38 



MANUAL OF BOTANY 



The asexual reproductive cells are frequently borne upon 

 the gametophyte. They may be ciliated cells, differing but 



Fig. 789. 



Fig. 789. A. sp. Xe\vl\' formed zoospore or zoogonidium of Vaucheria 

 sessilis escaping. B. Zoospore at rest after baviug lost its cilia, 

 c. First stage of germination, i). Filament of Vaucheria sessilis pro- 

 ducing oogonia, og, og. and autlieridium, h. w. Hj"aline root-like 

 process, forming a sort of mycelium, sq. Zoospore, which by germinating 

 has formed the filament. After Sachs. 



FiG.TeO 



, — f^p. \i\^^\q^ if at all, in appearance from 

 the gametes. These are kno^^oi as 

 zoogonidia, from their power of move- 

 ment. In other cases they are non- 

 motile. They are at first alwa3's 

 naked cells. Sometimes they are 

 developed in special organs, the goni- 

 clang ia ; in some forms any cell of 

 the thallus may produce them. 

 Where the sporophyte exists it gives 

 rise to spores similar in structm^e to 

 the gonidia. In many of the green 

 Algae the zj^gospore or oospore gives 

 rise to a number of zoospores, either 

 with or without preliminary cell-divi- 

 sion. It must in such cases be re- 

 garded as a very rudimentary sporo- 

 phyte. 



Vegetative reproduction is very 

 common in the lower forms ; in the 

 unicellular Algte, each cell-division is 

 often followed by a separation of the cells, showing thus the 

 simplest example of this mode. In Sphacelaria, gemmae are 



/•"/;/. 7'JO. Procarp of one of the 

 lied Seaweeds. //-.Trichogyne. 

 $}}• Spermatia. After Kny. 



