152 PEDIASTRUMS OF THE UNITED STATES. 



Family, PROTOCOCCACE^. 



Tells chlorophyllous, strictly unicellular, without terminal 

 growth, cither single, segregate or associated in families. 



Propagation by means of gonidia, which are of* two kinds, 

 maerogonidia the larger kind, and microgonidia the smaller form-; 

 they are ovate, the smaller, anterior end, somewhat protruding and 

 colorless, is provided with two ciliae; the posterior end is broadly 

 rounded and green. 



This family is subdivided into a Dumber of subfamilies, and 

 these again into genera. There remains for us to treat merely of 

 the subfamily Pediastreae, and genu- Pediastrvm. The features of 

 the subfamily and of the genus being in this case, the same, there- 

 fore 



Genus, PEDIASTRUM, Meyen. 



Cells united into definite families, known as coenobivms ; they 

 are plane, discoid or stellate, swimming free. A coenobiv/m is formed 

 of cells in a single or rarely, in part, double stratum which is con- 

 tinuous or perforated. Cells arc polygonal, with four or more side-; 

 those of the centre entire or often emarginate, and those of the 

 periphery often bilobed : the lobes cuneate, either simple or bidentate, 

 often produced into short hair-like end-. Cell contents green, 

 primarily homogeneous, then granular. 



In the propagation of the species the granular chlorophyllous 

 contents of the cells break up into -mall subspherical bodies; these 

 constitute the maerogonidia. which break through the membrane. 

 After a short period of motile life, they come to rest, then divide 

 and redivide, and become invested with a gelatinous covering; 

 the cells unite in a single layer, then gradually develop into the 

 form of the matured, or mother plant. 



P. simplex, Meyen. Plate L1II, figs. 17-20. 



Coenobium variously composed of from one to three circles 

 of subquadri lateral cells, those of the periphery with apex 

 more or less conically produced, cuspidate. 



