No. IO.] FRESH- WATER ALG.E. $J 



S. spinulosum Nag. (?). The common form of Soras- 

 trum in this region is shown in Fig. 213. It shows only one 

 spine at each corner. A less common form which may be a 

 different species is shown in Fig. 55. 



FAMILY III. HYDRODICTYACEyE. 



These plants are free-floating, non-motile ccenobia, com- 

 posed of cells arranged like a net or in a flat plate. Pediastrum 

 may have fifty cells and Hydrodictyon many hundreds. Multi- 

 plication by autocolonies. Reproduction by spores, which be- 

 come quiescent within the mother colony and then unite by 

 their extremities to form a new ccenobium. A fusion of 

 isogamous gametes to form a zygospore also sometimes occurs. 

 The two sub-families are probably not closely related. 



Key to Sub-families. 



Cells in a flat plate pediastrEyE 



Cells form a network hydrodictye^: 



SUB-FAMILY I. PEDIASTRE^. 



Microscopic plants composed of a number of small cells 

 united into a flat disk. Zodgonidia formed in the mother-cell 

 are liberated into an external vesicle, and there form new 

 ccenobia. 



Description of Genus. 



Pediastrum Meyen. The plane, discoid or stellate, free- 

 swimming ccenobium is formed of a single, rarely a double, 

 layer of green cells, perforated or continuous. The cells are 

 polygonal, with four or more sides ; the central cells are entire, 

 while the marginal cells are often bilobed ; the lobes are 

 wedge-shaped, simple or bidentate, sometimes drawn out into 

 hair-like ends. The genus is very abundant and variable. The 

 reproduction is as follows : The cell contents are at first 

 homogeneous, later becoming granular. The granular contents 

 divide into small zoogonidia, spherical or nearly so, which 

 break away from the mother-cell into an external vesicle. 

 After they have been motile awhile, they come to rest, and 

 then divide and redivide ; a gelatinous covering forms around 

 them, the cells arrange themselves into a single layer, and 

 gradually take on the shape of the mother plant. Autocolonies 



