176 



Volvocitiese 



arranged as a single layer within the periphery of an almost spherical, or 

 rarely ellipsoid, mucous investment (fig. 105 A}. The cells are globose, and the 

 bell-shaped chloroplast may possess more than one pyrenoid. In Pleodorina 

 (fig. 106) there is a differentiation into smaller, purely vegetative (somatic) 

 cells, which occupy the anterior region of the coenobium, and larger reproduc- 

 tive (or gonidial) cells which are alone capable of division. This differentiation 

 is possibly constant in Pleodorina californica Shaw ('94), especially in view 

 of the observations of Chatton ('11); but in P. illinoisensis Kofoid ('98) the 

 differentiation is not at all constant, and every intermediate state can be 

 observed between the coenobium as originally described and that of Eudorina 

 elegam 1 . In any case, the two forms are of great interest since they are links 

 between Eudorina and Volvox. 



K 



Fig. 104. A H, Pandorina Morum (Mull.) Bory ; A, normal colony; B, daughter-colonies 

 within the swollen mother-cell-wall, x 475 ; C H, gametes (g), formation of zygospore (z), 

 and its development (after Pringsheim). K, Slephanosphsera pluvialis Cohn, ordinary 

 vegetative colony (after Hieronymus, x 320). 



Platydorina, described by Kofoid ('99) from the United States, is one of 

 the most curious of the Volvocese, the coenobium consisting of a twisted plate 

 of 16 or 32 cells, the enveloping jelly being produced posteriorly into 3 or 5 



1 The author has examined large numbers of Pleodorinn illinoisensis from Yorkshire, the 

 west of Scotland, the west of Ireland, and Madras. The somatic cells may be entirely absent, 

 or they may vary in number from 1 to 12 ; moreover, every gradation in size occurs between the 

 reproductive and the somatic cells, and the latter are somatic or non-somatic according to the 

 degree of differentiation in size. In many specimens somatic and reproductive cells are indis- 

 criminately mixed. The author is inclined to agree with Powers ('05) that there are so many 

 transition-types, even in the same collection, between Eudorina elegans and Pleodorina 

 illinoisensis that the latter should be regarded merely as a state of the former. 



