296 



PART IV. — VEGETABLE TAXONOMY. 



formed, therefore, consists of an old and a newly-formed serai 

 cell. This mode of multiplication is illustrated in Fig. 493- 



a b c 



FlG. 493. — Cosmarium Botrytis, a Desmid, showing process of division, a, the mature 

 plant; b, the plant in an early stage of division, with two new semi-cells forming between 

 the old ones; c, the process nearly completed. Magnified about 500 diameters. 



Sexual reproduction takes place usually late in the season, 

 and consists in the conjugation of two cells which come together 

 for the purpose. The zygospore differs considerably in appear- 



b'" 



r—b° 



b-} 



t> IG. 404. — Conjugation of Cosmarium Botrytis. a, zygospore; b, 6', b" and b"', the 

 empty semi-cells of the two plants. Magnified about 500 diameters. 



ance in different species. Sometimes it is smooth and spheri- 

 cal, at others warty or tuberculate, and in still others spiny, as in 

 Fig. 494, which illustrates the sexual reproduction of Cosmarium 

 Botrytis. 



