24 PLANT STEUCTUKES 



(§ 13). The other cells are longer than in Ulothrix, each 

 cell containing a single nucleus and apparently several 

 chloroplasts, but really there is but one large complex 

 chloroplast. 



The cells of the filament have the power of division, 

 thus increasing the length of the filament. Any cell also 

 may act as a sporangium, the contents of a mother cell 

 organizing a single large asexual spore, which is a zoospore. 

 The zoospore escapes from the mother cell into the water, 

 and at its more pointed clear end there is a little crown of 

 cilia, by means of which it swims about rapidly (Fig. 7, 0). 

 After moving about for a time the zoospore comes to rest, 

 attaches itself by its clear end to some support, elongates, 

 begins to divide, and develops a new filament (Fig. 7, D, E). 



Other cells of the filament become very different from 

 the ordinary cells, swelling out into globular form (Fig. 7, 

 A, B), and each such cell organizes within itself a single 

 large egg (oosphere). As the egg is a female gamete, the 

 large globular cell which produces it, and which is differen- 

 tiated from the other cells of the body, is the oogonium. 

 A perforation in the oogonium wall is formed for the 

 entrance of sperms. 



Other cells in the same filament, or in some other fila- 

 ment, are observed to differ from the ordinary cells in 

 being much shorter, as though an ordinary cell had been 

 divided several times without subsequent elongation (Fig. 

 7, A, /, B, a). In each of these short cells one or two 

 sperms are organized, and therefore each short cell is an 

 antheridium. When the sperms are set free they are seen 

 to resemble very small zoospores, having the same little 

 crown of cilia at one end. 



The sperms swim actively about in the vicinity of the 

 oogonia, and sooner or later one enters the oogonium 

 through the perforation provided in the wall, and fuses 

 with the egg (Fig. 7, B, c). As a result of this act of fer- 

 tilization an oospore is formed, which organizes a firm wall 



