246 WISCONSIN PHYTOPLANKTON 



view broadly elliptic, with a distinct nodule at each pole. Cell wall 

 smooth, except for faint vertical striae just above the isthmus. 



Zygospores subspherical, with smooth walls (De Bary). 



Cells 20-24 /a long ; 45-48 fx. broad ; breadth at isthmus 40-42 fi. 



Found (rr). No Mans (rrr). 



The species is widely known under the name D. cylindricum. In 

 founding the genus Didymoprium Kiitzing, for some unknown rea- 

 son, gave the alga a new specific name, a name which Ralfs adopted 

 in his British Desmidieae. The specific name cylindricum antedates 

 the name Grevillii but the use of Ralfs as the starting point for 

 nomenclature of the Desmidiaceae necessitates the use of the latter 

 name. 



In the conjugation of this species zygospore formation takes place 

 within one of the old cells and not between the conjugating cells. 

 This feature, however, has not been considered a justification for 

 separating it from other species of the genus. 



GYMNOZYGA Ehrenberg 1841. 



Cells united to form long filaments. Filaments rarely twisted, 

 frequently enclosed by a gelatinous sheath. Cells barrel-shaped, not 

 compressed, length about twice the breadth, median constriction 

 slight, at times a small groove running around the cell; apices flat- 

 tened and without protuberances. Cell wall smooth or with vertical 

 striae near the poles and at the isthmus. Chloroplasts axial and with 

 radiating vertical laminae ; pyrenoids single and central. 



Zygospores spherical to broadly ellipsoidal, with smooth walls. 



In the division of the Gymnozyga cell there is the same girdle-like 

 thickening that pushes back into the old semicell at the time of divi- 

 sion that is found in Desmidium. 



Gymnozyga moniliformis Ehrenberg. PL 87, Fig. 11. 



Berichte Verh. Preuss. Akad. d, Wiss. Berlin 1840: 212. 1841. 



Cells barrel-shaped, united in permanent filaments, length about 

 twice the breadth, sinus a small semielliptic depression, isthmus very 

 broad; semicells with sides converging to a truncate apex, lateral 

 margins distinctly concave and with apical angles only slightly 

 rounded. Cell wall with vertical striae in the polar regions and at 

 times with other vertical striae at the bases of the semicells. Vertical 

 view circular. Chloroplasts axial, with 6-8 delicate laminate vertical 

 processes to the cell wall; pyrenoids single and central. (Facultative 

 planktont.) 



