218 ARCHER, ON DESMID1ACE.E. 



may be met with in which one of the component cells is 

 about double the dimensions of the others, while the normal 

 number is deficient by one; indicating, not the special in- 

 crease in size of one of the cells, as compared with the others, 

 but rather that the ultimate segmentation of the endochrome 

 within the original parent cell, preparatory to being con- 

 verted into zoospores, was in this one instance not fully 

 carried out. Sometimes a few marginal cells are wanting, 

 which may, perhaps, be explained in the same manner ; some- 

 times, however, they become accidentally removed by ex- 

 ternal forces. Indeed, it is hard to suppose an increase in 

 number of the constituent cells of a frond without its becom- 

 ing altered from a plane to an irregular structure, such as 

 takes place in Monostroma, Ulva, &c., the dimensions of the 

 frond itself, however, expanding by a simultaneous 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 

 two symmetrical halves, and that in the empty cells there is 

 no evidence of a suture ; unless, indeed, the slit or gasli 

 occurring in those cells which have produced zoospores, and 

 by which they have escaped, be an indication of its existence, 

 while they are characterised by merely being bidentate at the 

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

 internal being frequently of undefined outline. 



Lastly, so far as I am aware, conj ligation has never been 

 seen in this genus. I have myself noticed in Pediastrum. 

 Boryanwn the cell-contents of certain marginal cells retracted 

 from the external wall, and massed together into a green, 

 smooth, orbicular, spore-like body [resting spore ?] in the 

 centre. But as this took place, not in the 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 contents from one neighbouring cell to 

 another. 



The diagnosis given by Berkeley, in his f Introduction to 

 Cryptogamic 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 each of a new and old hall" cell. 

 Spores generated by the conjugation of two distinct indi- 

 viduals.''' The only point of difference in fcheabove definition 

 from Ralfs' 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 



