328 



FUNGI 



observed, however, and fertilisation takes place in the usual way. A 

 very small quantity of gonoplasm (not visibly differentiated) passes 

 over into the oosphere. The antherids and oogones arise together 

 in this species, and develop in close connection. The oosperms 

 form each a promycele, as described above. The sporangiophores of 



each branch 



FIG. 288. Simple sporophores of Phytophthora infestans de By. a, formation of first 

 spores (zoosporanges) at ends of branches*: b, two ripe spores on each branch and a third 

 being formed (x about 200). (After de Bary.) 



Phytoph|hora, which resemble those of Peronospora in general habit, 

 differ from them in the fact that each branch bears more than one pro- 

 pagating body not in chains, like Cystopus, but at intervals on the 

 branch. In P. infestans a propagating cell is produced at the apex of 

 and as it ripens a papilla-like swelling arises beneath it ; 

 the branch grows on and turns 

 the cell aside. These propagat- 

 ing cells are usually zoosporanges, 

 but not unfrequently they are coni- 

 diospores, differing from them in 

 no other respect than the pro- 

 duction of a germ-tube directly 

 instead of zoospores. The zoo- 

 spores are formed within and 

 escape directly from the zoospo- 

 range itself, as in Peronospora. 

 Phytophthora infestans has a special economic interest, as the cause 

 of the well-known r&ta|D-disease. The "disease first appears, as a rule, 

 on the green leaves of the potato plant in July or August, the sporangio- 

 phores emerging through the stomates. Sporanges are formed, under 

 favourable conditions of temperature, moisture, &c., in a few hours, are 



FIG. 289. Zoosporanges of Phytophthora infestans 

 de By. a, division completed ; b, escape of zoo- 

 spores ; c, free zoospores ; d, spores come to rest 

 and germinating (x 390). (After de Bary.) 



