FIELD BOOK OF PONDS AND STREAMS 



The dense green mats of "pond-scum" or "frog-spittle" 

 and the "blanket algae" of early spring are filamentous algse 

 which grow on the surface of sunny pools and form bubbles of 

 gas as they decay. In such "blankets" Spirogyra (Fig. 43) 

 usually predominates, but the threads of Zygnema (Fig. 43) 

 and other forms are interlaced with it, as well as a host of 

 desmids, diatoms, and minute animals. In spring pools 







Fig. 43. — Filamentous green alga;: i, Spirogyra; 2, 



Zygnema. 



"blanket algae" flourish all winter, sheltering an extensive 

 community of animals — small mayflies, midges, and crusta- 

 ceans feed on the algae and in turn furnish food for predacious 

 animals — young dragonfiies, beetles and the like. 



The similarity between certain green algae and animals has 

 already been mentioned. One of the largest of these animal- 

 like algae is Volvox (Fig. 44, i), whose beautiful spheres of cells 

 are but green dots to the naked eye, though sometimes they 



Fig. 44. — I, Volvox; 2, Eiiglena. 



are so crowded through the water that a glass of it will be col- 

 ored nile green with them. There are hundreds of cells in each 



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