WISCONSIN PHYTOPLANKTON 97 



a large anterior eyespot ; two cilia of equal length with two contractile 

 vacuoles at their base. Reproductive cells at first like the vegetative 

 cells, later with a more massive chloroplast that eventually contains 

 several pyreuoids. The eyespot and cilia of reproductive cells disap- 

 pearing when they are mature. 



Asexual reproduction by a simultaneous division of all reproductive 

 cells to form autocolonies. 



Sexual reproduction (which has been observed in one species only) 

 heterogamous, dioecious, by a division of all reproductive cells to form 

 plates of 64-128 biciliate, fusiform antherozoids ; or a development of 

 reproductive cells into oospheres. Zygotes smooth-walled. 



The validity of this genus has been questioned by many and the view 

 put forward that the alga is but a stage in the life history of Eudorina. 

 Grove (New Phytol. 14: 169-182. 1915) gives a full discussion of the 

 literature of Pleodorina. Although Eudorina elegans is found in a 

 large number of Wisconsin lakes, Pleodorina has been found in but 

 three of them. If Pleodorina is a developmental stage of Eudorina, it 

 is strange that, the "Pleodorina" stage has not been observed with 

 greater frequency. The young colonies look very much like those of 

 Eudorina and it is probably on this account that the two have been 

 confused. 



PLEODORINA CAT.TFORNICA Shaw. PI. 17, Figs. 1-3. 

 Bot. Gaz. 19: 279, pi. 27, figs. 1-9. 1894. 



Colonies containing 64-128 cells. Cells usually spherical and about 

 half vegetative and half reproductive. Reproductive cells becoming 

 4-5 times broader than the vegetative cells. (Facultative planktont). 



Vegetative cells 6-14 p. in diam. ; reproductive cells 6-34 //, in diam. 

 Colonies 40-400 /* in diam. 



Clam (ss), Mud (1) (cc), Sand (1) (r). 



VOLVOX L. 1758. 



Colonies always motile, spherical to ovoid, containing a large number 

 of cells arranged in a single layer just within the periphery of the 

 homogeneous, hyaline, gelatinous, colonial envelope. Cells differen- 

 tiated into those for vegetative purposes, asexual, and sexual repro- 

 duction. Vegetative cells close together or some distance from one 

 another, with or without cytoplasmic connections of varying thickness. 

 Cell shape spherical, ovoid, or disciform. Individual cells or groups 

 of cells sometimes enclosed by their own gelatinous envelope. Cells 

 with two cilia of equal length, 2-6 contractile vacuoles, one nuclei 





