Page:Transactions NZ Institute Volume 15.djvu/342

Mr. Smith (loc. cit.) says that Closterium ehrenbergii stands alone amongst the European Closteria in producing double zygospores. It is, therefore, not uninteresting to have to add to it in this respect a plant from New Zealand. But I have some doubt whether Mr. Smith's statement is altogether correct, in view of a noticeable feature in the conjugation of the next plant on my list, which affords, I think, foundation for a closer study of the phenomenon in connection with other plants of the genus. As a rule the conjugation of Closterium is, in a sense, simple enough: two fronds approach, join, open at a suture, and a zygospore is formed between them. If, as in C. rostratum, the fronds open at the median suture, the segments attached to the zygospore will be equal in length: should there be secondary sutures as in C. intermedium, the fronds may open at these and the segments will be unequal, but the inequality will be easily intelligible. In the case of

C. acerosum, Schrank,

the process, to a certain extent, resembles that in C. selenæum. That is to say, the segments attached to the zygospore are unequal, although there are no secondary sutures. The inequality is shown in my figure 18b, where each frond has one long arm and one very short one. This inequality is also shown in Ralfs' plate xxvii, but no reference is made to it in the text. Mr. Archer, in Pritchard's "Infusoria," likewise says nothing of it. Von Siebold, in the Journal of the Micros. Society, 1853, seems to refer to something of the kind, though I do not understand his expression: he speaks of "only the two upper and lower halves" coalescing, a phrase which may mean anything.

In the spring of last year I gathered on one occasion a small quantity of C. acerosum in conjugation. Although unable to watch the process from its commencement, I examined the gathering with great care. There must have been several hundreds of plants in it, and they were all surrounded with a common mucous envelope, and not segregated in pairs as in C. selenæum. When the mass was first placed on the slide many of the fronds were already in full conjugation, and many others had completed the process. A small proportion (less than one in ten), presented the normal form of the plant, with two equal arms, as in my figure 18a, the uppermost figure. A few more appeared as the second shown in fig. 18a, and the rest had still shorter arms, the greater number of all being as in my lowest figure, with one arm almost an equilateral triangle. Conjugation invariably occurred between two fronds of this last form, never in any of the others.

If, in the conjugating fronds, I had detected any folds or wrinkles in the cell-wall of the shorter arms, I could have concluded that in the process that arm, for some reason or other, shrank up. But no such folds were