a marginal series of initial cells ; but in the Bangiaceae there is no growing-point, all the cells being merismatic : branching, generally monopodial, but sometimes sympodial (e.g. Plocamium, Dasya) : adventitious roots common. Vegetative reproduction by gemmae (e.g. Monospora, Melobesia) is rare. Reproduction by means of asexually produced spores occurs throughout the sub-class ; the sporophyte (cystocarp) always produces spores (carpospores) ; the gametophyte usually produces gonidia (usually tetragonidia) except in the Lemaneacea3 and most Helminthocladiaceae ; the gonidia are, as a rule, not borne on indi- viduals which produce sexual organs (actual gametophytes), but on distinct individuals (potential gametophytes), though there are exceptions to this rule (e.g. Lomentaria kaliformis, Callithamnion corymbosum, Polysiphonia variegata, etc.). The gonidia are produced in unilocular gonidangia, either singly, or two together, or sometimes as many as eight, but most commonly in fours ; hence they are generally termed tetragonidia. The tetragonidia may be formed tetrahedrally in the gonidangium ; 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 gonidangia on the shoot is various. In simple monosiphonous forms (e.g. Callithamnion) the terminal cells of short lateral branches become gonidangia. In forms of 268 PART III. THE CLASSIFICATION OF PLANTS. more complex structure the gonidangia are developed internally, within the superficial layer of tissue. The gonidangia 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 Dasya, Chondriopsis, Polysiphonia) the gonidangia are confined to certain specially modified branches (gonidiophores) which are termed stichidia. The tetragonidia are set free as spherical unciliated cells without a cell-wall. The sexual organs are antheridia and procarps; they arc usually borne by distinct individuals, but in some cases on the same (e.g. Grateloupia, Halymenia, Hala- rachnion, Nemastoma, Dudresnaya coccinea and purpurifera, Glceosiphonia capillaris, Helminthora divaricata). ' The antheridia are small and unicel- lular; in the simple filamentous 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 parenchymatous forms they occur in groups on the surface ; in those forms in which the shoot is differentiated into stem and leaf (e.g. some Rhodome- lacese such as Polysiphonia fastigiata and nigrescent, Chondriopsis tenuissima) the antheridia are confined to the leaves, the FIG. 193. Portion of a branch of Dasya elegans, bearing a stichidium (s), with tetragonidangia (t); t' empty tetragonidangium. (x 25: after Kiitz- 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, ing.) spherical or oval, unciliated cells desti- tute of a cell-wall; they acquire a cell-wall at the time of fertili- sation; they contain no chromatophores, except in Bangiaceae. The procarp presents considerable variety of form and structure. It may be unicellular (e.g. Bangiacese, Cbantraiisia, Batracho- spermum, Lemanea, Nemalion), or multicellular, as is more com- monly the case. The unicellular procarp consists simply of a carpogonium prolonged (except, perhaps, in Bangia) into a filament termed the trichogyne. Various descriptions are given of the structure of the multicellular procarp ; however, it appears to GROUP 1. THALLOPHYTA: ALGM : RHODOPHYCE5]. 269 consist essentially of a unicellular carpogoniam (with a trichogyne) together with one or more specially differentiated auxiliary cells. In some cases (e.g. Dudresnaya coccinea, Squamariaceae), the carpogonium and the auxiliary cells are not developed in the same procarp, bat 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. The carpogonium is (except in the Bangiaceas) developed from the terminal cell of a lateral appen- dage; in some cases (e.g. Polysi- phonia fastigiata and nigrescens) the lateral appendage is a leaf, the whole or part of which goes to form the procarp ; in the Coral- linaceae the procarps are aggre- gated in receptacles. The sexual process consists in the fusion of the protoplasmic con- tents of a spermatium with those of a trichogyne. The spermatiura is brought by the water into con- tact with the projecting trichogyne to which it adheres, the sperma- FIG. 194. Sexual organs of Nemalion ( x 300) A ends of branches bearing a unicellular procarp t-o, and a group of antheridia s ; the trichogyne (t) of the procarp has two spermatia (s) adhering tium being at this time covered with a cell-wall ; the intervening cell-walls are absorbed at the point of contact, and the plasm of the spermatium the trichogyne. protoenters to it. .B early stage in the development of the cystocarp ; the fertilised carpogonium is undergoing growth and division. C nearly mature cystocarp, consisting of a number of short filaments each terminating in a carposporangium. The development of the cystocarp is direct. 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 unicellular ; indirectly, when it is developed from both carpogonial and auxiliary cells : the tricho- gyne takes no part in the development of the cystocarp, being shut off by a septum. 270 PART III. THE CLASSIFICATION OF PLANTS. The simplest mode of direct formation of the cystocarp occurs in the Bangiaceae ; 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, Chantransia, Lemanea), the carpogonium gives rise to a number of filaments, termed ooblastema-filaments, which bear a cluster of sporangia (Fig. 194). In the indirect formation of the cystocarp, the carpogonium fuses with one or more of the auxiliary cells. In the simplest case FIG. 195. Sexual organs of Spermofhamnion hermaphroditum. A Male and female organs ; c multicellular procarp ; t trichogyne ; t' 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). (e.g. Origartinaceae, Bhodymeniaceae, Spha3rococcea3, Rhodomelacese), the carpogonium fuses directly with the auxiliary cell (or cells), and from the latter the 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. Gelidiaceae) or from the auxiliary cells {e.g. Squamariaceae and other Cryptoneminae). In the Corallinaceae, where the procarps are aggregated in receptacles, only a single cystocarp is formed from the whole group of procarps. Some of the procarps appear to be altogether abortive, and only those toward the centre of the group have tricho- GROUP I. THALLOPHYTA: ALG2B : RHODOPHYCEJ;. 