178 



PLANT LIFE. 



Algae 



food-supply of fish. In fresh water, many 

 belonging to different groups assist in forming the frcQ- 

 f\oa.t\ng p lank ton. Of these the Desmids are especially 

 remarkable for the beauty and variety of their shapes. 

 They consist of a single cell, which is almost always 



Fig. 23. — Diatoms (1-7) and Desmids (8-12). (Diagrammatic.) (i) Gom- 

 phonema. (2) Cymbella. (3) Rhoicosphenia. (4) Meridion. (5) Synedra. 

 (6) Navicula. (7) Tabellaria. (8) Micrasterias. (9) Euastrum. (10 and 11) 

 Staurastrum spp. (12) Closterium. 



divided transversely, by a contraction towards the 

 middle, into two symmetrical parts or valves, and is cut 

 into rosettes of fringed circles, or into wheel-like stars, 

 often ornamented with tubercles, or reflex spines, or 

 hairs. Desmids are generally of a brilliant and vivid 

 green colour. Their shapes seem sometimes intended to 

 assist in their floating, sometimes to keep them from being 



