DESMIDS, DIATOMS, AND FKESH-WATER ALG.E. 103 



built up of many cells attached to each other by their 

 narrow ends, the single filament being considered a sin- 

 gle and entire plant. They have no roots, although 

 they may fasten themselves by one end to submerged 

 objects. Some are simple, straight, or curved cellular 

 threads ; some give off branches which generally resem- 

 ble the main plant or stem. Their color is usually 

 some shade of green, although a few purplish and 

 brownish ones are known. The following is a key to 

 those genera referred to in this book. 



Key to Genera, of Fresh-water Algce. 



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



2. Color pure green (d). 



a. Filaments branched (J). 



a. Filaments not branched (c). 



1). Branches with many, whorled, moniliform threads ; 



plant slippery. Batrachospermum, 1. 

 c. Moniliform, with larger scattered spherical cells. 



Anabcena, 2. 



c. Not moniliform ; bluish green ; twisting, bending, 



gliding. Oscilldria, 3. 



d. Green color in one or more spiral bands in each 



cell. Spirogyra, 4. 



d. Green color in two star-shaped masses in each cell. 

 Zygnema, 5. 



d. Green color not in patterns (e). 



e. Terminal cells with a colorless, hair-like bristle (/*). 

 e. Terminal cells without a bristle. Vaucheria, 6. 



