VEGETABLE KINGDOM. 2 There is one peculiar factor which enters into the problem of the classification of plants and materially adds to its complexity. It is the polymorphism of the individual: that is, the life-history is usually complex, the individual assuming different forms in various stages of its life-history. Thus, in the great majority of plants there is a well-marked alternation of generations,an alternation, that is, of a sexual form, the gametophyte, with an asexual form, the sporophyte (see REPRODUCTION, vol. xx. p. 423). And not only so, but in many cases one or other of these generations presents a number of different forms. Hence the true affinities of any individual cannot be regarded as satisfactorily ascer-tained unless its life-history is fully known; and, since in most cases the various forms are perfectly separate, and often quite dissimilar, there is difficulty in obtaining all the information necessary for determining the true system-atic position of a plant,a difficulty which has not yet been overcome in very many cases.
Comparatively little light is thrown on the affinities of existing plants by the information which has been accumu-lated with regard to the extinct fossil forms. In no case can the genealogy of an existing plant be traced as in the case of the horse among animals (see ANIMAL KINGDOM).
The vegetable kingdom is usually divided into the four following sub-kingdoms:I. THALLOPHYTA; II. BRYO-PHYTA; III. PTERIDOPHYTA; IV. PHANEROGAMIA (SPER-MAPHYTA).
All of these, except some Thallophyta, present a more or less clearly marked alternation of generiations. In all cases the more conspicuous form is considered to be "the plant." Thus, in the Thallophyta generally the plant is the gametophyte, the sporophyte being comparatively in-conspicuous and in many cases merely an appendage on the gametophyte; in the Bryophyta likewise the gameto-phyte is the plant, the sporophyte being an appendage on the gametopbyte. In the Pteridophyta and in the Phan-erogamia the plant is the sporophyte, the gametophyte being comparatively inconspicuous. In the Pteridophyta the gametophyte is still an independent organism; but in ascending from the lower to the higher forms it becomes more and more reduced. In the Phanerogamia the game-tophyte is still further reduced and becomes a inere appendage on the sporophyte.
SUB-KINGDOM. I. THALLOPHYTA.
This sub-kingdom includes the most lowly organized of plants. They are characterized by the total absence, or the imperfection, of that differentiation of the body into root, stem, and leaf which is so marked a feature in the higher plants, and by the simplicity of their internal struc-ture, especially by the absence of woody vascular tissue. In those Thallophyta which present an alternation of generations the gametophyte is generally the more con-spicuous, constituting the plant. The gametophyte is commonly capable of producing spores, not only sexually, but also asexually.
The sub-kingdom is naturally divided into two main classes, the ALGAE or SEA-WEEDS and the FUNGI, to which may be added, as a subsidiary group, the LICHENS. It is becoming usual to regard the Algae and Fungi as distinct sub-kingdoms; but it is preferable, as they have so much in common, to continue to regard them as classes of the sub-kingdom Thallophyta.
CLASS 1. ALGAE.There is so much variety in the form and structure of the Algae (see article ALGAE) that no more precise definition of them can be given than that they are Thallophytes which contain chlorophyll (see PHYSIOLOGY, vol. xix. p. 52). Though they characteristically live in water, this is by no means universally the case, for the natural habitat of many of them is damp soil. All Algae contain chlorophyll, but many of them contain other colouring-matters in addition,a feature which forms a convenient basis for classification. On this basis the Algae, are classified into the four following sub-classes:
I. Cyanophyceae, containing a bluish colouring-matter, phyco-cyanin, and having a blue-green colour.
II. Chlorophyceae, green Algae, containing no colouring-matter, except chlorophyll and its derivatives.
III. Phaeophyceae, containing a yellow or brown colouring-matter, phycophaein, and having a brown colour.
IV. Rhodophyceae, containing a red colouring-matter, phyco-erythrin, and having a red or purple, colour.
These four sub-classes are by no means co-extensive. The Cyanophyceae include, only very simple forms; the Chlorophyceae and the Phaeophyceae include a series of forms from the simplest to the most complex; the Rhodo-phyceae include only forms which, though their vegetative structure is frequently very simple, are comparatively highly developed as regards their reproductive organs.
Sub-Class I. Cyanophyceae, or Blue-Green Algae.The body may consist of a single, more or less nearly spherical cell, as in most of the Chroococcaceae (e.g., Gloeocopsa), or it may be a multi-cellular layer one cell thick (e.g., Merismopedia), or it may be filamentous, consisting of a single row of cells, as in Rivularia, Nostoc, Oscillaria. When the body is filamentous, it sometimes presents a distinction of base and apex, as in Rivularia ; and it is frequently branched, the branching being either spurious (e.g., Scytonema) or true (e.g., Sirosiphon). In most cases growth and cell-division go on in all the cells of the body, but in the Scyto-nemeae and Sirosiphoneae only at the apices. A characteristic feature of the sub-class is the more or less bulky mucilaginous cell-wall which invests the cells and filaments.
Reproduction is mainly effected in a purely vegetative manner. In the unicellular forms each cell-division necessarily leads to the formation of new individuals. In the flattened forms (e.g., Merismo-pedia), when the body reaches a certain limit of size, it simply breaks up into a number of separate portions. In the filamentous forms the body is marked out into segments by inert cells, termed heterocysts, which are quite different in appearance from the living cells of the filament. Eventually these segments, termed hormo-gonia, separate, and, escaping from the mucilaginous matrix, de-velop by growth and cell-division into new individuals. In many cases special reproductive cells, spores, are developed. Each spore is formed from a single cell of the body, which surrounds itself with a thick firm wall of its own. It is probable that ciliated spores, zoopores, are produced, but the existing observations on this point are inconclusive. There is no evidence of the existence of any form of sexual reproduction in this group.
The Cyanophyceae are divided into a number of orders:
Order 1. Chroococcaceae.
Order 2. Nostocaceae.
Order 3. Oscillariaceae
Order 4. Rivulariaceae.
Order 5. Scytonemeae.
Order 6. Sirosiphoneae (Stigonemeae).
It is doubtful to what extent these orders really represent distinct forms ; for there is some evidence that certain Cyanophyceae assume the different forms characteristic of several of these orders at various stages of their development and under various external conditions: in other words, some, at least, of the Cyanophyceae are polymorphic, and this necessarily renders their classification difficult.
The Cyanophyceae; resemble the Schizomycetes. (Fungi) in many respects, as, for instance, in their general structure, in their simple vegetative reproduction and in their spore formation, in the pro-duction of a bulky mucilaginous matrix, and in their polymorphism. On these grounds they are frequently placed along with the Schizomycetes in a distinct class under the name Schizophyla. But this arrangement does not seem to secure any special advantage. It is simpler to regard the Cyanophyceae and the Schizomycetes as parallel groups, the one belonging to the Algae, the other to the Fungi.
Sub-Class II. Chlorophyceae.The body may consist of a single cell (e.g., Protococcoideae), or it may be multinucleate and unseptate (e.g., Siphoneae), or septate and therefore multicellular (e.g., Char-aceae). It presents all degrees of morphological differentiation: thus, it may be spherical (as in Haematococcus and in Volvox), or flattened (as in Ulva), or filamentous (as in Spirogyra, Ulothrix, &c.), with a differentiation of root and shoot (as in (Edogonium) ; or it may even present rudimentary differentiation into stem, leaf, and root, as in the Siphoneae (e.g., Caulerpa) and Characeae. The structure of the body is peculiar in the Hydrodidyeae, being com-posed of a number of originally separate cells; such a body is termed a coenobium.
