400 



BOTANY 



PABT II 



Order 2. Protococcales 



These are unicellular green Algae, or their cells are united in colonies of various 

 form ; the vegetative cells have no cilia, and the cell or colony is consequently 

 non-motile. Usually each cell contains a nucleus and only one chloroplast. 

 Reproduction is by means of zoospores, in place of which in many genera non- 



D 



FKJ. 334. Volvox globator. A, Colony showing various stages of development of ova and 

 spermatozoids (x 165). B, Bundle of spermatozoids formed by division from a single 

 cell (x 530). C, Spermatozoids (x 530). D, Egg-cell surrounded by spermatozoids in 

 the mucilaginous membrane (x 265). (After F. COHN.) 



ciliated aplanospores are found. Sexual reproduction, when present, takes the 

 form of conjugation of similar gametes. It has only been demonstrated in certain 

 genera and appears not to have arisen in the simpler forms. 



The simplest forms belong to the genera Chlorococcum and Chlorella ( 31 - 32 ). 

 The cells of the former are spherical, and occur in fresh water and also on damp 

 substrata ; they frequently take part in the composition of Lichen thalli. Asexual 

 reproduction is by the production from a cell of a Dumber of biciliate zoospores 

 (Fig. 335) ; under certain conditions these are replaced by aplanospores without 

 cilia. Chlorella vulgaris (Fig. 336) is a widespread Alga, the small cells of which 



