Desmidiaceae 



375 



the two nuclei (in the Placoderm Desmids) the fusion-nucleus divides 

 mitotically twice, and after the completion of the second mitosis the proto- 

 plast also divides forming two bi-nucleated daughter-cells (figs. 236 5 and 

 237 D). In each of these cells the nuclei become differentiated into a ' large 

 nucleus ' and a ' small nucleus,' and during subsequent development the small 

 nucleus gradually disappears (fig. 237 E G), the large nucleus assuming a 

 median position and becoming the nucleus of the fully developed cell 

 (fig. 237 H). It seems probable that in the Placoderm Desmids two embryos 

 are formed from each zygospore (consult fig. 236 and 237 E), but in the 

 Saccoderm Desmids there is considerable variability in the number of embryos 

 which are produced on the germination of the zygospore. The observations 



H 



Fig. 237. Germination of the zygospore of Cosmarium sp. A, the ripe zygospore with the 

 nuclei of the gametes as yet uiifused. B, commencement of germination. C, second 

 mitosis of the fusion-nucleus. D, complete division into two cells, each with a large and 

 small nucleus. E, upper cell showing commencement of median constriction. F, one of 

 the two cells formed by germination ; large and small nuclei both in one semicell. G, further 

 stage with small nucleus gradually disappearing. H, final stage, the cell having assumed 

 its proper shape and the large nucleus having become median in position, ch, chloroplast ; 

 n, fusion-nucleus ; In, large nucleus; sn, small nucleus. All x 350 (after Klebahn). 



of De Bary tend to show that in the Gonatozygse only one embryo is produced 

 precisely as in the Zygnemacese. In the Spirotseniese there may be either 

 two or four. Four may be the general rule in this sub-family, but in various 

 species of Mesotsenium and in Gylindrocystis Brebissonii the number of 

 embryos may be only two, and in Mesotsenium caldariorum two is the usual 

 number (G. S. W., 15 ; also consult fig. 235 C G). 



The observations on the cell-division and germination of the zygospore of Mesotsenium 

 caldariorum are of particular interest in view of the attempt by Oltmanns to establish 

 within the Conjugatae a third family, the Mesotaeniaceae (consult p. 331). The removal of 

 the Desmidian Conjugates embraced in Oltmanns' Mesotseniaceso ( = Ltitkemiiller's Spiro- 

 ttenieae of the Saccodermte) from the Desmidiaceee appears to be contrary to their affinities. 



