DESMIDS, DIATOMS, AND FRESH-WATER ALG.E. 71 



nor circular ; 2 will not do, because the plant is not in 

 long threads ; 3 and 4 do not describe it, because it is 

 neither star -shaped nor formed of oval cells with two 

 bristles on each end ; but 5 calls for a green net often 

 visible to the naked eye, which describes the specimen, 

 giving the name of its genus, Hydrodictyon, and refer- 

 ring the student to the Algse, Section III. of this chap- 

 ter. After using this preliminary Key for a few times, 

 he will be able to .decide at a glance through the micro- 

 scope to which section his specimen belongs. 



Key to the Desmids, Diatoms, and fresh-water Algce. 



1. Plants formed of a single, crescent-shaped, spherical, 



barrel - shaped, oblong and constricted, or circular 

 and flattened, cell, sometimes arranged side by side 

 in long ribbons, but seldom end to end ; color green 

 or brown (a). 



2. Plants formed of many cells arranged end to end in 



long threads ; coloring matter usually green, often 

 in spiral bands or other patterns on the cell-wall (d). 



3. Plants formed of several green cells grouped in the 



shape of a flat disk with six to many short blunt star- 

 like points ; floating free. Pedidstrum (Algce, III.). 



4. Plants formed of two to eight narrowly -oval green 



cells placed side by side, each terminal cell with 

 two curved colorless bristles; floating free. See* 

 nedesmus (Algce, III.). 



5. Plants forming a green net visible to the naked eye. 



Hydrodictyon (Algce, III.). 



