The Potato Disease. 513 



within the tissues of the diseased plant. They are borne 

 upon short foot-stalks, Avliich spring from the mycelial 

 threads. The fertilization of the oosphere witliin the oogo- 

 nium is effected by the antlieridium, wliile they arc yet 

 attached to the mycelium, or after one orbotli liavc become 

 free. "When the two bodies accidentally touch, the anther- 

 idium forces a small branch or tube into tlie wall of the 

 oogonium, and discliarges into the oosphere a part of its 

 contents. AVhen the oosphere thus fertilized become ma- 

 tui'ed, it is the oospore, or resting-spore. 



The perfected resting spore is a slightly egg-shaped body, 

 about one-thousandth of an inch in diameter. It is fur- 

 nished with two coats, the inner one being termed the 

 endospore, and the outer one, which is almost black in color, 

 and strongly warted, the exospore. After the potato plant 

 has been destroyed by the fungus, every part of its parasite 

 perishes except the dark brown, warted resting-spores, just 

 described ; and these, except such of them as are lodged 

 in the tubers, find their way, through the dscay of the leaves 

 and otlier parts of the affected plant, into tlie earth to 

 hibernate. When they awake to renewed life in tlie suc- 

 ceeding summer, they must germinate in the damp, warm 

 earth; and if no potato plants are near, they must perisli ; 

 but, if they are able to reach these plants with their corro- 

 sive mycelium, it penetrates at once the tubers or the roots. 

 Beneath the soil simple and swarm-spores cannot be pro- 

 duced ; but into the superior parts of the plant the mycel- 

 ium soon grows, and produces both these forms of fruit. 

 These are carried through the air into the breathing pores; 

 17 



