166 



Volvocinese 



Family Sphserellacese. 



Although but recently proposed (Schmidle, '03) and including only two 

 genera, the unicellular Sphterella (= Htematococcus) and the colonial Stepha- 

 nosphxra, this family appears to be established upon a sound basis. There 

 are two leading characters which distinguish the Sphaerellacese from the 

 Volvocacese : (1) the form of the protoplast with its outstanding branched 

 processes, and (2) the numerous contractile vacuoles. The presence of more 

 than one pyrenoid in the chloroplast is an important although not a dis- 



Fig. 96. A 0, vegetative individuals ( = zoogonidia) of Sphwrellu lacustris (Girod.) Wittr. A, 

 surface view ; B, optical section (after Schmidle) ; C, zoogonidiura from a ' snow-water- 

 medium' (after Wollenweber). D F, Sphwrella Draebakensis (Wollenw.) G. S. West. 

 D, vegetative cell (zoogonidium) in optical section ; E, gamete ; F, resting cell (all after 

 Wollenweber). G and H, portions of individual cells of Stephanosphsera pluvialis Cohn, to 

 show general cytology; G, after Wollenweber; H, after Schmidle. All x about 1000. 

 n, nucleus ; cv, contractile vacuole ; py, pyrenoid ; st, stigma ; v, vacuole. 



tinctive character. The peculiar nature of the protoplast, with its branched 

 processes, has resulted in a corresponding modification of the cell-wall, which 

 consists of a firm outer membrane, extending around all the branched tips 

 of the processes, and an inner mucilaginous portion filling up the intervening 

 space between the protoplast and the outer wall. 



In Sphterella the protoplasmic extensions are generally distributed all 

 round the protoplast (fig. 96 A D), but in Stephanosph&ra they are mostly 

 confined to the two extremities of the elongated cell (fig. 104 K). In both 

 genera the chloroplast is more or less peripheral and appears to be reticulate 

 in the mature cell, this condition being brought about by the formation of 