Vegetative multiplication, though not universal, is not uncommon in this sub-class. Reproduction by means of asexually-produced spores is very general, and the spores are commonly ciliated zoo-spores. A sexual process has been observed in members of every order of this sub-class. It consists either in the fusion of similar sexual cells, when it is said to be isogamous; or a large non motile female cell (oosphere.) is fertilized by a small motile male cell (antherozoid), when it is said to be oogamous. The isogamous pro-cess usually takes place between free-swimming ciliated cells (plano-gametes) ; but in the group of the Conjagatae it takes place between gametes which are not set free into the water and are not ciliated (see REPRODUCTION). In those Chlorophyceae in which both sexual and asexual spore-formation occurs, there is generally a distinction of sexual and asexual forins, so that the life-history presents all alternation of generations. Thus, in Acetabularia the plant is the sporophyte, producing asexually non-motile spores, which on ger-mination give rise to sexual reproductive cells or gametes. Each of these spores represents a sexual form or gametophyte. Again, in Coleochaete the plant is the gametophyte, producing the, sexual organs, the sporophyte being represented by a small flattened plate of cells, developed froin the fertilized female cell or oospore, which never produces any sexual organs, but only spores.
The Chlorophyceae may be classified as follows:
Order 1. Protococcoideae.Unicellular plants; the body frequently spherical and unattached, but presenting in some forms a distinc-tion of base and apex, and then it is attached by the base. Re-production by vegetative division, or by zoospores, or by a sexual process, the sexual cells being similar planogametes. This order may be divided into two families,the Protococcaceae and the Palmellaceae, the distinction being that in the latter the cell-walls are swollen and mucilaginous, so that the cells are held together, whereas in the former the cells are free. Vegetative multiplication by division is universal in the Palmellaceae, but is commonly wanting in the Protococeaceae.
Order 2. Volvocineae.Unicellular or multicellalar plants; body free-swimming by means of cilia, either spherical or a flat plate.
a. Isogamous forms: Pandorina, Stephanosphaera (spherical), Gonium (flattened).
b. Oogamous forms: Volvox, Eudorina, Chlamydomonas (unicellular).
Order 3. Hydrodicteae.Multicellular plants ; body unattached, a net (Hydrodictyon) or a flat plate (Pediastrum), formed by the combination of originally separate cells (a coenobium), The sexual process is isogamous.
Order 4. Siphoneae.Unseptate multinucleate plants ; body vesi-cular and unbranched, or filamentous and branched, assuming most various forms, presenting distinction of base and apex, at-tached by base ; sometimes (Caulerpa) presenting differentiation into root, stem, and leaf. The sexual process is isogamous or oogamous.
a. Isogamous forms: Codieae, Dasycladeae, Caulerpeae, Botrydieae.
b. Oogamous forms: Vaucheriaceae.
Order 5. Confervoideae.Septate multicellular plants (body uni-cellular in some Desmids) ; cells uni- or multi-nucleate; body fila-mentous, branched or unbranched, sometimes presenting distinc-tion of root and shoot, and then attached by the root, or a flat plate or hollow tube of cells. The sexual process is isogamous or oogarnous.
a. Isogamous forms
_. Gametes not free-swimming or ciliated: Conjugatae (including Desmidieae, Mesocarpeae, Zygnemeae).
_. Gametes free-swimming and ciliated.
Body filamentous, unbranched ; Ulothricaceae.
Body filamentous, branched: Cladophoreae, Chaetophoreae.
Body a flat or tubular layer of cells: Ulvaceae.
b. Oogamous.forms: body filamentous
_. Sexual organs undifferentiated: Sphaeropleae.
_. Sexual organs differentiated.
Aedogonieae ; body unbranched (except Eulbochaete); oogonium without trichogyne; sporophyte, a single cell (oospore).
Coleochaeteae ; body branched ; oogonium with trichogyne ; sporophyte multicellular.
Order 6. Characeae.Multicellular plants ; bod. presenting differentiation into leaf, stem, and root, There is no asexual produc-tion of spores; the sexual process is oogamous, the sexual organs being highly differentiated.
The distinction of the genera, and to some extent also that of the orders, is rendered difficult in many cases by polymorphism. For instance, it has been ascertained that many plants which have been referred to the Protococcoideae are simply forms of some of the Confervoideae, Siphoneae, &c. ; and similarly some of the Ulvaceae have Ulothricoid forms, Still, there can be no doubt that each of the above orders includes some autonomous forms.
With regard to the affinities and phylogeny of the orders of Chlorophyceae, the Protoccoideae must be taken as the starting-point from which the other orders have sprung, their evolution having taken place in various directions. Beginning with the simple spherical Protococeaceae, the origin of the Volvocineae from these by cell-division can be readily traced through Chlamydomonas, the higher forms being simply motile multicellular Protococeaceae. Closely allied to the Volvocineae are the Hydrodictyeae. The Siphoneae can be readily traced back to the simplest Protococcaceae through the attached Protococcaceae, such as Characium. The Confervoideae are probably also derived from the Protococcaceae ; and a certain relationship between some members of this orderthe Cladophoreaeand the Siphoneae is indicated by the multinucleate cells of the former. Thus, Valonia, which is septate, and therefore really be-longs to the Confervoideae, closely resembles some of the, Siphoneae in its general habit, The phylogeny of the Characeae is obscure, but it is probable that they have sprung from the Confervoideae, possibly from the Cladophoreae. It is of interest to note that the large cells of the Characeae become multinucleate.
Sub-Class III. Phaeophyceae.The form of the body is very vari-ous; it may consist of a single cell; when multicellular it may be filamentous and branched, or a flattened expansion, or cylindrical or vesicular, hollow or solid. It presents also various degrees of morphological differentiation: in some forms it is quite undifferen-tiated; in others it presents a differentiation of base and apex, and is then attached by the base ; in others it presents indications of differentiation into root, stem, and leaf. Vegetative multipli-cation is common only in the lowest forms; in the higher it occurs in some cases, and is effected by the abstriction of modified members of the parent, termed gemmae. The existence of a sexual process has been ascertained in several forms ; but in many others further investigation is required to determine its presence or absence. In those forms in which it occurs it may be either isogamous or oogam-ous; the isogamous process may take place between free-swimming gametes, or between gametes which are not free-swimming or ciliated. The life-history of the plants of this group is imperfectly known; but it has been ascertained that in some there is, and in others there is not, an alternation of generations.
The Phaeophyceae may be classified as follows:
Order 1. Diatomaceae.Unicellular plants, either free, or con-nected into filaments or masses by mucilage. Reproduction, vege-tative by division or by means of asexually-produced spores (auxo-spores); or sexual isogamous by the fusion of non-ciliated gametes. The cell-wall is impregnated with silica.
Order 2. Syngeneticae.Body unicellular, the cells being held together by mucilage. Reproduction by division and by asexually--produced spores. The order includes the two forms, Chromophyton and Hydrurus. The former is distinguished by being unattached and by the motility of its spores, which have a single long cilium. Hydrurus grows attached, and the spores are not motile.
