46 



WISCONSIN PHYTOPLANKTON 



Fig. 5. Micrasterias radiosa Ealfs (x400). A plankton specimen from Grasmere, 

 England, in the G. S. West collection. 



than that of the preceding series, ultimate lobelets deeply emarginate 

 and bifid, inferior lateral lobes sometimes with only two series of in- 

 cisions. Cell wall smooth. Vertical view fusiform, with central por- 

 tion slightly elevated and truncate. Lateral view of semicells linear- 

 oblong, with apex tridenticulate. Chloroplast extending to ultimate 

 lobelets, with apex of portion in polar lobe deeply incised, with two 

 conspicuous vertical ridges extending from polar apex to the nucleus 

 and secondary ridges bordering the major incisions; pyrenoids nu- 

 merous, 50-65, in all parts of the chloroplast. 



Zygospores unknown. 



Cells 172 [x long ; 174 fi broad at the base of the semicells, 50 /a at 

 the apex of the polar lobe ; isthmus 20 jn broad. 



Not found in the plankton of Wisconsin lakes. 



W. & G. S. West have discussed the nomenclature of this alga and 

 shown that what Ralfs calls M. radiosa is not M. radiosa (Lyngbye) 

 Agardh but Micrasterias Sol (Ehrenberg) Kiitzing (Euastrum 8ol 

 Ehrenberg). In spite of the fact that the name of M. Sol has priority, 

 the ruling that Ralfs' British Desmidieae is to be taken as the start- 

 ing point for the nomenclature of the Desmids requires the retention 

 of the specific name radiosa. 



It is distinguished from other deeply incised circular species by 

 the deeper incisions, greater subdivision, and undulate margins of 

 the lobes. 



