O'Meara — Heport on the Irish Dudomacece. 241 



frustules were poured ; the prolongations became gradually more and 

 more constricted at the base, until ultimately they were completely 

 cut off from the mucous sac, in which the frustules remained folded, in 

 a state of perfect inanition. The process described was completed 

 within the space of a few hours, so that in innumerable instances I was 

 able to trace it from beginning to end — that is, from the commence- 

 ment of conjugation up to the formation of the sporangia. Smith refers 

 to cases of Cocconema cistula, and also of Synedra radians, haying been 

 found aggregated in great numbers and enclosed in mucous sacs simi- 

 lar to what has been described in the case of Diatoma vulgare ; and 

 all three cases seem to me to represent the same phase of conjugation: 

 and I am disposed to think that, as in Diatoma Tulgare, so in the 

 other two cases, the encysted fi'ustules were not, as Smith considered 

 them, young fi'ustules in course of development from a sporangium, 

 but parent frustules preparing to produce sporangia. 



Instances of conjugation in any of its varied forms are rarely to be 

 met with. "WTien Smith published his Synopsis, in 1856, cases had 

 been observed in thirty species, included in seventeen distinct genera ; 

 and during the inteiwal of fifteen years that had elapsed when Pfitzer 

 published his work, " Uber Bau und Entwicklung der Bacillariaceen," 

 only twenty-eight cases had been added to the list, exclusive of that 

 of Diatoma vulgare, making a total of sixty-one. This remarkable fact 

 Smith thus endeavours to account for : "During conjugation the process 

 of self-division is aiTested, the general mucous envelope or stratum 

 produced during self-division is dissolved, and the conjugating pairs 

 of fi'ustules become detached from the original mass ; they are thus 

 more readily borne away and dispersed in the surrounding currents, or 

 by the movements of worms or insects, and their detection becomes in 

 consequence more casual and difficult." It is not improbable, however, 

 that the mode of collecting, and the time that is often suffered to elapse 

 before the collection is submitted to investigation, may have more to do 

 with the fact. And, in confirmation of this view, I would mention 

 that, although I have for very many years been engaged in the study 

 of the Diatomacese, and have made innumerable collections at all 

 seasons of the year, I have not been so fortunate in observing instances 

 of conjugation as some friends whose collections have been made with 

 a view to the discovery of other organisms. Their gatherings are 

 usually made in large bottles containing a considerable quantity of 

 water, by which the specimens may be preserved for a long time in 

 their normal state — my gatherings being put up in minute bottles with 

 little water, so that the \igour of the frustules is greatly abated be- 

 fore an opportunity of examining them may be afforded. As to the 

 seasons of the year in which conjugation is most likely to occur, the 

 facts hitherto accumulated do not afford much infoi-mation. Besides 

 the case of Diatome vulgare which I observed in conjugation in the 

 month of August, seventy-two observations, with specification of date, 

 have been recorded, making seventy-three in all. Of these, twenty- 

 three occurred in spring, twenty in summer, twenty-four in autumn, 



