DESMIDS, DIATOMS, AND FRESH-WATER ALG^. lOI 



curs SO' profusely at times that it might be collected 

 by the wheel-barrow load. It is not uncommon else- 

 where. 



The masses which the 

 Algae in general form are 

 usually composed of great 

 numbers of lorig. threads, 

 commonly called filaments, 

 and matted together, prob- 

 ably by their rapid growth, 

 among other causes. Each 

 filament is built up of many' 

 cells attached to each other 

 by their narrow ends, the 

 single filament being for „ „ „ j j- . . ■ , . 



*^ ' Fig. 8i. — Hydrodictyon utnculatum. 



convenience, considered a 



single and entire plant. The Algae have no roots, al- 

 though they may fasten themsel,ves by one end to sub- 

 merged objects, while a few single-celled terrestrial 

 forms have minute branching filaments which may be 

 called rootlets, growing from them and penetrating 

 the ground (Botrydiuni). Some are simple, straight, 

 or curved cellular threads; some give off branches 

 which generally resemble the main plant or stem. 

 Their color is usually some shade of green, although a 

 few purplish and brownish ones are known. The fol- 

 lowing is a key to those genera referred to in this book. 



Key to Genera of Fresh-water Algm. 



1. Color brownish-green, bluish, or olive (a). 



2. Color pure green {d). 



a. Filaments branched {b). 

 a. Filaments not branched {c). 