Order 3. Phaeosporeae.Multicellular plants ; the body is fila-mentous and branched or flattened, always presenting differentiation of base and apex, and in some cases more or less well-marked dif-ferentiation into root, stem, and leaf, usually attached by the base. Reproduction, vegetative by gemmae or by means of asexually-pro-duced zoospores, or by a sexual process which is essentially isogamous, the gametes being ciliated, but in the higher forms tend-ing to become oogamous. The principal families of the Phaeosporeae, are Ectocarpeae, Sphacelarieae, Mesogloeaceae, Desmarestieae, Scyto-siphoneae, Cutleriaceae, and Laminarieae. The filamentous type of structure obtains in the Ectocarpeae, Sphacelarieae, Mesogloeaceae, and Desmarestieae. The filaments may consist of single rows of cells (monosiphonous), as in most Ectocarpeae, or of several rows of cells (polysiphonous), as in the Mesogloeaceae; or they may be polysiphon-ous with a more or less well-developed cortical layer, as in the Sphacelarieae, in which case there is also a large apical cell. The structure of the Desmarestieae (e.g., Desmarestia and Arthrocladia) is similar to that of the Sphacelarieae, but the growing point is not apical, as in the Sphacelarieae, but intercalary, as in the Ectocarpeae. In some of these forms (e.g., Cladostephus among the Sphacelarieae) there is an indication of the differentiation of the shoot into stem and leaf. The type of structure obtaining in the Scytosiphoneae, Cutleriaceae, and Laminarieae is that of a flattened expansion of parenchymatous tissue, though in the Laminarieae there is a certain degree of histological differentiation, as is indicated by the presence of elongated cells forming structures resembling the sieve-tubes of the vascular plants. In some of the Scytosiphoneae there is a large internal cavity, so that the body presents a cylindri-cal or vesicular form. Vegetative reproduction by gemmae is only known in the Sphacelarieae. Reproduction by means of asexually-produced spores (zoospores) is known to occur in the majority of the Phaeosporeae, but it has not yet been detected in some forms (e.g., Arthrocladia, Scytosiphon, Phyllitis, Colpomenia). The asexual organs are unilocular sporangia. The sexual organs (gamet-angia) are multilocular. They have been found in all the families except the Laminarieae, but not yet in all the genera (e.g., Aspero-coccus). The gametangia are all quite similar, except in the Cut-leriaceae and in Tilopteris (a genus allied to Desmarestia), in which also the gametes differ in appearance. A sexual process has only been observed in a few species of Ectocarpeae and Scytosiphoneae (Ectocarpus siliculosus and E. pusillus, Giraudia sphacelarioides, Scytosiphon lomentarius), and in the Cutleriaceae. A full account of the sexual process in these forms is given in the article, REPRO-DUCTION, vol. xx. p. 425.
Order 4. Dictyotaceae.Body multicellular, thalloid, flattened, ribbon-like, as in Dictyota, Phycopteris, Dictyopteris, or broader and fan-shaped, as in Taonia, Padina, Zonaria; usually attached by the base, but by root-hairs developed on the under surface in Zonaria. Asexual reproduction by spores, formed four (tetraspores) in each unilocular sporangium ; spores not motile, Though a sexual pro-cess has not been observed, there are apparently male and female organs, antheridia and oogonia. The supposed male cells are set free, and are not motile; the supposed female cells are not set free. The sporangia and the sexual organs are borne on distinct indi-viduals, and in some genera (e.g., Dictyota, Taonia) the male and female organs are borne on distinct individuals.
Order 5. Fucaceae.Body multicellular; generally differentiated into root and shoot; shoot usually thalloid, cylindrical, or flattened, but differentiated into stem and leaves in Sargassum. No asexual production of spores. Sexual organs antheridia or oogonia. Male cells numerous, set free, ciliated ; female cells (oospheres) either one (Pycnophycus, Himanthalia, Cystoseira, Sargassum), two (Pelvetia), four (Ozothallia), or eight (Fucus) in an oogonium, set free, not motile.
The life-history of the Phaeophyceae is in most cases imperfectly known. In the Fucaceae, since there is no asexual formation of spores, there is no alternation of generations, and there is no indi-cation of any other kind of polymorphism. In those forms in which the sexual and asexual organs are restricted to separate indi-viduals, as in Cladostephus (Sphacelarieae), in the Cutleriaceae, and in the Dictyotaceae, there may be an alternation of generations. In the Cutleriaceae it is, in fact, highly probable. In the one genus, Zanardinia, the, two generations are similar, differing only in the nature of their reproductive organs. In Cutleria, the other genus, the two generations are dissimilar, the form known as Cutleria, being the gametophyte, whereas the sporophyte appears to be a form which has been regarded as a distinct genus, under the name of Aglaozonia. In some genera of Dictyotaceae (Taonia, Padina, Dictyopteris) there is a further indication of polymorphism, in that the spore (whether tetraspore or oospore) produces on germination a spherical or cylindrical protonematoid body, from which spring the shoots of the characteristic form.
Since the life-history of the Phaeophyceae is so imperfectly known, the relations of the various orders cannot be accurately determined. The Diatomaceae and the Syngeneticae are apparently isolated groups. The families of the Phaeosporeae form a series leading from the simple isogamous Ectocarpeae; through the Cutleriaceae to the oogamolas Fucaceae. There are also points of resemblance in the vegetative parts between some of the Scytosiphoneae and the Laminarieae.
Sub-Class IV. Rhodophyceae.This sub-class includes the single order Florideae.
Order Florideae.Multicelltilar plants; body flattened or filamentous ; when filamentous, either monosiphonous or polysiphon-ous, with or without a cortex ; thalloid, or with indications of differentiation into stem and leaf. Asexual reproduction, by means of unciliated naked spores, which are usually tetraspores (wanting in Lemanea), produced in unilocular sporangia. Sexual reproduction by means of male and female organs, termed antheridia and procarpia; the procarpium is generally multicellular, but sometimes unicellular (Bangiaceae, Nemalieae), is always closed, and generally consists of two parts, the carpogonium or sporogenous portion and the trichogyne or receptive portion. The male cells are non-motile and have a cell-wall (spermatia) ; there is no differentiated female cell in the procarpium. After fertilization the carpogenous cell (or cells) divides to form the mother-cells of spores, and each mothercell gives rise to a single naked spore (carpospore), which is not ciliated and is usually non-motile; in the Bangiaceae the carpospores exhibit amoeboid movements for a time. The group of sporogenous cells formed from the fertilized carpogenous cell (or cells) is termed a cystocarp; in many cases these cells become surrounded by an investment developed from the adjacent vegetative tissue. The structure of the cystocarp is very simple in the Bangiaceae, consisting only of eight spore. mother-cells. The principal orders of the Florideae are Bangiaceae, Lemaneaceae, Nemalieae, Ceramiaceae, Corallineae, Rhodomelaceae, Cryptonemiaceae, Rhodymeniaceae, Wrangeliaceae, Squamariaceae, Sphaerococcaceae.
With regard to the life-history of the Florideae, it is generally considered to present a well-marked alternation of generations, the plant being the gametophyte, the fructification (cystocarp) formed in consequence of fertilization being the sporophyte. Further polymorphism has been detected in the genera Lemanea, and Batracho-spermum. Here the carpospore does not at once give rise to the plant, but to a filamentous or flattened body, which in Batracho-spermum reproduces itself by spores and from which the parent form springs as lateral branches. This filamentous form appears to be identical with species of the genus Chantransia ; it is therefore known as the Chantransia-form.
Mutual Affinities and Phylogeny of Sub- Classes of Algae.
Comparison of the four sub-classes shows that, whereas the Chlorophyceae and the Phaeophyceae present two fairly complete series of parallel forms, ranging from unicellular undifferentiated to multi-cellular more or lcss highly differentiated organisms, the Cyanophyceae, consist only of comparatively simple forms in which sexual reproduction is unknown, and the Rhodophyceae of comparatively complex forms in all of which asexual process obtains. Ithasbeen suggested by Cohn, Berthold, and others that there is a close re-lation between the lowest Rhodophyceae (Bangia) and the higher filamentous Cyanophyceae (Scytonemeae, Sirosiphoneae), and that possibly the latter may have been derived from the former. On this view the Cyanophyceae and the Rhodophyceae, taken together, constitute a series which is on a level with those of the Chloro-phyceae and of the Phaeophyceae. On comparing, these three series together, it will be observed that they present a general correspond-ence, in that the main types of form are to be found in all. Thus, the Chroococeaceae among the Cyanophyceae, the Palmellaceae among the Chlorophyceae, and the Syngenetieae among the Phaeophyceae are corresponding groups; similarly, the Corfervoideae among the Chlorophyceae correspond to the Ectocarpeae, and other filament-ous Phaeophyceae, and to various forms of Rhodophyceae, such as Bangia, Porphyra, Griffithsia, and Callithamnion ; and Chara (Chlorophyceae) resembles, at least in its vegetative structure, such forms as Cladostephus (Phaeophyceae) and Polysiphonia (Rhodophy-ceae). There are, however, forms belonging to some of the sub--classes which have no representatives in others. Thus the orders Siphoneae, Volvocineae, Hydrodictyeae, belonging to the Chlorophyceae, are unrepresented in the other sub-classes, and this is true also of the Fucaceae and the Laminarieae among the Phaeophyceae. The Conjugate Chlorophyceae also form a peculiar group, the only corresponding forms being the Diatomaceae (Phaeophyceae), which present many points of resemblance to the Desmidieae.
