144 WISCONSIN PHYTOPLANKTON 



Desmidium Swaetzii C. a, Agardh. PI. 88, Figs. 1-2, 

 Systema Algarum 9. 1824; Ealfs, Brit. Desm. 61, pi. 4, figs. A-F. 1848. 



Cells united in permanent filaments ; filaments spirally twisted (one 

 complete turn every 12-20 cells), enclosed by a broad firm gelatinous 

 envelope that frequently is radially fibrillar. Cells slightly under 

 medium size, breadth about twice the length, moderately constricted, 

 sinus with inner portion having sides subparallel and apex rounded, 

 outer portion widely open, isthmus very broad; semicells broadly 

 trapezoidal, with lateral margins bearing in the median portion an 

 acute upturned corner, semicells apices plane, without a depression 

 that forms open spaces between adjacent cells. Cells separated by a 

 flattened gelatinous cushion that is thickest at the outer margin. 

 Vertical view triangular, with sides concave but slightly tumid in 

 the median portion, angles broadly rounded (subcircular). Cell wall 

 smooth. Chloroplast w^ith a fairly massive central axial mass and 

 with broadly ovate projections running to the angles of the cells, pro- 

 jections incised about half the distance from the apex to the central 

 mass; pyrenoids in each lobe and at times in the central mass. 

 ( Ty choplanktont. ) 



Zygospores subspherical to ellipsoidal, with broadly rounded poles, 

 smooth-walled ( Ralf s ) . 



Cells 13-17.5 jx long ; 35-42 ^ broad ; 27-36 ix broad at isthmus. 



Beaverdam (rrr). Diamond (rrr). Devils (rrr), Fish (rrr). Found (rr), 

 George (rrr), Horseshoe (rrr), Howey (rrr), Jag (rrr), Lae Court Oreilles (rrr), 

 Little Doctor (rrr), Meta (rrr). Sand (rrr). Thousand Island (rrr). 



This, the first member of the family to be described, is sharply 

 differentiated from other species of the genus except D. Aptogonum 

 De Brebisson. It is readily distinguished from D. Aptogonum by 

 the lack of open spaces between contiguous cells. 



2. Desmidium Aptogonum De Brebisson. PL 88, Figs. 3-4. 



Mem. soc. acad. d. sc. de Falaise 1835: 268, pi. 2. 1835; W. & G. S. West, Trans. 

 Linn. Soc. 2nd. Ser. Bot. 5: 233, pi. 12, fig. 24. 1896. 



Aptogonum Desmidium (Ehrenberg) Ealfs, Brit. Desm. 64, pi. 32, figs. lA-lH. 

 1848. 



Cells imited in permanent filaments that are spirally twisted (one 

 complete turn in every 15-20 cells) ; filaments enclosed at times by 

 a firm gelatinous envelope. Cells somewhat under medium size, 

 breadth about twice the length, moderately constricted, sinus with 

 sides subparallel and apex rounded, outer portion widely open, 

 isthmus very broad ; semicells trapezif orm, with lateral margins bear- 

 ing at the base an acutely upturned angle ; semicell apices with a deep 

 broad semielliptic to subrectangular depression that forms an elliptic 

 to subrectangular open space between adjacent cells. Vertical view 

 triangular, with sides slightly concave and angles broadly rounded. 

 Cell wall smooth. (Tj^choplanktont.) 



Zygospores spherical to broadly elliptic, with smooth walls (Ralfs). 



Cells 18-20 fji long ; breadth 31-43 /x ; breadth at isthmus 23-35 /z. 



Zygospores 22-27 /. long, 20-25 /x broad (W. & G. S. West). 



Devils (rrr). 



