WISCONSIN PHYTOPLANKTON 143 



of filament broad, at times with a radial structure. (Facultative 

 planktont.) 



Zygospores spherical, with smooth dark yellow walls (Lagerheim). 



Cells 14-20 fi long ; 17 fx broad. 



Diameter of zygospores 30 fi. 



Beaverdam (r), Duck (rrr), Fish (rrr), Horseshoe (rrr), Kimball (rrr), 

 Lindy (rrr), Little Doctor (rrr), Meta (rrr), Mud Hen (rrr), Muskallonge (rr), 

 Eooney (rr), Sand (rr). 



The most conspicuous difference between this and the foregoing 

 species is the transverse double ridge near the apex of the semicell. 

 In most specimens this ridge appears to be a homogeneous band but 

 in some cases it is a transverse double ring of angular granules. 



DESMIDIUM C. A. Agardh 1824. 



o' 



Cells united to form long filaments. Filaments usually spirally 

 twisted and enclosed in a broad tough gelatinous envelope. Median 

 constriction generally shallow and poorly defined. Cells not marked 

 compressed, usually with a radial symmetry; breadth mostly about 

 twice the length; semicells trapezoidal in front view and with the 

 apices plane or depressed in the median portion and forming an 

 elliptic open space between adjacent cells. Lateral view identical 

 with the front view or subrectangular to octagonal. Vertical view 

 triangular, quadrangular or broadly elliptic. Cell wall not orna- 

 mented. Chloroplasts axial, with one or more pyrenoids. 



Zygospores formed between the conjugating cells or within the wall 

 of one of the two conjugating cells; spherical to ellipsoidal; with 

 smooth walls. 



Although the genus does not contain many species the shape of the 

 cells is somewhat variable and at first glance the species do not ap- 

 pear to belong together. At one time or another attempts have been 

 made to break up Desmidium into smaller genera but the current 

 practice is to place in the same genus all filamentous species without 

 apical processes that have smooth walls and develop a girdle-like 

 thickening at the time of division that pushes back into the old semi- 

 cells. 



KEY TO THE SPECIES. 



Cell apices plane. 



Vertical view angular (1) -D- Swartsii 



Vertical view elliptic (4) D. GrevillU 



Cell apices with broad depressions. 



Median constriction well marked (2) D. Aptogonum 



Median constriction poorly defined (3) Z>. Baileyi 