With regard to the phylogeny of the Algae, the possible derivation of the Rhodophyceae from the Cyanophyceae has already been pointed out. Assuming this, there are three series of Algae closely allied in their origin, in each of which evolution has proceeded on more or less divergent lines, thus leading to the development of a number of corresponding forms in the three series. It is not impossible that the Chlorophyceae and the Phaeophyceae may also have sprung from the Cyanophyceae, not, like the Rhodophyceae, from the higher forms, but from the lower. But, if the Cyanophyceae be regarded, not as primitive, but as degenerate, formsand for this view there are good groundsthen the simpler Chlorophyceae are probably to be regarded as the forms from which the others have been derived.
Literature of Algae.Special papers referred to:Cohn, Beiträge zur Physiologie der Phycochromaceen, 1867; Berthold, "Die Bangiaceen," in Fauna und Flora des Golfes von Neapel, 1882; Zopf, Zur Morphologie der Spaltpflanzen, Lelpsic, 1882; Hansgirg, Physiologische. und Algologische Studien, Prague, 1887. Of general works consultHarvey, Phycologia Britannica; Cooke, British Fresh water Algae, London, 1882; Thuret and Bornet, Notes Algologiques, i, ii, 1882-86, and Études Phycologiques, 1878; Falkenberg, "Algen " in Schenks Handbuch der Botanik, ii., Breslau, 1881-84; Hauck, "Die Meeresalgen," in Rabenhorsts Kryptogamen-Flora, 2d ed., Leipsic, 1885; Goebel, Outlines of Classification and Special Morphology, Oxford, 1886. These works contain references to all the more important isolated papers on the Algae.
CLASS II. FUNGI.In view of the description of these plants given in the article FUNGUS (q.v.), it is unnecessary to define them here further than as Thallophytes which are devoid of chlorophyll.
The classification followed here differs in detail from that given in the previous article. It is as follows:
Sub-Class I. Myxomycetes.
Sub-Class II. Schizomycetes.
Sub-Class III. Phycomycetes.
Order 1. Chytridiaceae.
Order 2. Mucorini.
Order 3. Entomophthoreae.
Order 4. Ustilagineae.
Order 1. Ancylisteae.
Order 2. Peronosporeae.
Order 3. Saprolegnieae.
Sub-Class. IV. Ascomycetes.
Order 1. Gymnoasceae.
Order 2. Pyrenomycetes.
Order 3. Discomycetes.
Sub-Class V. Aecidiomycetes.
Order 1. Uredineae.
Sub-Class VI. Basidiomycetes.
Order 1. Tremellini.
Order 2. Hymenomycetes.
Order 3. Gasteromycetes.
The chief points of difference between this classification and that given in the article FUNGUS, besides the introduction of some addi-tional orders, are the division of the Phycomycetes into Zygomycetes and Oomycetes, the removal of the Ustilagineae from the Uredineae to the Phycomycetes, and the inclusion of the Erysipheae and Tuberaceae in the Pyrenomycetes, this order being, considered as containing all the Ascomycetes which have well-developed, but more or less completely closed, apothecia (perithecia),in a word, all Ascomycetes which are neither Gymnoasceae nor Discomycetes. The division of the Phycomycetes into Zygomycetes and Oomycetes is necessary in order to distinguish those forms which are isogamous, and in which the sexual process is one of conjugation (see REPRODUCTION), from those which are oogamous, and in which the sexual process is one of fertilization, the former constituting the group Zygomycetes, the latter the group Oomycetes. The change in position of the Ustilagineae is made in accordance with the views of recent writers, such as De Bary and Brefeld, the Ustilagineae being regarded as sexually degenerate Zygomycetes.
A brief description of the newly introduced orders, namely, the Chytridiaceae the Entomaphthoreae, the Ancylisteae, and the Gymnoasceae, may be given. The Chytridiaceae are extremely simple Fungi, consisting in some forms of a single spherical cell and in others of a small mycelium. The Entomophthoreae have a well--developed mycelium, which, unlike that of most Phycomycetes, is septate. The Ancylisteae are merely the simplest forms of the Oomycetes. The Gymnoasceae are characterized by the simple struc-ture of their fructification, the asci (or, in some cases, the single ascus) not being surrounded by an investment of sterile iissue, This group includes typical forms, such as Gymnoascus and Eremascus, as well as aberrant forms, such as Exoascus, Saccharomyces, &c.
Mutual Affinities and Phylogeny of Sub-Classes of Fungi.
The Myxomycetes and the Schizomycetes are so peculiar that they cannot be connected in any way with the other sub-classes of Fungi. Beginning, then, with the Zygomycetes, there can be no doubt that the Chytridiaceae are closely connected with the Mucorini by such forms as Polyphagus and Zygochytrium, and with the Ancylisteae among the Oomycetes. On the other hand, they are connected with the Ustilagineae (especially Protomyces) by Clado-chytrium. The Entomophthoreae seem to be most nearly related to the Mucorini. The Ancylisteae are closely related to the Perono-sporeae (especially Pythium), and these again to the Saprolegnieae. Coming now to the Ascomycetes, there is an obvious similarity between the simpler Gymnoasceae and the Mucorini. Thus in Eremascus the sexual organs are quite similar, and the sexual process is one of conjugation, as in the Mucorini ; but there is this difference, that the product of the sexual process in Eremascus is an ascus, whereas in the Mucorini it is a zygospore (see REPRODUCTION). Those Ascomycetes which, like Pyronema, have differentiated sexual organs, show some resemblance to the Oomycetes. The Uredineae appear to be closely connected with the Ascomycetes on the one hand, the aecidium being homologous with the apothecium, and on the other hand with the Tremellini among the Basidiomycetes, through the Tremelloid Uredineae (Leptopuccinieae), which have lost their aecidia, possessing only asexually-produced spores (teleutospores). The Hymenomycetes and the Gasteromycetes appear to form two parallel series starting from the Tremellini.
In attempting to express these relations in terms of phylogeny, the first question which arises is as to whether or not the Chytri-diaceae are to be taken as the primitive forms. It is possible to regard them, not as primitive forms, but as degraded Mucorini, their degradation being due to their aquatic habit; but there are no conclusive grounds for this assumption. Then there is the ques-tion as to the origin of the Ascomycetes. De Bary inclines to the opinion that they are derived from the Oomycetes (especially Peronospora), on account of the similarity of the sexual organs of such Ascomycetes as Podosphaera to those of the Peronosporeae. But how, on this view, is the development of Ascomycetes with similar sexual organs to be accounted for? It seems more reasonable to trace the Ascomycetes back through Eremascus to the Mucorini, and to as-sume that the differeniiation of the sexual organs arose in the Ascomycetous series. The following scheme expresses the phylogeny of the Fungi as suggested in the foregoing remarks.
