156 The Scottish Naturalist. 



object. It is also of the greatest specific importance. For the zygo- 

 spore, as it is called, presents uniformly the same characteristics for 

 a given species. Hence, if two species of Desmids look so much 

 alike that one might be disposed to consider them to be one and the 

 same species, yet if we find them conjugated, and the zygospores of 

 the two prove different, this shows that they are distinct. It will 

 readily be admitted, therefore, that it is of no little consequence to 

 the investigator to find species conjugated. Still, many species have 

 never been found conjugated, though every year is reducing the 

 number of these. In the vast majority of cases, however, there is 

 little difficulty, with due care, in discriminating species, even when 

 not conjugated. But, when perfected, what is the object of the 

 zygospore, and what becomes of it ? Wolle, the author of an im- 

 portant work on the Desmids of the United States (published three 

 years ago), gives as his opinion that multiplication by cell-division 

 tends to impoverish the species, and that the process of conjugation, 

 and the consequent formation of a zygospore, is necessary for its 

 regeneration. I am not prepared to controvert this idea; but I must 

 remark on it that many species that I have never seen to conjugate 

 show no signs of degeneration or impoverishment, by which, I sup- 

 pose, he means a slight falling away from the type. There must be 

 some reason why the two modes of increase are provided ; but 

 apparently it has still to be discovered. As to what becomes of the 

 zygospore, Wolle very well summarizes the views of De Bary, as 

 follows : " The next step, so difficult to be traced, is the opening of 

 the wall of the zygospore, setting free small spheres of sarcode ; as 

 they issue, they enlarge and acquire a gelatinous or thin membranous 

 wall. The wall thickens and the sphere enlarges, the contents con- 

 strict, first in one direction, and then transversely to the plane of the 

 first incision ; these plants develop and set free two or four new 

 plants, in size and form like the mother cell, except in the cytioderm 

 or membrane ; this is not granular, but smooth, and remains so 

 until after the multiplication by division takes place. After the first 

 division, the new semi-cells assume the characteristic granular surface; 

 the result of this first division is two plants, each composed of one 

 granulated and one smooth semi-cell. The second division wiU 

 make two perfect cells, and two that retain the smooth semi-cell ; 

 the third division produces eight cells, all of which except the original 

 two semi-cells will be of typical form." 



These views of De Bary were published in his treatise on the 

 Conjugate in 1858, and practically no advance has been made since. 

 Notwithstanding the hundreds of zygospores observed by myself, I 



