38 



MANUAL OP BOTANY 



The asexual reproductive cells are frequently borne upon 

 the gametophyte. They may be ciliated cells, differing but 



Fia. 789. 



Fig. 789. A. sp. Newly formed zoospore or zoogonidium of Vaucheria 

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

 c. First stage of germination. D. Filament of VaucJieria sessilis pro- 

 ducing oogonia, 0(7, og^ and autheridium, h. w. Hyaline root-like 

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

 has formed the filament. After Sachs. 



Fra.790. 



little, if at all, in appearance from 

 the gametes. These are known as 

 zoogonidia, from their power of move- 

 ment. In other cases they are non- 

 motile. They are at first always 

 naked cells. Sometimes they are 

 developed in special organs, the goni- 

 dangia ; in some forms any cell of 

 the thallus may produce them. 

 Where the sporophyte exists it gives 

 rise to spores similar in structure to 

 the gonidia. In many of the green 

 Algse the zygospore 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 Algse, each cell-division is 

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

 simplest example of this mode. In Sphacelaria, gemmae are 



Fig. 79LI. Procarp of one of the 

 Bed Seaweeds, /r. Trichogyne. 

 sp. Spermatia. After Kny. 



