132 



SYSTEMATIC BOTANY. 



OOSPORES. 

 165. Tlie third division is the Oosporeee, or plants 

 characterized by the production of a large female cell, 

 called the oogonium (Fig. 247, 0), in which is contained 

 one or more round masses of protoplasm, the oospheres. 

 These are fertilized by the contents of special, smaller 

 male cells, called antheridia (Fig. 247, a). In some 

 cases the protoplasm of the latter is transferred to the 

 oosphere by direct contact ; in other cases, it is first broken 

 up into motile bodies, called spermaiozoids, which approach 

 and fuse with the oosphere. The oosphere becomes the 



oospore, with a hard (and generally colored) covering, or 

 exospore, and is capable of germinating after a certain 

 period of rest. One mode of non-sexual reproduction in 

 this group is presented by the genus CEdogonium, where 

 large zoospores are produced (Fig. 248). Another mode 

 is the formation oi conidia, as in the White Rust {Cystopus), 

 where they are formed under the epidermis, and in the 

 genus Peronospora (to which the potato disease, Perono- 

 spora infestans, belongs), where they are produced on the 

 ends of branching hyphse that grow through the stomates 

 (Fig. 250). 



Fig. 247. Reproduction by the formation of an oospore in Peronospora Alsinea- 

 rum: o, the oogonium; a, antheridium; sp, tlie oospore. 



