VEGETABLE KINGDOM.2 There is one peculiar factor which enters into the problem of the classification of plants and materially acids 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 !ptmetophyte, with an asexual form, the sporophyte (see 1.1limamucTios, 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 ascertained 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 systematic 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 accumulated 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. T HA LLOPHYTA ; II. Ilincopit YTA ; lll. PTEILIDOPHYTA ; IV. PHANEROGAMIA (SPERMA PHYTA).
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 I]ryophyta likewise the gametophyte is the plant, the sporophyte being an appendage on the gametophyte. In the Pteridophyta and in the Phanerevantia 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 Plionerogamia the gametophyte is still further reduced and becomes a mere appendage on the sporophyte.