Desmidiacede 



373 



from the dislocated isthmus as a green protoplasmic vesicle containing the 

 nucleus and the more or less disintegrated chloroplasts. The fusion of the 

 gametes results in a zygospore which develops a cell-wall with three distinct 

 layers. The inner layer consists of cellulose and is thin and colourless ; the 

 middle layer is brown and firm and possibly cutinized ; the outer layer 

 consists mostly of cellulose and may be quite smooth (fig. 233 K and'O; 

 fig. 235 </) or covered with variously arranged warts or spines (fig. 233 L 

 N,PQ; fig. 234(7). 



Sometimes more than two cells have participated in the formation of a zygospore, the 

 latter having been formed by the union of three ( W. W., 5 91 A ; W. & G. S. W., '97 A) or 

 even four (Turner, '93, t. x, f. 16e) gametes. Cytological details are entirely lacking. 



Fig. 235. A, germination of zygospore of Mesotsenium chlamydosporum De Bary, showing four 

 embryos, x 390 (after De Bary). B and C, germinatiou of zygospore of Cylindrocystis 

 Brebissonii Menegh.; B, with four embryos; C, with two embryos, x 390 (after De Bary). 

 D I, germination of zygospore of Mesotsenium caldariorum (Lagerh.) Hansg., x 500 ; D G, 

 the usual germination with the formation of two embryos ; H and I, the exceptional 

 germination with the formation of four embryos. J, zygospore of Desmidium cylindricum 

 Grev., x500. 



In many cases conjugation takes place immediately after vegetative division and before 

 the young semicells have arrived at maturity, the conjugation actually occurring between 

 the two daughter-cells which have resulted from the division of the original mother-cell. 

 The present author has observed this to take place in Micrasterias denticulata, Penium 

 didymocarpum (vide figs. 233 L and 234 D and E and several species of Closterium), and the 

 same phenomenon was observed many years ago by Archer. Conjugation between adjacent 

 cells (lateral conjugation) in the filamentous species of Desmids has been recorded in the 

 genera Sphxrozosma and Spondyloxium, but such cases are exceptional and very rare. 



In a few cases double zygospores are normally produced. The best known species are 

 Closterium lineatum Ehrenb. (fig. 234 F\ Cl. Ralfsii var. hybridum Rabenh., Cosmarium 

 diplosporum (Lund.) Liitkem., and Penium didymocarpum Lund. (fig. 234 D and E}. In the 



