PHYCOMYCETES— (a) OOMYCETES 



409 



those harvested. But such tubers often decay during the winter. 

 If however, they are not heavily infected and are used as " seed ' 



d 



9 



J 



Fig. 308. 



Stages of germination of one of the sporangia of Phytophthora. a, the ripe 

 sporangium in water, b, protoplasmic contents breaking up into blocks, which 

 separate and escape (c, d) as minute kidney-shaped zoospores (e), each with two 

 cilia. /, g, the zoospores coming to rest, and losing its cilia, h, i, j, k, stages of 

 germination of the zoospore. Highly magnified. (After Marshall Ward.) 



potatoes for a new crop, the mycelium perennating in the tubers provides 

 a source of infection in that sporangiophores develop on short aerial 





Fig. 309. 



Germination of zoospores of Phytophthora on the epidermis of Potato. At (a) 

 the germ- tube is entering a stoma. At (c) it bores directly through the cell-wall. 

 Highly magnified. (After Marshall Ward.) 



