418 



A TEXTBOOK OF BOTANY 



[On. X 



FIG. 286. Pediastrum 

 species; X 300. 



(Drawn from a micro- 

 photograph by Haeckel.) 



usual gelatinous material. This ma- 

 terial prevents too great desiccation, 

 and also permits rapid absorption 

 of water when available; and thus 

 this plant can live in markedly 

 dry places. In general structure and 

 mode of reproduction it resembles 

 Gloeocapsa (Fig. 276), but shows higher 

 evolutionary grade in its distinctly 

 differentiated chromatophore and nu- 

 cleus, while related kinds produce motile 

 swarm spores and gametes. Numerous other forms nearly 

 as simple occur, some of which are symbiotic in habit. 

 Several of the species enter into the composition of Lichens 

 (page 461), and one small form (Chlo- 

 rella vulgaris) lives in the tissues of 

 some lower animals, especially fresh- 

 water Sponges and the Hydra, to 

 which it imparts the green color. 



While typically unicellular and sepa- 

 rate, some members of this order form 

 colonies containing many individuals. 

 Certain ones lack cilia, and merely flrif t 

 with the current, as in case of the 

 beautiful Pediastrum (Fig. 286). Much 

 larger is the remarkable Water-net or 

 Hydrodictyon (Fig. 287), of which the 

 cylindrical colonies, comprising thou- 

 sands of cells and sometimes attaining 

 a length of a meter, float freely in 

 quiet ponds, with one end buoyed up 

 to the surface by bubbles of oxygen 

 caught in the interior. The net orig- 

 inates within a parent cell by the 

 formation of great numbers of zoo- 

 spores, which arrange themselves Kemer.) 



FIG. 287. The Water- 



