In this Division a well-marked alternation of generations is to be found. The development of the first or sexual generation (_gametophyte_),[16] which bears the sexual organs, antheridia and archegonia, commences with the germination of the spore, and consists, in the Liverworts, of a thallus, but in the true Mosses of a filamentous protonema, from which the Moss-plant arises as a lateral bud. The second or asexual generation (_sporophyte_), developed from the fertilised oosphere, consists of a sporangium and stalk. =The sexual generation, the gametophyte.= The protonema in the Liverworts is very insignificant, and not always very sharply demarcated from the more highly developed parts of the nutritive system. In the true Mosses the protonema is well-developed, and consists of a branched, alga-like filament of cells, the dividing cell-walls being always placed obliquely. In the parts exposed to the light it is green, but colourless or brownish in those parts which are underground (Fig. 186). The protonema is considered to be a lower form of the stem, and grows in the same manner by means of an apical cell; at its apex it may directly develope into a leaf-bearing stem, or these arise from it as lateral branches (Fig. 186 _k_). The more highly differentiated part of the vegetative system, the “Moss-plant,” which is thus developed from the protonema, is in the “thalloid” Liverworts generally a dichotomously-branched thallus without any trace of leaf-structures (Fig. 194); in _Marchantia_ (Fig. 197) and others, scale-like leaves (_amphigastria_) are found on the under surface. The higher Liverworts and the Leafy-Mosses are differentiated into a filamentous, ramified stem with distinct leaves arranged in a definite manner, resembling the stem and leaves of the higher plants (Figs. 186, 195, 200). _True roots are wanting_, but are biologically replaced by _rhizoids_. These are developed on the stems or thallus: in the Liverworts they are unicellular, but in the Leafy-Mosses generally multicellular and branched. In the latter group they are considered identical with the protonema, and may become true protonema, and new plants may be developed from them (Fig. 186 _b_). [Illustration: FIG. 186.--_A_ Lower portion of a Moss-plant with rhizoids (_r_), one of which bears a reproductive bud (_b_). The dotted line indicates the surface of the ground; the portions projecting above this become green protonema (_p_); _k_ is a young Moss-plant formed on one of these. _B_ Germinating spore of _Funaria hygrometrica_, with exospore still attached. _C_, _D_ Older stages of the protonema.]