Protococcacece 



227 



Genus Oocystis Nag., 1845. The cells are ellipsoid, sub- 

 cylindrical or panduriform, with a firm cell-wall which commonly 

 possesses a nodular thickening at each pole. There are usually 

 several parietal chloroplasts in each cell, destitute of pyrenoids. 

 In some cases, however, there may be one pyrenoid in each 

 chloroplast. Multiplication is by autospores, which are generally 

 retained for some time within the greatly swollen wall of the 

 mother-cell. Sometimes several generations are contained within 

 one large mother-cell-wall. 



Fig. 97. A and B, Oocystis solitaria Wittr., from Ben Lawers, Perthshire. C and 

 D, 0. crassa Wittr.; C, from Lanlivery Moor, Cornwall; D, plankton form 

 from Lough Beg, Ireland. E and F, 0. panduriformis W. & G. S. West; E, 

 from Pilmoor, N. Yorks. ; F, from near Clifden, Ireland. G, 0. elliptica West, 

 from Derryclare Lough, Gal way, Ireland. (All x 485.) 



There are about ten British species of the genus, some of which are widely 

 distributed in the quiet waters of ponds and lakes. 0. solitaria Wittr. (length 

 of cells 15 48 p.; breadth 9'5 25 p.; fig. 97 A and B) is undoubtedly the 

 commonest species, although 0. elliptica West (fig. 97 G) is abundant. 

 0. parva West & G. S. West (length 6 12 p. ; breadth 4 7 p.) is the smallest 

 species and 0. gigas Arch, (length 41 5O5 p. ; breadth 32'5 40 p.) is the 

 largest. 0. crassa Wittr. is not common except in the plankton; fig. 97 C 

 and D. 0. asymmetrica W: & G. S. West is another solitary species. 



Chodat states that 0. gigas Arch, and 0. panduriformis W. & G. S. West 

 (fig. 97 E and F) are merely stages in the life-history of Eremosphcera, but 

 this statement I do not accept. These plants often occur in localities from 

 which Eremosphcera is absent and they reproduce themselves in the manner 

 of true species of Oocystis. Moreover, I invariably find the life-history of 

 Eremosphcera to be remarkably free from polymorphic forms (vide page 229). 



152 



