374 



Conjugate 



last-named species conjugation occurs between four cells resulting from two consecutive 

 divisions of one individual (G. S. W., '04). Each of the two zygospores is formed by the 

 union of a distinct pair of gametes. In the two species of Closterium in which double 

 zygospores occur, each semicell produces a gamete and each of the two zygospores results 

 from the fusion of a pair of gametes, one from a semicell of each individual. The cytology 

 of this extraordinary conjugation is not yet worked out and would obviously prove of great 

 interest. In Spirotxnia condensata Breb. two distinct and separate zygospores are pro- 

 duced from a single pair of conjugating cells (Archer, '67). 



A double zygospore of Closterium rostratum Ehrenb. has been recorded by Lagerheim 

 ('86), but although this species conjugates more frequently than any other Closterium, this 

 is the only recorded instance of a double zygospore, which must therefore be regarded as 

 abnormal. 



456 



Fig. 236. Germination of the zygospore of Closterium sp. 1, zj'gospore just before germination, 

 the nuclei of the gametes not having yet fused ; 3, the first mitosis of the fusion-nucleus ; 

 3, first division of nucleus completed ; 4, the second mitosis ; 5, completed division of 

 protoplast into two cells, each showing a large nucleus and a small nucleus ; 6, further 

 stage in germination, the cells beginning to assume a definite shape. All x 308. (After 

 Klebahn, from Oltmanns. ) 



Klebabn ('90) has observed the formation and germination of parthenospores in a 

 species of Cosmarium, and Borge ('13) has also noted the production of parthenospores in 

 abortive attempts at conjugation in Cylindrocystis Brebissonii Menegh. 



The zygospore rests for some time before germination. The first obser- 

 vations of importance on the germination of the zygospore were those of 

 De Bary ('58), and further details with regard to certain genera have been 

 furnished by Millardet (70) and especially by Klebahn ('90). The nuclei of 

 the gametes lie side by side for some time before fusion (figs. 236 1 and 

 237 A). During the conjugation of most Desmids the chloroplasts are quite 

 disintegrated, but in the ripe zygospore there is evidence in some cases that 

 two chloroplasts are reorganized (fig. 237 A and B cA). Much more detailed 

 investigation is, however, required on this point. Soon after the fusion of 