271 gynes, whilst others seem to have only auxiliary cells : after fertilisation, the carpogonia of the central procarpia fuse with each other, and with the auxiliary cells of the other procarpia, forming a large cell from the periphery of which the carposporangia, constituting the cystocarp, are developed. The cell-fusions alluded to above are frequently considered to be of the nature of a sexual process. For instance, in Dudresnaya coccinea, the procarp bearing the trichogyne is regarded as a trichophore, whilst the procarp which includes the auxiliary cell, but has no trichogyne, is regarded as the carpogonium proper : the ooblastema-filament which grows from the former and fuses with the latter, is, from this point of view, a trichnphoric tube which conveys the fertilising substance of the spermatium from the trichophore to the carpogonium. From the point of view of the foregoing paragraphs, the fusion of carpogouial with auxiliary cells is simply of nutritive importance. Though both views are tenable, the latter is strongly supported by the fact that in certain forms (e.g. some Gelidiacese such as Wrangelia, Pterocladia) the ooblastema-filaments fuse with vegetative cells of the body. 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 is set free as a somewhat spherical unciliated cell destitute of a cell- wall, and germinates without any quiescent period. The germination of the tetragonidia 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. Batrachospermum and Lemanea are exceptions to the general rule that the germinating carpospore gives rise directly to the adult form, and afford good examples of heteroblastic embryogeny (see p. 14). In Batrachospermum, the carpospore gives rise to a small flattened embryo, from which there arise monosiphonous filaments ; these filaments constitute what is termed the Chantransia-form which reproduces itself by means of gonidia ; from the Chantransia- form, the Batrachosperraum-plant springs as a lateral branch, and, producing roots, becomes independent. In Lemanea the course of development is essentially the same, only that the Chantransia-form does not produce gonidia. The life-history of the Rhodophyceee is generally considered to 272 PART III. THE CLASSIFICATION OF PLANTS. present an alternation of generations ; the plant is the gameto- phyte (either actual or potential) and the cjstocarp is the sporophyte. In Batrachospermum and Lemanea the life-history is complicated by the polymorphism of the gametophyte. The Rhodophycese are almost exclusively marine ; the only fresh-water forms are Batrachospermum, Lemanea, and species of Chantransia, Bangia, and Hildenbrandtia. The sub-class, sometimes also termed FLORTDEE, is subdivided into a number of orders, the limits of which are at present but imperfectly defined, of which the following are the principal : Series I. NEMALIONIN^E. Order 1. Helminthocladiaceae : principal genera, Batracbospermum, Chantransia, Helminthocladia, Nemalion, Scinaia, Helminthora. 2. Lemaneaceae : Lemanea. ,, 3. Gelidiaceae : principal genera, Caulacanthus, Pterocladia, Geli- dium, Wrangelia, Naccaria. Series II. GIGARTININ^:. Order 4. Gigartinaceae : principal genera, Phyllophora, Gigartina, Chon, drus, Iridasa, Gymnogongrus. ,, 5. Khodophyllidaceae : principal genera, Catenella, Bhodophyllis. Series III. KHODYMENIN.E. Order 6. Spbaerococcaceae : principal genera, Gracilaria, Sphaarococcus, Hypnea. ,, 7. Khodymeniaceae : principal genera, Ehodymenia, Lomentaria, Plocamium, Chylocladia, Champia. ,, 8. Delesseriaceae : principal genera, Delesseria, Nitophyllum. ,, 9. Bonnemaisoniaceae : principal genera, Bonnemaisonia, Lepto- phyllis. ,, 10. Bhodomelaceas : principal genera, Polysiphonia, Bbodomela, Khytiphlcea, Chondriopsis, Dasya, Laurencia, Vidalia. 11. Ceramiaceae : principal genera, Ceramium, Spermothamnion, Callithamnion, Griffithsia, Ptilota, Monospora. Series IV. CRYPTONEMIN.E. Order 12. Gloeosiphoniaceae : Glceosiphonia, etc. ,, 13. Grateloupiaceae : principal genera, Grateloupia, Cryptonemia, Halymenia. 14. Dumontiacese : principal genera, Dumontia, Dudresnaya. 15. Nemastomaceae : principal genera, Schizymenia, Nemastoma, Furcellaria. ,, 16. Bhizophyllidaceaa : Poly ides, Khizophyllis. 17. SquamariaceaB : principal genera, Peyssonnelia, Petrocelis, Hildenbrandtia. Order 18. Corallinaceffi : principal genera, Corallina, Melobesia, Lithepbyllum, Litbothamnion. Series V. PORPHYRINJE. Order 19. Bangiaceae: principal genera, Bangia, Porpbyra, Erytbrotricbia. GROUP 1. THALLOPHYTA : FUNGI. 273 CLASS H. FUNGI. This class, like the preceding, includes many very simple organisms, as well as others of tolerably high development. None of them contain chlpEOphyJl ; 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. Some are parasitic, such as the Rusts and Smuts, and absorb these complex carbon-compounds from other living organisms, whether plants or animals. Others are saprophytes, absorbing these compounds from the remains of ^KcTorganisms, or from organic substance formed by living organisms : the numerous 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 destroying, or apparently injuring them. They commonly live with Algae, forming Lichens (see p. 319) ; 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 ccenocytic. In the former case it is small and roundecT 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 nnseptate, as in the Phycomycetes^in which case the body resembles in structure that of the Siphonaceae among the Green Algae (see p. 238). 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 ccenocytes (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. 222). In some of the more complex forms, the hyphae of the repro- v. s. B. T 274 PART 111. THE CLASSIFICATION OF PLANTS. ductive organs form compact masses of tissue of a somewhat parenchymatous appearance, in which there is no differentiation of tissue-systems, but the