CHAPTER XXV. 



GREEN ALGAE (CHLOROPHYCEAE). 



THE Brown and Red Algae form natural and coherent groups of plants, 

 characterised by their colour, but more particularly by their propa- 

 gative organs. Green Algae are a more heterogeneous assemblage 



of forms. Some are marine : others 

 live in fresh water. Some are uni- 

 cellular, some colonial (Protococcales, 

 Volvocales) : others, which are multi- 

 cellular, consist of a simple filament, 

 with various degrees of its branching 

 (Ulothricales) ; or they may form 

 widened flat expansions (Ulvaceae) : 

 others again are coenocytic, not being 

 partitioned into cells (Siphonales), or 

 only partially septate (Siphonocla- 

 diales). The variety of their form 

 and structure is matched by the 

 diversity of their propagation. Some 

 multiply by simple fission (Proto- 

 coccus viridis, Fig. 328) : others 

 undergo conjugation of equal, non- 

 motile cells (Conjugatae) ; but most 

 of them produce motile zoospores and gametes, the latter showing in 

 more than one natural series evidence of a progressive sexual differen- 

 tiation (Ulothricales, Siphonales, Fig. 335). The effect of a general 

 study of them is to suggest that they may all represent steps in 

 advance from the Flagellatae ; and that they represent many distinct 

 lines in which an increasing complexity of development of their 

 encysted phase, and a differentiation of sex have been independently 

 achieved. 



390 



FIG. 328. 



Cells of Protococcne, some isolated, others 

 resulting from recent division, are still 

 coherent. ( x 730.) 



