VEGETABLE KINGDOM 125 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 yametophyte, 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 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 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 gametophyte. In the Pteridophyta and in the Plian- 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 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 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 ALG.-E or SEA- WEEDS and the FUNGI, to which may be added, as a subsidiary group, the LICHENS. It is becoming usual to regard the Alyx 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. ALG^:. There is so much variety in the form and structure of the Algx (see article ALG.E) 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 Al>/;v 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 Algx are classified into the four following sub-classes : I. Cyanophycese, containing a bluish colouring-matter, fhyco- cyanin, and having a blue-green colour. II. Chlorophycese, green Algse, containing no colouring-matter, except chlorophyll and its derivatives. III. Phseophyceae, containing a yellow or brown colouring-matter, phycopbasin, and having a brown colour. IV. Rhodophycese, containing a red colouring -matter, phyco- ery thrin, and having a red or purple colour. These four sub-classes are by no means co-extensive. The Cyanophycese, include only very simple forms ; the Chlorophycese, and the Phxophycex include a series of forms from the simplest to the most complex ; the Rhodo- phycese, include only forms which, though their vegetative structure is frequently very simple, are comparatively highly developed as regards their reproductive organs. Sub-Class I. Cyanophycese, or Blue-Green Algas. The body may consist of a single, more or less nearly spherical cell, as in most of the Chroococcaceai (e.g., Glceoccipsa), 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- nemcas 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- pcdia), 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, zoosporcs, 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 Cyanophycesa are divided into a number of orders : Order 1. Chroococcaccx. Order 4. Rivulariacese-. ,, 2. Nostocaccx. ,, 5. Scytoncmese. ,, 3. Oscillariacees. ,, 6. Sirosiphoneae. (Stigo-ncmese). It is doubtful to what extent these orders really represent dis tinct forms ; for there is some evidence that certain Cyanophyccx 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 Cyanophyceee are polymorphic, and this necessarily renders their classification difficult. The CyanoplnjcciK 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 Schizo- 3S in a distinct class under the name Schizophyta. Hut this arrangement does not seem to secure any special advantage. It is simpler to regard the Cyanophycese and the Schizomycetes as parallel groups, the one belonging to the Algx, the other to the Fungi. Sub-Class II. Chlorophycese. The body may consist of a single cell (e.g., Protococco ideas), or it may be multinueleate and unseptate flattened (as in Ulvo), or filamentous (as in Spirogyra, Utolhrix, 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 Sij honcec (e.g., Caulcrpa) and C/iaraccas. The structure of the body is peculiar in the Hydrodidycie, 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 asexualry-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