216 ARCHER, ON DESMIDIACE.E. 



or statements often given are based rather on assumption 

 than on actual experience, because (Pediastrum excepted) I 

 do not find authorities given or references made to published 

 figures or recorded observations. Indeed, 1 am disposed to 

 think it not improbable that, in several instances, what is 

 meant by the authors alluded to is another and, I appre- 

 hend, a distinct phenomenon, but which is described as, and, 

 as I imagine, erroneously called, the formation of the motile 

 bodies or active gonidia, known amongst the algse as " zoo- 

 spores." 



It is indeed likely that, by some, arguing from analogy, 

 the assertion is based on the history of the propagation by 

 zoospores, as it occurs in Pediastrum, as described by A. 

 Braun {vide " Rejuvenescence in Nature," ' Ray Soc. Pub.,' 

 1853). In that genus this process occurs in the following 

 manner, of which it may not be out of place very briefly to 

 remind my hearers : — In this plant the frond consists, as is 

 well known, of a cluster of cells, disposed in a single plane, 

 generally concentrically — the marginal ones laterally and ex- 

 ternally, and in some species the innermost also laterally 

 notched. From the cells of this frond the zoospores are not 

 emitted singly, as in numerous other algre, but the entire 

 number, formed by the subdivision of the endochrome of 

 each into four, eight, sixteen, thirty-two, or sixty-four, or 

 even one hundred and twenty-eight portions, escape from the 

 parent cell, still involved in its inner membrane ; and it is 

 within this that they eventually settle down and arrange 

 themselves into a fiat cluster, resembling that from a cell of 

 which they themselves originated, each zoospore becoming 

 one of the component, mostly more or less notched or 

 bidentate, cells of the new frond. These spores are called by 

 the German writers " macrogonidia." Other fronds, how- 

 ever, give birth to smaller, more numerous, and more active 

 spores, called " microgonidia," of which the further history 

 after their escape is unknown. Notwithstanding that in all 

 our text-books, in which this genus is spoken of, it is referred 

 to the Desmidiacese, I have myself some time since come to 

 the conclusion that Pediastrum is not a Desmidian at all, and 

 I shall endeavour briefly to bring before you the considera- 

 tions which seem to lead to such a conclusion. 



I am, of course, aware of the difficulty sometimes met with 

 in satisfactorily embracing certain organisms within the terms 

 of what may occasionally appear as perhaps somewhat 

 arbitrary diagnostic characteristics ; and, while the ac- 

 knowledged fact cannot be overlooked, that no linear arrange- 

 ment can ever properly express the whole of the natural aud 



