218 



Chlorococcinese 



It is quite possible that the Pediastreae and the Hydrodictyeae have no 

 very close affinity, but the phylogenetic relationships of these Algae are at 

 present very obscure. 



Fig. 144. A E, Euastropsis Richteri (Schmidle) Lagerh. A and B, x 633 ; CE, showing 

 formation of young coenobia, very highly magnified (after Lagerheim). F, four cells of a 

 variety of Pediastrum duplex Meyen, stained to show the nuclei (n) and the pyrenoid (py), 

 x 580. G, part of ccenobium of P. Boryanum, showing escape of zoogonidia (z) into the 

 vesicles (v) just before swarming. H, P. simplex Meyen var. clathratum (Schroet.) Chod., 

 showing the numerous fine bristles stained by Loffler's cilia-staining method, x 316 (after 

 Petersen). 



Sub-family PEDIASTRE^E. The Algae of this sub-family are microscopic 

 in size, consisting of a number of small coenocytes firmly united to form a flat, 

 disc-like coenobium. The principal genus is Pediastrum (figs. 143 and 144 

 F H), with a world- wide distribution in ponds, pools and lakes. The 

 coenobium is always free-floating, consisting of a single layer of small cceno- 

 cytes 1 . Either .these are closely united or there are perforations of variable 

 size between them which give the coenobium a sieve-like aspect (fig. 144 H). 

 In Pediastrum simplex var. reticulatum and P. duplex var. reticulatum the 

 perforations of the plate-like colony are much larger than the coenocytes 



1 In certain monstrous forms the coenobium may consist of a double layer of coenocytes in 

 certain parts, or it may sometimes be very irregular. (Consult G. S. W., '07.) 