With the exception of The Basidiomycetes, the sub-classes of Fungi present resemblances to various forms of Algae, as might be expected, for the Fungi must have sprung from the Algae. The Myxomycetes resemble the Hydrodictyeae, in that the body (plas-modium) is a coenobium, formed by the aggregation of originally separate cells, The similarity of the Schizomycetes to the Cyano-phyceae has been already mentioned (p. 125). The Phycomycetes resemble the Siphoneae in their typically unseptate multinucleate structure; but the Mucorini approach the Conjugate Algae, and the Oomycetes the ooganious Siphoneae, in the sexual process. The simpler Chytridiaceae closely resemble the Protococaceae in their general form, as also in the production of ciliated zoospores. The Ascomycetes, especially those in which male cells (spermatia) are formed, and probably also the Aecidiomycetes, resemble the Rhodo-phyceae in many important features. In both groups the female, organ contains no distinctly differentiated female cell (oosphere); and the effect of fertilization is to cause the female organ to grow into a sporogenous fructification (apothecium, cystocarp), which con-stitutes the asexual generation or sporophyte in the life-history.
With regard to the derivation of the Fungi from the Algae, it appears that there are at least two distinct origins. The Schizomy-cetes doubtless arose from the Cyanophyceae; but, as pointed out above, the Schizomycetes cannot be regarded as having given rise to higher forms of Fungi. These sprang independently from the Algae; and probably the Chytridiaceae were derived from the Proto-coccaceae; and from the Chytridiaceae, the higher forms were developed as indicated in the above sketch of the phylogeny of the Fungi, the. evolution of the Fungi proceeding along much the same lines as that of the Algae, and thus giving rise to forms which have their representatives in the Algae, and terminating in the Basidiomycetes, which are altogether and peculiarly Fungal. The origin of the Chytridiaceae from the Protococcaceae is suggested by the fact that many of the latter are "endophytic": that is, they inhabit the tissues of higher plants ; and the Chytridiaceae may be regarded as Protococcaceae which have become truly parasitic, and have consequently lost their chlorophyll.
Additional Literature to Fungi.De Bary and Woronin, Beiträge zur Mor-phologie und Physiologie der Pilze, 4th and 5th series, 1881-82; Brefeld, Un-tersuchungen, parts v.-vii., 1883-88; De Bary, Comparative Morphology and Biology of the Fungi, Oxford, 1887.
SUBSIDIARY GROUP, LICHENES.As pointed out in the article FUNGUS, a Lichen is a compound organism consist-ing of a Fungus and an Alga living symbiotically. In that article only those Lichens are considered in which the Fungus belongs to the Ascomycetes; but Lichens are now known in which the Fungus belongs to the Basidiomycetes.
The Lichens may be classified as follows:
-Ascolichenes (Ascomycetous Lichens).
1. Discolichenes (Discomycetous Lichens).
2. Pyrenolichenes (Pyrenomycetous Lichens).
Basidiolichenes (Basidiomycetous Lichens).
1. Hymenolichenes (Hymenomycetous Lichens).
2. Gasterolichenes (Gasteromycetous Lichens).
Literature.Massee, " On Gasterolichenes," in Phil. Trans., vol. cixxviii., 1887.
SUB-KINGDOM II. BRYOPHYTA (Muscineae).
The Bryophyta may be characterized as plants which pre-sent a definite alternation of generations, the plant being the gametophyte and the fructification or sporogonium the sporophyte. The sporophyte is not independent, but remains permanently attached to the gametophyte. The shoot of the gametophyte is sometimes thalloid; but more frequently it is differentiated into stem and leaf. The shoot of the sporophyte is not differentiated into stem and leaf, though there is in some cases an indication of such differentiation. The gametophyte commonly reproduces its like by means of gemmae; the female organ is an arche-gonium. The Bryophyta are divided into two classes-the HEPATICAE or LIVERWORTS and the MUSCI or MOSSES. (For details see MUSCINEAE.)
Mutual Affinities and Phylogeny of Mosses and Liverworts.
In consequence of the well-marked alternation of generations in these classes, it is essential to trace their resemblances in both generations.
a. Gametophyte (plant).The higher Liverworts (foliose Jungermannieae) resemble the Mosses in that the shoot is differentiated into stein and leaf; but there is this general difference, that the shoot of those Liverworts has dorsi-ventral symmetry, whereas that of the Mosses has radial symmetry. The connecting form is afforded by Haplomitrium, which alone among the foliose Liverworts has radial symmetry.
b. Sporophyte (sporogonium).The main differences between the sporogonium of the Liverworts and that of the Mosses are these: the structure of the sporogonium is simpler in the Liverworts than it is in the Mosses; in the former it usually has no columella and produces elaters, whereas in the latter a columella is always present, and there are no elaters. In the Liverworts the sporogonium re-mains enclosed in the enlarged venter of the archegonium (calyptra) until the spores are ripe, but in most Mosses the developing sporo-gonium bursts the calyptra at an early stage. A connecting form is afforded by the sporogonium of Anthoceros (Liverwort), which has a columella and bears stomata like the sporogonia of the Mosses, and in which the elaters are rudimentary. Sphagnum and Archi-dium among Mosses resemble the Liverworts in that the sporo-gonium is enclosed in the calyptra until near maturity.
Although there are so many points of resemblance between them, there is no evidence to show that the Mosses and Liverworts form one continuous series. This might perhaps be assumed if the gametophyte alone were considered; but it cannot when the sporo-phyte is taken into account as well. The more important features in the comparative morphology of the sporophyte are as follows. Taking first the Liverworts, in the Riccieae proper (Riccia, Oxymitra) both the epibasal and hypobasal halves of the oospore (fertilized female cell) form sporogenous tissue. In the allied Corsinieae, the Marchantieae, and the Anthoceroteae the epibasal half alone gives rise to sporogenous tissue, the hypobasal half forming an organ of absorption and attachment which is the true foot. In the Jungermannieae, likewise, the sporogenous tissue is developed entirely from the epibasal half of the oospore, which also gives rise to the seta or stalk of the sporogonium ; but the hypobasal half is aborted and gives rise to only a rudimentary foot; in some foliose forms a false foot is formed by a dilatation of the base of the seta. In the Mosses there is not in any family such a simple sporophyte as that of the Riccieae among the Liverworts; but there are forms which present the same morphological difference as that which in the Liverworts distinguishes the Marchantieae and the Anthoceroteae, on the one hand, from these Jungermannieae, on the other. Thus, among Mosses the Sphagnaceae, Andreaeaceae, and Phascaceae have a true foot, whereas in the Bryineae there is a spurious foot at the base of the seta, as in some Jungermannieae. Taking all the foregoing facts into account, it is clear that the Riccieae are the most primi-tive of existing Muscineae, and it is probably from them that the true. Mosses, as well as the other Liverworts, have been derived.
The following phylogeny may be suggested. As regards the Liverworts, there can be no doubt that the Marchantieae are to be traced back through the Corsinieae to the Riccieae, and it is probable that, as Leitgeb points out, the frondose Jungermannieae were de-rived from the Corsinieae through the Rielleae. With regard to the Anthoceroteae, Leitgeb seems to suggest that they may have been derived through Notothylas from the Jungermannieae ; but in opposi-tion to this must be set the difference between the two groups as regards the morphology of the foot. Probably the Anthoceroteae arose independently from the Corsinieae. The Mosses are to be traced back to the Anthoccroteae, on account of the general similarity in the structure of the sporogonium. The genus Anthoceros shows its special relation to the Sphagnaceae in the development of the sporogenous cells from the amphithecium, whereas the genus Noto-thylas resembles the other Mosses in that the sporogenous cells are developed from the endothecium. The following scheme will serve to illustrate these remarks.
