220 PART III. THE CLASSIFICATION OF PLANTS. 



GROUP I. 

 THALLOPHYTA. 



THIS group includes the more lowly-organised plants. As already 

 mentioned, the alternation of generations is here either irregular or 

 wanting. When the alternation of generations is irregular, the 

 irregularity is mainly due to the fact that the gametophyte is 

 capable of reproducing itself, it may be through several successive 

 generations, by means of asexually-produced reproductive cells 

 (gonidia ; see p. 3) : this production of gonidia by the gametophyte 

 does not occur in any other group of plants, and in this group it 

 frequently happens that a gametophyte which produces gonidia 

 bears no sexual reproductive organs, and is, therefore, not an actual, 

 but a potential gametophyte. When an alternation of genera- 

 tions is wanting, its absence may either be due to the fact that, 

 as in the lowest Algae and Fungi, sexual reproduction has not yet 

 made its appearance ; or it may be due to the fact that, as in 

 certain Alga? (e.g. Spirogyra, Fucus, Chara), the product of the 

 sexual process is a cell (zygospore or oospore; see p. 80), which 

 gives rise directly to a gametophyte, so that no sporophyte is 

 developed. 



The morphology of these plants is such that the body, whether 

 of the sexual or the asexual form, is generally a thallus, though in 

 certain cases there are more or less distinct indications, especially 

 in the gametophyte, of that differentiation of the body into root, 

 stem, and leaf, which is so familiar in the sporophyte of the 

 Pteridophyta and Phanerogamia. In those forms in which the 

 sexual organs are differentiated, the female organ may be an 

 oogonium, or a procarp, or an archicarp, but it is never an arche- 

 gonium. 



These plants are further characterised by the simplicity of their 

 structure : the body may be unicellular, or coenocytic and unseptate 

 or incompletely septate (see p. 89), or it may be multicellular. 

 One conspicuous structural feature (shared, however, with the 

 Bryophyta), is the absence of lignified cell-walls, the cell-walls con- 

 sisting generally of some form of cellulose, and being frequently 

 mucilaginous. In the lower forms, vegetative reproduction by 

 some mode of cell-division is not uncommon. 



The division of the group into the two classes Algae and Fungi 

 appears to be artificial, inasmuch as it is based upon a single 



