272 MICROSCOPIC FORMS OF VEGETABLE LIFE. 



teristic "horns." "Within about four or five hours after the 

 escape of the gonidia,. the cluster has come to assume mucli 

 more of the distinctive aspect of the species, the marginal cells 

 having grown out into horns (e) ; still, however, they are not 

 very closely connected with each other; and between the cells of 

 the inner row, considerable spaces yet intervene. It is in the 

 course of the second day, that the cells become closely applied 

 to each other, and that the growth, of the horns is completed, so 

 as to constitute a perfect disk, like that seen at F, in which, 

 however, the arrangement of the interior cells does not follow 

 the typical plan. 



167. The varieties which present themselves, indeed, both as 

 to the number of cells in each clustei", and the plan on which 

 they are disposed, are such as to baiHe all attempts to base spe- 

 cific distinctions 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 repre- 

 sented in Fig. 74, d, under the name oi Pediastrum pertusum, is 

 in reality nothing else than a young frond of P. granulatum, in 

 the stage repi'esented in Fig. 73, b, but consisting of 32 cells. 

 On the other hand, in Fig. 74, e, we see an emptied frond of P. 

 grajiulatum, exhibiting the peculiar surface-marking from which 

 the name of the species is derived, but composed of no more 



Various species (0 ai Fmmf^bttm ; — a, P. tdras ; b, c, P. birj.dialum ; D, P.pertusum; E, empty 



frond of P. granulatum. 



than 8 cells. And instances every now and then occur, in which 

 the frond consists of only 4 cells, each of them presenting the 

 two-horned shape. So, again, in Fig. 74, b and c, are shown two 

 ^■arieties of Pediastrum biradiatum, whose frond is normally com- 



