168 PEDIASTRUMS OF THE UNITED STATES. 



Family, PROTOCOCCACE.E. 



Cells chlorophyllous, strictly unicellular, without terminal 

 growtli, either single, segregate or associated in families. 



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

 macrogonidia the larger kind, and microgonidia the smaller forms ; 

 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 number of subfamilies, and 

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

 the subfamily Pediastreae, and genus Pediastrum. The features 

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

 therefore 



Genm, PEDIASTEUM, Meyen. 



Cells united into definite families, known as coenobiums; they 

 are plane, discoid or stellate, swimming free. A coenobmm is 

 formed of cells in a single or rarely, in part, double stratum, 

 which is continuous o'r perforated. Cells are polygonal, with 

 four or more sides ; those of the center entire or often emargi- 

 nate. and those of the periphery often bilobed ; the lobes cuneate, 

 either simple or bidentate, often produced into short hair-like 

 ends. Cell contents green, primarily homogeneous, then granular. 



In the propagation of the species the granular chlorophyllous 

 contents of the cells break up into small subspherical bodies; 

 these constitute the macrogonidia, 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 gela- 

 tinous covering ; the cells unite in a single layer, then gradually 

 develop into the form of the matured, or mother plant. 



P. SIMPLEX, Meyen. Plate LXIY, figs. 17-20. 



Coenobium variously composed of from one to three circles 

 of snbquadrilateral cells, those of the periphery with apex 

 more or less conically produced, cuspidate. 



Var. a. Composed of six simple cells radiately connected, 

 fig. 17. 



Yar. b. (P. Sturmii, Eeinsch.) Composed of six cells, with 

 an open space in the center, fig. 18. 



