24 DUBLIN NATTJILVL HTSTOTIT SOCIETY. 



expanding by a siraultaneons increase all over. If I be right, then, 

 Pediastrum does not coincide with the second clause of Ralfs' diagnosis. 

 As to the next clause, an inspection of any species of Pediastrum 

 will manifest that the cells are not composed of tAvo symmetrical halves, 

 and that in the empty cells there is no evidence of a suture ; unless, 

 indeed, the slit or gash occurring in those cells which have produced 

 zoospores, and by which they have escaped, be an indication of its 

 existence, while they are characterized by merely being bidentate at the 

 external margin of often the outside row of cells only, while the internal 

 are frequently of undefined outline. 



Lastly, so far as I am aware, conjugation has never been seen in this 

 genus. I have myself noticed in Pediastrum Boryanum the cell-contents 

 of certain marginal cells retracted from the external wall, and massed 

 together into a green, smooth, orbicular, spore-like body [resting spoi-c ?] 

 in the centre. But as this took place, not in neighbouring cells, and no 

 cells being empty or disturbed in form, it could not be imagined to be 

 any process of conjugation effected by the transmission of the con- 

 tents from one neighbouring cell to another. 



The diagnosis given bj^ Berkeley, in his " Introduction to Crypto- 

 garaic Botany," is as follows : — " Cells void of silex, free, or forming 

 brittle threads or minute fronds, increased by the formation of two new 

 half cells in the centre, so that the two new cells consist eacli of a new 

 and old half cell. Spores generated by the conjugation of two distinct 

 individuals." The only point of difference in the above definition from 

 Ealfs' is the introduction of the characteristic of the two new half cells 

 during division being interposed between the old ones ; but as in a few 

 species this can only be a matter of just inference, not of direct proof, 

 but of which indeed there cannot be any reasonable doubt, it cannot 

 always be insisted on. But as to Pediastrum, I have before intimated 

 that, so far as I can see, the component cells do not increase in number 

 at all, and therefore in that respect cannot agree with the terms of either 

 diagnosis. 



The figured outline of the cells, often, however, confined to the mar- 

 ginal series, yet wanting as they do bilateral symmetry, seems then the 

 reason why Pediastrum has been placed amongst the Desmidiaceae. 

 But, whilst arguing against the claims of this genus, as such, I own I 

 am myself unaware of where else to place it. Its affinity "v^dth Sydro- 

 dictyon utriculatum seems sufficiently striking. That plant, with what, 

 liowever, must appear questionable propriety, has been associated with 

 the Siphonacea) (Micrographic Dictionary), a family of which Vauche- 

 ria may, perhaps, be assumed as typical. Possibly Pediastrum and its 

 allies, with Hydrodictyon, may prove a distinct family near Volvoci- 

 nacese, with which they seem perhaps connected through Pandorina 

 and Gonium, by certain points of similarity in their development, or in 

 which at least certain parallel phases seem noticeable. 



I had written so far of the present paper some months back, and 

 have read it as I then wrote it. Since then I met with " Gattungen 

 einzelliger Algen physiologisch und systematisch bearbeitet" von C. 



