23 NATURAL HISTORY SOCIETY OF DUBLIN. 



a figured outline cannot be assumed as an essential, though, it is very- 

 frequent, and affords classificatory characters. 



Is there any evidence of such a mode of cell-increase in Mesotae- 

 nium ? I fancy there is. I refer to PL I., Pig. 6, in which the chlorophyll- 

 plate is divided and a division of the cell itself is taking place. Then 

 the two portions of the chlorophyll-plate appertaining one to each 

 daughter- cell, in place of presenting in side, i.e. edge-view, the some- 

 what fusiform figure, pointed at both ends, which is seen in a fully grown 

 cell, now show the end near the septum which halves the mother-cell 

 to be bluntly rounded. I conclude, therefore, that its recovery of the 

 fusiform and pointed outline at the blunt ends may be due to a new 

 growth onwards in that direction, therefore accompanied by the growth 

 of a new half cell, the older halves of each remaining unchanged. But 

 I imagine the probability of a new half to the chlorophyll-plate being 

 formed in continuation of an old one is strengthened by the fact, that in 

 dividing cells, when discernible, the corpuscle therein imbedded is found 

 near the end towards the septum, and in fully-grown cells at the middle. 

 I therefore suppose it must be inferred that this change of position of the 

 embedded corpuscle is not due to any inherent movement of its own, but 

 to an elongation of plate and cell at the end nearest to which it at first 

 lies, or that at which division has only recently taken place. A similar 

 argument I before applied as some proof of the Desrnidian nature of my 

 Leptocystinema Kinahani* Again, I have stated that in our present 

 plant, Pigs. 9 to 1 4, in the act of conjugation, a shedding of the parent 

 cell-wall takes place, accompanied by a splitting, as it were, through a 

 suture, indicating, as in the Desmidiacese, the point of union of the half- 

 cells. It may be worth noting, too, that in Mesotcenium chlamydosporum 

 the free inner spore, finally formed within the zygospore, seems to find 

 a parallel in the similar occurrence in Tetmemorus Icevis. All these 

 characters point strongly to the Desmidiacese. 



But, on the other hand, Mesotcenium Braunii and M. violascens (De 

 Bary) seem to conjugate by complete participation of the parent-cell- 

 membranes in the act; cases, too, in the present plant are not rare in which 

 the parent membrane cannot be detected, but it must, in such cases, 

 have become either lost or dissolved. Al. Braunf seems to consider 

 that the genus Palmogloea (Kiitz) is more Palmellacean than Desrnidian, 

 but thinks that the greatest distinction between Cylindrocystis and Pe- 

 nium is the participation in the former of the outer cell membrane in 

 the act of conjugation. But our plant presents an example of a " Pal- 

 mogloea " in which the cell-membrane does not co-operate in the conju- 

 gative act. 



But, except that they are Conjugate, exhibiting, according to De 

 Bary's researches, the character, dwelt on by him, of capability of self- 

 division in all the daughter-cells originating from the Zygospore, I do 



* " Natural History Review," 0. S., vol. v., p. 243. 

 f Op. cit. (English translation), p. 135. 



