All of these, except some Thallophyta, present a more or less clearly marked alternation of generations. 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 inconspicuous and in many cases merely an appendage on the gametophyte; in the Bryophyta likewise the gametophyte is the plant, the sporophyte being an appendage on the gametophyte. In the Pteridophyta and in the Phanerogamia 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 gametophyte is still further reduced and becomes a mere 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 structure, especially by the absence of woody vascular tissue. In those Thallophyta which present an alternation of generations the gametophyte is generally the more conspicuous, 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 I. 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, phycocyanin, 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 Rhodophyceae 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., Gloeocapsa), or it may be a multicellular layer one cell thick (e.g., Merismopedia), or it may be filamentous, consisting of a single row of cells, as in Riυularia, 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., Merismopedia), 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 hormogonia, separate, and, escaping from the mucilaginous matrix, develop 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, zoospores, 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 4. Rivulariaceae. ,, 2. Nostocaceae. ,, 5. Scytonemeae.

„ 3. Oscillariaceae. ,, 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 production 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 Schizophyta. 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 IL 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., Characeae). 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 Oedogonium); 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 Hydrodictyeae, being composed 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 zoospores. 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 process usually takes place between free-swimming ciliated cells (planogametes) ; but in the group of the Conjugates 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 forms, so that the life-history presents an alternation of generations. Thus, in Acetabularia the plant is the sporophyte, producing asexually non-motile spores, which on germination 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 from 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 distinction of base and apex, and then it is attached by the base. Reproduction 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 cellwalls 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 Protococcaceae. Order 2. Volvocineae.— Unicellular or multicellular 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 (uni cellular).

Order 3. Hydrodictyeae.— 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 vesicular and unbranched, or filamentous and branched, assuming most various forms, presenting distinction of base and apex, attached 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 unicellular in some Desmids); cells unior multi-nucleate; body filamentous, branched or unbranched, sometimes presenting distinction 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 oogamous.

a. Isogamous forms—

α. Gametes not free-swimming or ciliated: Conjugates (including Desmidieae, Mesocarpeae, Zygnemeae). β. Gametes free-swimming and ciliated.

Body filamentous, unbranched: Ulothricaceae.

Body filamentous, branched: Cladophoreae, Chaeto-plioreae.

Body a flat or tubular layer of cells: Ulvaceae. b. Oogamous forms: body filamentous—

a. Sexual organs undifferentiated: Sphaeropleae. β. Sexual organs differentiated.

Oedogonieae ; body unbranched (except Bulbochaete); oogonium without trichogyne; sporophyte, a single cell (oospore).

Coleochaeteae ; body branched; oogonium with trichogyne; sporophyte multicellular.

Order 6. Characeae.— Multicellular plants; body presenting differentiation into leaf, stem, and root. There is no asexual production 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 Protococcaceae, the origin of the Volvocineae from these by cell-division can be readily traced through Chlamydomonas, the higher forms being simply motile multicellular Protococcaceae. 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 order—the Cladophoreae —and the Siphoneae is indicated by the multinucleate cells of the former. Thus, Valonia, which is septate, and therefore really belongs 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 various; 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 undifferentiated; 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 multiplication 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 oogamous; 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 connected into filaments or masses by mucilage. Reproduction, vegetative by division or by means of asexually-produced spores (auxospores) ; 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 filamentous and branched or flattened, always presenting differentiation of base and apex, and in some cases more or less well-marked differentiation into root, stem, and leaf, usually attached by the base. Reproduction, vegetative by gemmae or by means of asexually-produced zoospores, or by a sexual process which is essentially isogamous, the gametes being ciliated, but in the higher forms tending 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 cylindrical 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 {gametangia) 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 Cutleriaceae 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 siticulosus 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 Reproduction, 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 process 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 individuals, aud 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 indication of any other kind of polymorphism. In those forms in which the sexual and asexual organs are restricted to separate individuals, 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 oogamous 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.— Multicellular 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...