CHAPTER XXII. 



GREEN ALGAE (CHLOROPHYCEAE). 



Green Algae are a heterogeneous assemblage of forms. Some are 

 marine : others live in fresh water. Some are unicellular, some 

 colonial (Chlorococcales, Volvocales) : others, which are multicellular, 

 consist of a simple filament, with various degrees of its branching 

 (Ulotrichales) ; or they may form widened flat expansions (Ulvaceae) : 

 others again are coenocytic, not being partitioned into cells or only 

 partially septate as in the Siphonales. A frequent feature is the 

 enlargement of the chloroplast into a body often of complicated shape. 

 One or more of these chromatophores may be present in the cell, and 

 each may contain one or more highly refractive pyrenoids, spherical 

 bodies which act as centres for the formation of starch. The variety 

 of the form and structure of the plants is matched by the diversity 

 of their propagation. Some multiply by simple fission [Pleurococcus 

 Naegelii) : others undergo conjugation of equal, non-motile cells 

 (Conjugatae) ; but most of them produce motile zoospores produced 

 in cells which may be called sporangia, and gametes produced in cells 

 which may be called gametangia. The latter show in more than one 

 natural series evidence of a progressive sexual differentiation (Ulotri- 

 cales, Siphonales, Fig. 275). The effect of a general study of these 

 Algae is to suggest that they may all represent steps in advance from 

 the Protophyta ; and that they contribute many distinct lines in 

 which an increasing complexity of development of their encysted 

 phase, and a differentiation of sex have been independently achieved. 

 A few examples will be described which illustrate the great variety of 

 structure and propagation that these plants show. 



Volvocales and Chlorococcales. 



The Volvocales include unicellular and colonial organisms which 

 are typically motile throughout the vegetative phase or readily resort 



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