272 



VARIATION AMONG PEDIASTRE^. 



as to constitute a perfect disk like that seen at f, in which, how- 

 ever, the arrangement of the interior cells does not follow the 

 typical plan.* 



210. The varieties which present themselves, indeed, both as to 

 the number of cells in each cluster, and the plan on which they are 

 disposed, are such as to baffle all attempts to base specific distinc- 

 tions on such grounds ; and the more attentively the Life-history of 

 any one of these Plants is studied, the more evident does it appear 

 that many reputed Species have no real existence. Some of these, 

 indeed, are nothing else than mere transitory forms ; thus it can 

 scarcely be doubted that the specimen represented in Fig. 117, d, 

 under the name of Pediastrum pertusum, is in reality nothing 

 else than a young frond of P. gramdatum, in the stage repre- 

 sented in Fig. 116, E, but consisting of 32 cells. On the other 

 hand, in Fig. 117, e, we see an emptied frond of P. gramdatum, 

 exhibiting the peculiar surface-marking from which the name of 

 the species is derived, but composed of no more than 8 cells. And 

 instances every now and then occur in which the frond consists of 



Fig. 117. 



Various species (?) of Pediastrum .—a, P. tetras; b, a, P. biradiatuni; 

 d, P. pertusum ; e, empty frond of P. granulatum. 



only 4 cells, each of them presenting the two-horned shape. So, 

 ao-ain in Fig. 117, b and c, are shown two varieties of Pediastrum 



* See Prof Braun on " The Phenomenon of Rejuvenescence in Nature," 

 published by the Ray Society in 1853 ; and his subsequent Memoir, 

 " Algarum Unicellularum Genera nova aut minus cognita," 1855. 



