246 



THE FUNGI 



B 



several two-ciliate zoospores. If the conidium has germinated 

 011 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 enter 

 the host through one of the breathing 

 pores or 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 

 oogoniuni 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 oospore, which rests during 

 the winter, 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 (Phytophihora infestans) has a dif- 

 titication of the potato 



blight (Phytophthora ferent type of conidial fructification from 



infestans) Albugo. The hyphse emerge from the 



A, the air spores (conidia) leaves through the stoiuata (Fig. 217, A), 



J -, f*-l O \ O /' 



formed on long nla- , . -,. -IPI-.LI 



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

 from the interior of the j n immense quantities. These air spores 



potato leaf through the . , , 



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

 opment of zoospores in na ting in moisture develop zoospores (Fig. 



a conidium; a single . 



zoo-spore is shown at the 217, B), which infect new hosts, as in 



right.-- After Scheuck 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 



FIG. 217. Conidial fruc- 