The phylogeny of the Muscineae can be traced with exceptional clearness in both generations. With regard to the gametophyte, it must be remembered (see MUSCINEAE) that at its first develop-ment from the spore it is a protonema, which is either filamentous, as usually in Mosses, or a flattened expansion, as generally in Liver-worts, the shoot which bears the sexual organs being a secondary development on the protonerna. Both these forms of protonemia bear a striking resemblance to various Coufervoid Chlorophyceae, and there can be little doubt that it is from this group that the Muscineae have arisen, This view is supported by the close resem-blance of the simple sporophyte of Riccia among the Muscineae to that of Coleochaete among the Confervoid Chlorophyceae. There is, however, this difference, that in Riccia the cells resulting from the division of the oospore are differentiated into an external sterile layer, forming the wall of the capsule, and an internal mass of sporo-genous cells, whereas in Coleochaete all the cells are sporogenous.
SUB-KINGDOM III. PTERIDOPHYTA
In these plants the gametophyte is a filamentous or tuberous, or more commonly a membranous, flattened pro-thallium, exhibiting no differentiation into stem and leaf. The sexual organs are antheridia and archegonia, as in the Muscineae. The sporophyte becomes quite independent of the gametophyte, constituting the plant. The shoot is always differentiated into stem and leaf, and the tissues are highly differentiated.
The sub-kingdom is divided into three classes,FILICINAE, EQUISETINAE, LYCOPODINAE.
Each of these classes includes two series of forms,the homospor-ous, in which all the spores are alike, and the heterosporous, in which the spores are of two kinds, namely, large spores (macrospores) and small spores (microspores). On germination the spore of the homo-sporous forms gives rise to a well-developed prothallium, which be-comes quite free from the spore, and usually bears both antheridia (male) and archegonia (female). The spores of the heterosporous forms give rise on germination to comparatively inconspicuous pro-thallia, which remain connected with the spores, and bear either exclusively male or exclusively female organs. Thus, in the hetero-sporous forms the gametophyte is represented by a male and a female organism, the former being much the smaller. The male prothal-lium is always developed from a microspore, the female prothallium from a macrospore (See REPRODUCTION).
CLASS 1. FILICINAE.The Filicinae are characterized by having relatively large and few leaves. The spore-bearing or fertile leaves (sporophylls) are generally similar to the foliage leaves, and are not aggregated on special shoots, so as to form flowers. The embryo sporophyte has no suspensor; but it has generally (except in Salvinia) a primary root.
Series A.. Homosporous Filicinae.This series consists of the sub-class Filices, which includes the Ferns. It seems to be certain that all existing Ferns are homosporous, and apparently all the known fossil forms are homosporous also, though it is not impossible that there may have been heterosporous Filices. The orders of exisd Ferns are arranged in two categories, according to the mode of development of the sporangia.
Leptosporangiate Filices: the sporangium is developed from a single epidermal cell:
Order 1. Hymenophyllaceae.
Order 2. Cyatheaceae.
Order 3. Polypodiaceae.
Order 4. Gleicheniceae.
Order 5. Schizaeaceae.
Order 6. Osmundaceae.
Eusporangiate Filices: the sporangium is developed from a group of epidermal cells:
Order 1. Ophioglossaceae.
Order 2. Marattiaceae.
Series B. Heterosporous Filicinae.As far as is known at present, this series contains the sub-class Hydropterideae or Rhizo-carpae. In addition to the various peculiarities connected with their heterospory, its members are characterized by the development of an investment round the clusters of sporangia (sori), the whole forming a sporocarp. They are all Leptosporangiate.
The Hydropterideae are arranged in two orders, according to the structure of the sporocarp.
Order 1. Salviniaceae: wall of sporocarp formed as a superficial outgrowth of the sporophyll; each sporocarp contains a single sorus either of macrosporangia or of microsporangia (Salvinia, Azolla).
Order 2. Marsileaceae: the wall of the sporocarp is formed by a portion of the sporophyll ; each sporocarp contains several sori ; and each sorus includes both macrosporangia and microsporangia (Marsilea, Pilularia).
It is possible that Isoetes represents the heterosporous euspor-angiate Filices (see infra).
CLASS 2. EQUISETINAE.These plants are characterized by their rudimentary foliage leaves arranged in whorls at the nodes, and by their highly modified sporophylls, which are aggregated together at the ends of shoots, so as to form cone-like flowers. The embryo sporophyte has no suspensor, but a primary root.
Series A. Homosporous Equisetinae.All the existing forms belong to this series and constitute the order Equisetaceae; they all belong to the genus Equisetum. The gametophyte is a green flattened prothallium, like that most commonly found in the Filices; but it is much branched, and generally dioecious.
Series B. Heterosporous Equisetinae.There are no existing heterosporous Equisetinae ; it is probable that some fossil plants, such as Annularia and Asterophyllites, represent the extinct hetero-sporous forms.
CLASS 3. LYCOPODINAE.The Lycopodinae are generally charac-terized by their small and numerous foliage leaves, which are not arranged in whorls, though they are almost entirely wanting in Psilotum. The sporophylls are not highly modified, but they are frequently aggregated at the ends of shoots, so as to form cone-like flowers. So far is the embryology of this class is known at present, the embryo sporophyte has a suspensor, but no primary root.
Series A. Homosporous Lycopodinae.
Order 1. Lycopodiaceae: the sporangia are borne on sporophylls (Lycopodium, Phylloglossum).
Order 2. Psilotaceae: the sporangia are borne on the stem (Psilo-tum, Tmesipteris).
Series B. Heterosporous Lycopodinae.
Order 1. Selaginellaceae (Selaginella).
The existing heterosporous Lycopodinae are usually considered to consist of a family, termed the Ligulatae, which includes the two genera Selaginella and Isoetes. This arrangement is not accepted here, for the reason that Selaginella and Isoetes have nothing in common beyond the ligule, and the fact that they are both heterosporous vascular cryptogams. It is in fact a question whether or not Isoetes should be included in the Lycopodinae at all ; for it has few and large leaves, and the embryo sporophyte has no suspensor, but it has a primary root. This being the case, it is impossible, to frame any definition of the group Lycopodinae which would apply to all the members, if Isoetes be included in it. In many features Isoetes resembles the Filicinae, and especially the Hydropterideae. It may be suggested here that Isoetes is a representative of the heterosporcus eusporangiate Filicinae. The only member of the Lycopodinae which Isoetes, at all resembles is Phylloglossum.
Mutual Affinities and Phylogeny of Pteridophyta.
Beyond the characteristics which they possess in common, and which have caused them to be collected into one class, there are no special points of contact between the three sub-classes of the Pteridophyta. The classes have each its own well-marked habit; and it therefore appears probable that they are to be regarded as equivalent branches of the Pteridophyta, springing from a common origin, in each of which a differentilition of homosporous and heterosporous forms has taken place. The relation of the homosporous to the heterosporous forms in each class is a point of interest. There is little or nothing to be said on this subject as regards the Equisetinae. But in the Filicinae it appears probable that the simpler heterosporous forms have sprung from the simpler homosporous forms, that is, the Salviniaceae from the Hymenophyllaceae; and in the Lycopodinae; that Selaginella has sprung from Lycopodium. Assuming that Isoetes belongs to the Lycopodinae, its nearest ally would appear to be Phylloglossum; but this point must remain undecided until the whole life-history, especially the embryogeny, of Phylloglossum is known, and its true systematic position thereby fixed.
