568 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



The varieties which present themselves, indeed, both as to the 

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

 posed, are such as to baffle all attempts to base specific 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 be 

 scarcely doubted that the specimen represented in fig. 429, D, under 

 the name of Pediastrum pertusum, is in reality nothing else than a 

 young frond of P. granulatum in the stage represented in fig. 428, E, 

 but consisting of thirty-two cells. On the other hand, in fig. 429, E, 

 we see an emptied frond of P. granulatum, exhibiting the peculiar 

 surface-marking from which the name of the species is derived, but 

 composed of no more than eight cells. And instances every now 

 and then occur in which the frond consists of only four cells, each of 



FIG. 429. Various species (?) of Pediastrum: A, P. tetras; B, C, P. Ehrenbergii'y 

 D, P. pert usum ; E, empty frond of P. granulatum. 



them presenting the two-horned shape. So, again, in fig. 429, B and 

 C, are shown two varieties of Pediastrum Ehrenbergii, whose frond 

 is normally composed of sixteen cells ; whilst at A is figured a form 

 which is designated as P. tetras, but which may be strongly suspected 

 to be merely a four-celled variety of B and C. Many similar cases 

 might be cited ; and the Author would strongly urge those micro- 

 scopists w r ho have the requisite time and opportunities, to apply 

 themselves to the determination of the real species of these groups 

 by studying the entire life-history of whatever forms may happen to 

 lie within their reach, and noting all the varieties which present them- 

 selves among the offsets from any one stock. The characters of such 

 varieties are diffused by the process of binary subdivision amongst 

 vast multitudes of so-called individuals. Thus it happens that, as 

 Mr. Ralfs has remarked, ' one pool may abound with individuals of 

 Staurastrum dejectum or Arthrodesmus incus having the mucra 



