WISCONSIN PHYTOPLANKTON 183 



as branchlets. Zoospore formation confined to terminal and subtermi- 

 nal cells. (Tychoplanktont). 



Diam. main cells 75-100 /*, branchlets 35-50 /x. 



Mendota (rr). 



Division AKONTAE. 



Cells solitary or in filaments, variously shaped, with one or more 

 grass-green chloroplasts that usually contain more than one pyrenoid. 



Asexual reproduction ordinarily by vegetative cell division, rarely by 

 aplanospores, never by zoospores. 



Sexual reproduction isogamous, always by aplanogametes, never by 

 ciliate zoogametes. 



The lack of cilia in every type of reproduction is the great character- 

 istic of the division. It includes but one order, the Conjugales, which 

 forms a very homogeneous though diverse group. 



Order CONJUGALES. 



Cells solitary or in simple filaments; cell shape various, cylindrical 

 without median constrictions or bilaterally symmetrical and usually 

 v;ith a median constriction dividing the cell into two symmetrical halves. 

 Cell wall smooth or variously ornamented, with an inner firm portion 

 and an outer gelatinous portion that is very copious at times. Chloro- 

 plasts 1-8, variously shaped, axial or parietal, with one to several pyre- 

 noids. Nucleus single, at the center of the cell. 



Asexual reproduction by vegetative cell division or aplanospores. 



Sexual reproduction isogamous, by aplanogametes. 



KEY TO THE FAMILIES. 



Cells cylindrical, without a median constriction, united to 



form simple filaments ZYGNEMACEAE 



Cells solitary or in filaments, divided into two symmetri- 

 cal halves, generally with a median constriction DESMIDIACEAK 



(See Part II) 



Family ZYGNEMACEAE. 



Cells always cylindrical, with ends flattened or replicate, serially 

 arranged to form simple filaments that occasionally have simple lateral 

 outgrowths. Filaments generally free-floating though at times attached 

 by a modification of terminal cell into a basal cell with short rhizoidal 

 processes. Wall of cell firm, homogeneous, with a narrow or copious, 

 homogeneous or radially fibrillar, gelatinous covering. Cell contents 

 consisting of a single nucleus, and one to several chloroplasts that are 



