No. IO.] FRESH-WATER ALG/E. 2"J 



The class may conveniently be divided into orders, as in- 

 dicated by the following key : 



Key to Orders. 



i. Thallus coenocytic (. e., non-cellular but with many 



nuclei) SIPHONALES 



Thallus filamentous and septate, or unicellular, or 

 expanded 2 



2. Thallus filamentous, though filaments may unite in a 



plane. In the Conjugatse some are unicellular and 



not filamentous 3 



Thallus expanded, membranous ULVALES 



Thallus neither expanded nor filamentous '. 



PROTOCOCCALES 



3. Cell division by intercalation of new cells producing 



transverse striation CEDOGONI ALES 



Cell division of ordinary type 4 



4. Filaments attenuated and commonly ending in a 



bristle CH^TOPHORALES 



Filaments not ending in a bristle 5 



5. Chloroplasts single, substellate, with one pyrenoid. 



Filaments may fuse in a plane: . . . .SCHIZOGONIALES 

 Chloroplasts single, reticulated or band-shaped, with- 

 out pyrenoids MICROSPORALES 



Chloroplasts numerous, parietal, each with a pyre- 

 noid CLADOPHORALES 



Chloroplasts single or several, large and of some 

 definite shape, with pyrenoids. The entire contents 

 of two cells unite to form a single zygote. .CONJUGATE 



ORDER I. PROTOCOCCALES. 



Single-celled green Algae, without terminal growth or 

 branches, and without vegetative generation of cells ; either 

 single or in flocks or families. Sometimes the cells of the 

 families indefinitely increase in number, and form daughter- 

 families. At other times there is a definite number associated 

 together to form colonies called cocnobia. Even when ap- 

 parently closely united, each cell has the power of reproduction, 

 and therefore the plants are essentially unicellular. 



