GROUP I. THALLOPHYTA : ALG.E. 237 



GROUP I. 



THALLOPHYTA 



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

 mentioned, the alternation of generations is here either irregular 

 or wanting. The morphology of these plants is such that the body 

 is generally a thallus, though in certain cases there are more or 

 less distinct indications 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, ccenocytic and unseptate 

 or incompletely septate (see p. 63), or it may be multicellular. 

 One conspicuous structural feature (shared, however, with the 

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

 consisting 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 Algse and Fungi 

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

 character, the presence (Algse) or absence (Fungi) of chlorophyll. 

 But the division is really natural, since this one character is 

 correlated with various others. It is, indeed, becoming usual to 

 regard the Algse and the Fungi as altogether distinct groups : but 

 it appears to be preferable to continue to regard them as classes 

 of the group Thallophyta, inasmuch as the Fungi have doubtless 

 arisen from the Algse, and since they possess many features in 

 common. 



CLASS I.ALGM. 



Many of these are plants of the simplest structure, which either 

 live in water in the form of green, blue-green, red, or brownish 

 filaments or masses of cells, or clothe damp surfaces, such as rocks, 

 walls, or the bark of trees, with a covering of one or other of these 

 colours. In the sea they attain often a very considerable bulk ; 

 some of them are of a very beautiful red or brown colour, and 



