218 



Ohlorococcmeae 



It is quite possible that the Pediastrese and the Hydrodictyese have no 

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

 present very obscure. 



H 



Fig. 144. A E, EiKistropsis Ricliteri (Schmidle) Lagerh. A and P>, x 633 ; C E, showing 

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

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

 x 580. G, part of ccenobium of P. Boryanvm, 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 ccenobium. The principal genus is Pediastrum (figs. 143 and 144 

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

 ccenobium is always free-floating, consisting of a single layer of small cosno- 

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

 size between them which give the ccenobium 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 ccenocytes 



1 In certain monstrous forms the ccenobium may consist of a double layer of ccenocytes in 

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



