22 DUBLIN NATUEAL HISTORY SOCIETY. 



draw attention to what is stated in books on the subject of the occur- 

 rence of zoospores in this family. I believe every writer in our text- 

 books on microscopic organisms, when touching on Desmidiacese, states 

 it as a fact, that, like various other algae, they are propagated by zoo- 

 spores ; while some go more or less into details, I am induced to say, 

 very deferentially, that I think the descriptions or statements often 

 given are based rather on assumption than on actual experience, be- 

 cause (Pediastrum expected) I do not find authorities given or refer- 

 ences made to published figures or recorded observations. Indeed, I am 

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

 meant by the authors alluded to, is another, and I apprehend, 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 algae as ''zoospores." 



It is indeed likely that, by some, arguing from analogy, the asser- 

 tion is based on the history of the propagation by zoospores, as it occur- 

 in Pediastrum, as described by L. Braun (vide " Eejuvenescence in 

 Natui-e," Eay. Soc. Pub., 1853). In that genus this process occurs in 

 the folloAving maimer, 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 con- 

 centrically — the marginal ones laterally and externally, and in some 

 species the innermost also laterally notched. Prom the cells of this 

 frond the zoospores are not emitted singly, as in numerous other alg£e 

 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 ceU, still 

 involved in its inner membrane ; and it is within this that they even- 

 tually settle down and arrange themselves into a flat cluster, resembling 

 that from a cell of which they themselves originated, each zoospore be- 

 coming one of the component, more or less notched, cells of the new 

 frond. These spores are called by the German writers " macrogonidia." 

 Other fr-onds, however, give birth to smaller, more numerous, and more 

 active spores, called " microgonidia," of which the farther 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 Desmidiacete, I 

 have myself some time since come to the conclusion that Pediastrum' is 

 not a Desmidian at aU, and I shaU endeavour briefly to brino- before 

 you the considerations which seem to lead to such a conclusion. 



I am, of course, aware of the difficulty sometimes met with in satis- 

 factorily embracing certain organisms within the terms of what may 

 occasionally appear as perhaps somewhat arbitrary diagnostic charac- 

 teristics; and, while the acknowledged fact cannot be overlooked, that 

 no linear arrangement will ever properly express the whole of the na- 

 tural and mutual affinities of organic objects ; and while at the same 

 time I will not deny, in regard to certain organisms which seem to be 

 incongruously united with certain groups or families, that it sometimes 

 happens, while our present state of knowledge as to their nature and 