Various attempts have been made to trace back the sporophyte of the Pteridophyta to that of the Muscineae, the points of contact being the Hymenophyllaceae on the one hand and Anthoceros on the other; but these attempts cannot be considered to have been successful. The differences between the known forms of sporophyte in the two classes are too great to be explained away on any hypothesis. The case is different with regard to the gametophyte; there are, in fact, many points of resemblance between the prothallium of the Pteridophyta and the gametophyte of the Muscineae. As Goebel and Bower have pointed out, the prothallium of some species of Trichomanes is a branched filamentous structure, closely resembling Moss protonema; and in most ferns, as also in Equisetum and Lycopodium, the prothallium. is at first filamentous. The filamentous prothallium gives rise to one or more flattened expansions, on which the sexual organs, or at least the archegonia, are developed (archegoniophores). Thus there are indications in the gametophyte of many Pteridophyta of that distinction of protonema and sexual shoot which is so characteristic of the gametophyte of the Muscineae. But this remarkable correspondence in the gametophyte does not warrant the conclusion that the Pteridophyta have sprung from the Muscineae. It rather indicates that they are groups having a common origin in the Confervoid Chlorophyceae, but diverging from the first in the relation of the sporophyte to the gametophyte, so that, whereas in the Muscineae the sporophyte remains attached to the gametophyte throughout its whole existenee, in the Pteridophyta the sporophyte develops as an independent organism. There can be little doubt that in the primitive Pteridophyta the two generations resembled each other, and that, as evolution proceeded, they became more and more widely dissimilar, the gametophyte losing, the sporophyte gaining, in both morphological and histological differentiation. An indication of this is still afforded by the general resemblance of the sporophyte of Phylloglossum to the gametophyte of Lycopodium cernnum as described by Treub, and of the sporophyte of Lycopodium Phlegmaria to its gametophyte (Treub). It will probably be found, when material is obtained for investigating it, that the sporophyte.and the gametophyte of Phylloglossum are very similar.
The phylogeny of the Pteridophyta, as suggested in the foregoing remarks, is indicated in the following table:
Literature of Pteridophyta.Affinities and phylogeny:Prantl, Untersuchungen zur Morphologie der Gefässkryptogamen, i.-ii., Leipsic, 1875-81; Goebel, Annales du Jardin Botanique de Buitenzorg, vii., 1887; Bower, "On some Normal and Abnormal Developments of the Oophyte of Trichomanes," in Annals of Botany, vol. i., 1888; Id., On the Morphology of Phylloglossum Drummondii, in Phil., Trans., part II., 1885 ; Treub, "Recherches sur les Lycopodiacées," in Ann. du Jardin Botanique de Buitenzorg, iii.-iv., 1884-86; Id., Annals of Botany, i., 1888; Kny, "Die Entwickelung der Parkeriaceen," in Nova Acta d. k. Leop. Akad., xxxvii., 1875. For general works, see Goebel, Outlines of Classification and Special Morphology, Oxford, 1887; Sadebeck, "Die Gefässkryptogamen," in Schenks Handbuck der Botunik, i., 1879.
SUB-KINGDOM IV. PHANEROGAMIA.
These plants are commonly known as "flowering plants"; but they are more correctly designated "seed-bearing plants," or Spermaphyta, for the production of a seed is the one feature which distinguishes the members of this sub-kingdom from those of the other sub-kingdoms.
There is a definite alternation of generations in the life-history of these plants, but it is obscured by the extreme reduction which the gametophyte has undergone (see REPRODUCTION,). The sporophyte is the plant; a suspensor is formed in its embryogeny; and it is heterosporous. The sporophylls are aggregated on special shoots, frequently with other floral leaves (bracteoles, perianth leaves), to form flowers. The two kinds of sporangia are in all cases borne on distinct sporophylls. In the microsporangium there are numerous fertile sporogenous cells, each of which produces four microspores; in the macrosporangium there are generally but few sporogenous cells, of which only one is usually fertile, and this one produces a single macrospore without division, The microspores are set free from the sporangium producing them, whereas the macrospore is not. It is this last peculiarity which determines the formation of the seed.
In the above account of the asexual reproductive organs of the Phanerogamia, the terms employed are those which are applied in the case of the heterosporous Pteridophyta; the advantage gained by the use of these terms is that the true homologies of the reproductive organs in the two sub-kingdoms are kept in view. The relation between these terms and those more commonly in use is as follows:stamen = leaf bearing microsporangia (microsporophyll); carpel = leaf bearing macrosporangia (macrosporophyll); pollen-sac = microsporangium ; anther = sorus of microsporangia; ovule (nucellus) = macrosporangium; pollen-grain = microspore ; embryo sac = macrospore. Stamens and carpels are commonly borne by the same individual and in the same flower ; but in many cases they are confined to distinct individuals (dioecious) or to distinct flowers (diclinous) on the same individual.
Inasmuch as these plants are heterosporous, the sexual generation is represented by male and female gametophytes. The male gametophyte, formed by the germination of the microspore (pollen-grain), consists of two or sometimes three or four cells, one of which becomes the male organ (pollen tube = antheridium of other sub-kingdoms), while the rest have nothing to do with reproduction, but constitute the rudimentary vegetative portion of the prothallium. The female gametophyte, commonly known as the endosperm (=female prothallium), is formed in the interior of the macrospore or embryo sac, and usually does not come at all into relation with the exterior. From certain cells of this endosperm one or more female organs are formed, which are equivalent to, and in some Phanerogams closely resemble, the archegonia of the Pteridophyta ; each of these female organs possesses, as its essential constituent, the female cell (oosphere, ovum).
The process of fertilization (see REPRODUCTION) depends upon the pollen-grains being brought near to the ovules, so that, when the pollen-grains germinate, the pollen tubes may be able, in the course of their growth, to come into close relation with the female cells which the ovules contain. In many cases the transference of the pollen-grains is effected by the wind ; the plants in which this is the case are, characterized by their inconspicuous flowers, and are termed anemephilous. In other cases the transference is effected by means of insects; the plants in which this is the case are termed entomcphilous, and are characterized by their conspicuous flowers, in which the perialith leaves are highly developed and brightly coloured, to attract the visits of insects,an end which is further attained by the secretion of scented volatile oils and of sugary liquid (iiectar). It is the conspicuous flowers of these entomophilous Phanerogamia which have earned for the whole group the designation of "flowering plants," which, however, is by no means generally applicable.
The Phanerogamia are divided into two classes,the GYMNOSPERMAE and the ANGIOSPERMAE.
CLASS I. GYMNOSPERMAE.The carpels are sometimes absent, and, when present, they do not form an ovary; hence the ovules are naked. There is no stigma, consenquently the pollen-grain comes into direct contact with the ovule. The whole of the female prothallium is formed in the embryo sac before fertilization. The female organ is in most cases a fully developed archegonium. The embryo is developed from a portion only of the fertilized female cell or oospore (meroblastic embryogeny); frequently several embryos are developed from one oospore (polyembryony). The flowers are dioecious or diclinous.
This class consists of the following orders:
Order 1. Cycadaceae: characterized generally by the unbranched stem and by the large branched leaves. The order includes nine genera:Cycas, Dioon, Ceratozamia, Zamia, Macrozamia, Encephalarlos, Stangeria, Bowenia, and Microcycas. The genus Cycas is peculiar, in that the carpels are not borne on special shoots, so as to form flowers, but on the main sboot, in the same way as the foliage leaves. The stamens of Cycas, as also both the stamens and carpels in the other genera, are highly modified sporophylls which are borne on special shoots forming cone-like flowers.
Order 2. Coniferae: characterized by the much-branched stem and by the very numerous, small, unbranched leaves. The staminal flowers are generally cone-like. This order includes the Pines, Firs, Larches, Yews, Cypresses, &c., arranged in the following families:
Series A. Pinoideae: the carpellary flowers are cones:-
Fam. 1. Abietineae.
Fam. 2. Araucarieae.
Fam. 3. Cupressineae.
Fam. 4. Taxodineae.
Series B. Taxoideae; the carpellary flowers are not cones:-
Fam. 1. Taxineae.
Fam. 2. Podocarpeae.
Order 3. Gnetaceae: habit various; the flowers are not cones, and have a perianth. This order includes the three genera Ephedra, Gnetum, and Welwitschia. In Gnetum and Welwitschia the female organ (archegonium) is reduced to a single cell, the oosphere.
