246 



THE FUNGI 



several two-ciliate zoospores. If the conidium has germinated 

 on the proper host after a rain or heavy dew, the zoospores 

 swim over the moist surface, and finally coming to rest they 

 put forth delicate germ tubes that pene- 

 trate the epidermis of the host through 

 one of the stomata. The sexual organs are 

 generally found in portions of the leaves 

 and stems which become much swollen 

 and colored reddish or purplish. The large 

 oogonium forms a single egg and is ac- 

 companied by a single antheridial filament 

 which develops from the hypha below 

 (Fig. 216, D). The antheridial filament 

 puts forth a tube-like process which en- 

 ters the oogonium and discharges one or 

 more nuclei into the egg, fertilizing it. 

 The fertilized egg develops heavy walls, 

 becoming an ob'spore, which rests during 

 the whiter, and on germinating in the 

 spring produces a large number of zoo- 

 spores that infect new hosts. 



The potato blight, or rot. The potato 

 blight (Phytophthora infestans) has a dif- 

 ferent type of conidial fructification from 

 Albugo. The hyphse emerge from the 

 leaves through the stomata (Fig. 217, J.), 



FIG. 217. Conidial fruc- 

 tification of the potato 

 blight (Phytophthora 

 infestans) 



A, the air spores (conidia) 



formed on long fila- .,. 



ments which grow out and conidia are formed freely in the air 

 from the interior of the in immense quantities. These air spores 



potato leaf through the ... \ . , , 



stomata; B, the devel- are distributed by the wind, and germi- 

 nating in moisture develop zoospores (Fig. 

 217, B), which infect new hosts, as in 

 Albugo. Cloudy, wet, and windy seasons 

 are naturally especially favorable to the spread of the potato 

 blight. The green parts of a blighted potato plant wither, and 

 the potatoes either cannot be formed, or rot in the ground. The 



opment of zoospores in 

 a conidium; a single 

 zoospore is shown at the 

 right. After Schenck 





