HOW TO KNOW THE FRESH-WATER ALGAE 



84b Cells not arranged to form flat plates 



88 



85a Cells forming circular plates (sometimes irregularly subcircular), 

 the marginal cells sometimes different in shape from those within. 

 Fig. 70 PEDIASTRUM 



Figure 70 



Fig. 70 a, Pediastrum simplex (Mey- 

 en) Lemm.; b, P. tetras (Ehr.) Ralfs; 

 c # P. hirodiotum var. emarginatum 

 fa. convexum Pres.; d. P. Boryanum 

 (Turp.) Menegh.; e, P. obfusum 

 Lucks. 



Although there are many species of this 

 genus all may be identified by the plate- 

 \ like arrangement of cells. The plate may 

 be continuous with internal cells different in 

 shape from those at the periphery, or there 

 may be interstices with all cells in the colony 

 about the same shape. One, P. tetras (Ehr.) 

 Ralfs, forms plates of but 4 cells. Rarely 

 a 2-celled colony may appear, in which case 

 it might be mistaken for Euastropsis (Fig. 33). 



85b Cells not arranged to form circular plates 86 



86a Cells triangular or ovoid, forming guadrangular plates, bearing 

 one or more spines. Fig. 71 TETRASTRUM 



Fig. 71. 

 Chod. 



Tetrostrum heterocanthum (Nordst.) 



There are probably only three species of this 

 genus repotted from the United States. They 

 occur in the euplankton and are readily identi- 

 fied by their arrangement in flat plates of four, 

 the cells bearing 1 to 4 spines on the outer free 

 walls. 



Figure 71 



86b Cells rectangular or trapezoidal or if oval, without spines 



87 



55 