Mutual Affinities and Phylogeny of Gymnospermae.
There is but little resemblance between the Cycadaceae and the other two orders. Gingko or Salisburia, a remarkable genus which is usually placed in the Taxineae, though it is in many respects peculiar, resembles the Cycadaceae in the structure of the arche-gonium, in the fleshy character assumed by the outer portion of the seed-coat, and in the fact that the development of the embryo does not begin until after the seed has been shed. Probably the fossil plants known as the Cordaitaceae may be regarded as a group intermediate between the Cycadaceae and the Coniferae. The Coniferae and the Gnetaceae are more closely allied, so much so in fact that the latter have been regarded by various writers as a family of the Coniferae. The structure of the flowers in Ephedra is very similar to that in Taxus.
The connexion of the Gymnosperms with the Pteridophyta is un-doubted. The development of the microsporangima in the Gymno-sperms is the same as that of the sporangium. of Lycopodium. The development of the macrosporangium presents in most cases the characteristic Phanerogamic features; but in certain cases (Ceratozamia longifolia, Cupressus sempervirens, Callitris quadri-valvis) there is a mass of sporogenous cells, as in the sporangia of the Pleridophyta, though only one of these cells is fertile, and it produces a single macrospore without division. In fact, there is a marked general resemblance in the development of the macrospor-angium between these species and Isoetes. Another point of importance is the fact that, although the female prothallium (endosperm) is under ordinary conditions entirely and permanently enclosed in the macrospore (embryo sac)a feature which is charac-teristic of the Phanerogamia---yet in the Cycadaceae, if the first--formed archegonia are not fertilized, the prothallium resumes its growth and bursts the macrospore, coming to the surface of the ovule and turning green in consequence of exposure to light. Tak-ing into consideration their general habit and the peculiarities in development of their sporangia, it appears that the Cycadaceae find in the Eusporangiata Filices their nearest allies among the Pteridophyta. This suggestion presupposes the existence of hetero-sporous Eusporangiate, although all the known forms are homo-sporous. But this difficulty will be met, if, as suggested above, it turns out that Isoetes really belongs to the Eusporangiate Filices. The Coniferae resemble more nearly the Lycopodinae, such as Lyco-podium, and Selaginella, in their general habit and in the cone-like form of their flowers. However, the position of the pollen-sacs on the under Surface of the peltate sporophyll is a feature which connects them with the Equisetinae; but, on the other hand, the ligular outgrowth on the upper surface of the macrosporophyll in Araucaria, which in the Abietineae becomes the large ovuliferous scale, is another point of resemblance to Selaginella and Isoetes. In view of these resemblances to the Lycopodinae and the Equisetinae, it is hardly possible to trace back the Coniferae to either of these groups: it is more probable that they sprang from forms of Pterido-phyta intermediate in character between the Lycopodinae and the Equisetinae. The mutual relations and the phylogeny of the three genera of Gnetaceae cannot at present be determined with any degree of probability. The one point which affords any clue is the resemblance in general structure between the flowers of Gnetum and those of the Taxineae. The flower of Welwitschia, more than that of any other Gymnosperm, resembles that of an Angiosperm, especially in the fact that the staminal flower contains a rudi-mentary ovule.
CLASS II. ANGIOSPERMAE.The carpel or carpels form an ovary, in which the ovules are enclosed. A portion of the carpel forms a stigma, which receives the pollen-grains. The pollen-grains germinate on the stigma, and therefore do not come into direct relation with the ovules. A part only of the female prothallium (primary endosperm) is formed before fertilization, the remainder (secondary endosperm) after fertilization. The female organ is a reduced archegonium, consisting merely of the female cell (oosphere). The general occurrence of both macrosporophylls and micro-sporophylls in the same flower is a characteristic feature of this group, as is also the whorled arrangement of the floral leaves.
The Angiospermae are divided into two sub-classes.the Mono-cotyledons and the Dicotyledons, according to the number of the primary leaves or cotyledons of the embryo ; hence, in the former sub-class the first leaves are alternate, in the latter opposite. In the Monocotyledons, with few exceptions, the growing-point of the embryonic stem is lateral; in the Dicotyledons it is always terminal. There are other distinguishing features, such as the parallel vena-tion of the leaves of Monocotyledons and the reticulate venation in Dicotyledons ; but they are not sufficiently constant to be of much taxonomic value.
The Monocotyledons may be conveniently arranged in the following series:
Series 1. Nudiflorae: usually diclinous or dioecious; perialith wanting or rudimentary. This series includes the Spadiciflorae (Aroids, Palms, Naiadeae) and the Glumiflorae (Grasses, Sedges).
Series 2. Petaloideae: usually monoclinous ; perialith present, and usually well-developed and coloured. This series includes the Lilies, Orchids, Irises, &c.
The Dicotyledons are usually classified as follows:
Series 1. Monochlamydeae: usually diclinous or dioecious ; perianth absent or simple.
Series 2. Polypetalae: usually monoclinous; perianth usually con-sists of calyx and coloured corolla, the petals being free.
Series 3. Gamopetalae: usually monoclinous ; perianth usually consists of calyx and coloured corolla, the petals being coherent.
Phylogeny of the Angiospermae.
There can be no doubt that the Angiogermae have been derived from the Pteridophyta; but it is a question whether they have had an independent origin from that group or whether they are to be traced back to it through the Gymnosperms. In view of the wide gulf which separates the Angiosperms from even the highest Pteri-dophyta, and of the affinities of the Gymnosperms to the Pteri-dophyta on the one hand and to the Angiosperms on the other, the latter suggestion would appear to be the more probable.
Although the Monocotyledons and the Dicotyledons have so many features in common, it is probable that they have not had the same origin. The Gymnospermous forms to which the Dicoty-ledons are most nearly allied are the Coniferae, and Gnetum arnong the Gnetaceae. The marked Angiospermous characters of Gnetum, and its general similarity in habit to a Dicotyledonous plant, afford some ground for regarding this genus as the starting-point of the Dicotyledons. But more probably the Dicotyledons are to be traced back to the Coniferae, and the Gnetaceae to be regarded as a lateral offshoot of the Coniferm, with an Angiospermous tendency, but not leading on to higher forms. The Gymnosperms which most re-semble the Monocotyledons are the Cycadaceae; and, if it be assumed that the Angiosperms have sprung from the Gymnosperms, it is to this order of the latter class that the Monocotyledons are to be traced. The Monocotyledons in which this resemblance is most conspicuous are those, such as the Aroids and Palms, which belong to the series Nudiflorae.
Literature.On the Phanerogamia, see Goebel, Outlines of Classification and Special Morphology, Oxford, 1887; Le Maout and Decaisne, Traité de Botanique (English ed. by Hooker, 1868); and Engler and Pranti, Die Pflanzenfamilien, Leipsic (in course of publication).
On the phylogeny of plants, see Nägeli, Theorie d. Abstammungslehre, Munich, 1884; and Saporta and Marion, LÉvalution du Règne Vègètal (Int. Sci. Series, vols. xxxix. and lii.), Paris, 1881-85. Descriptions of the numerous natural orders will be found in the various text-books which treat of systematic botany, as well as in the above. (S. H. V.)
FOOTNOTE (page 124)
2 The problem of classification beng essentially the same in the animal and vegetable kingdoms, the reader is refeered to the general discussion of it in the article ANIMAL KINGDOM.
The above article was written by Sydney Howard Vines, M.A., D.Sc., F.R.S.; President of the Linnaean Society of London; Sherwood Professor of Botany, Oxford; Fellow of Magdalen College; Fellow and Lecturer of Christ's College, Cambridge, 1876; Reader in Botany, Cambridge, 1883; author of Lectures on the Physiology of Plants, A Student's Text-book of Botany, papers in various scientific journals, etc.