DESMIDS 



209 



each containing one or two chloroplasts, and not uncommonly 

 separated by a median constriction, where the single nucleus 

 is situated {e.g. Cosmaritim, Fig. 114, D, E). The wall is often 

 richly sculptured or provided with spinous outgrowths (Fig. 

 114, A, D). In Closterium (Fig. 114, G), species of which are 



Fig. 114. — Diverse Desmids. A, Micrasterias. B and C, Eiiastrum. 

 D and E, Cosmariiim {in E the top left-hand figure shows the cell 

 in side-view ; the lower left-hand figure the cell in end-view). 

 F, Staurastrum (the right-hand figure shows the end-view) (after 

 Ostenfeld). G, Closterium. H, Cylindrocystis. I, zygospore of 

 Cosmarium meneghini (after West). J, Desniidhim (filamentous). 

 K, Pleurot(snium. The cell-contents are shown only in E-H. 

 »., nucleus ; p., pyrenoids. 



very frequent, each half of the, usually semilunar, cell is occupied 

 by a chloroplast consisting of a central rod which contains a 

 row of pyrenoids {p.) and bears a number of I'adiating longi- 

 tudinal plates ; the latter appear as dark green streaks when the 

 cell is viewed from the surface. The small vacuoles, commonly 

 seen at either end of the cell, enclose minute crystals of gypsum 

 14 



