10 



WISCONSIN PHYTOPLANKTON 



Cells fairly large, length 6-8 times the breadth, curvature of outer 

 margin conspicuous, inner margin with central portion tumid and 

 apical portions with approximately the same curvature as the outer 

 margin ; apices bluntly rounded. Cell wall colorless, without a median 

 girdle; smooth. Chloroplasts with G-8 longitudinal ridges and an 

 axial row of 5-9 pyrenoids that vary greatly in size. Terminal 

 vacuoles conspicuous. (Tychoplanldont.) 



Zygospores ovoid, smooth; enclosed in a laminated gelatinous en- 

 velope (Lundell). 



Distance between apices 244-252 fx ; breadth 46-50 ix. 



Monona (rr). 



4. Closterium acerosum (Shrank) Ehrenberg. PI. 53, Fig. 1. 



Symbolae Physicae seu Icones et Descriptiones etc. pi. 2, fig. 9. 1828 (sec. Nordst. 

 Index Desm.) ; Ralfs, Brit. Desm. 164, pi. 27, figs. 2A-2L. 1848. 



Cells fairly large, length 10-13 times the breadth ; median portion 

 straight and apical region slightly curved; outer margin curved but 

 inner margin straight; gradually attenuated to subacute or rounded 

 apices. Cell wall colorless, rarely brownish in older specimens ; with- 

 out a median girdle; smooth, or in rare cases with delicate striae. 

 Chloroplast with two longitudinal ridges in front view of cell and 

 an axial row of 10-12 pyrenoids. Terminal vacuoles with numerous 

 granules. ( Tychoplanktont. ) 



Zygospores spherical and smooth-walled (Ralfs). 



Length 288-328 ix ; breadth 19-22.5 ix. 



Diameter of zygospore 63 fx (Ralfs). 



Camp (rr), Center (rrr). 



5. Closterium gracile De Brebisson. 



in Chevalier, Des microscopes et de leur usage 272. 1839; Mem. de la Soc. imp. d. 

 So. nat. de Cherbourg 4: 155, pi. 2, fig. 45. 1856; W. & G. S. West, Monogr. Brit 

 Desm. 1: 166, pi. 21, figs. 8-12. 1904. 



Cells fairly small, acicular, very delicate, length about thirty times 

 the breadth; median portion straight and with sides parallel, ter- 

 minal portions gracefully attenuated to blunt apices. Cell wall color- 

 less; without a median girdle; smooth. Chloroplast with 5-7 pyre- 

 noids. 



Fig. 1. Closterium gracile De Brebisson (x610). Prom a herbarium specimen in 

 the British Museum of Natural History collected by De Brebisson. 



Zygospores spherical to subquadrate ; with a smooth wall. 

 Length 130-190 /x, breadth median portion 3.4^6 fx- breath apices 

 1.2-2.4 /x (W. & G. S. West). 



Diameter of zj^gospore 20-27.5 /n (W. & G. S. West). 



Not found in the plankton of Wisconsin lakes. 



