156 FUNGI. 



species as P. effusa. The conditions which control this form 

 of germination cannot be indicated, since some conidia which 

 germinate after this manner will sometimes be found mixed 

 with others, the majority of which furnish zoospores. It may 

 be that the conidia themselves are in some sort of abnormal 

 condition. 



In all the species examined the conidia possess the power of 

 germination from the moment of their maturity. The younger 

 they are the more freely they germinate. They can retain this 

 power for some days or weeks, provided they are not entirely 

 dried. Dessication in an ordinary temperature seemed sufficient 

 to destroy the faculty of germinating in twenty- four hours, when 

 the conidia had been removed from the leaves on which they 

 were produced. They none of them retained the faculty during 

 a few months, hence they cannot preserve it during the winter. 



The germs of Peronospora enter the foster plant if the spores 

 are sown upon a part suitable for the development of the 

 parasite. It is easy to convince one's self that the mycelium, 

 springing from the penetrating germs, soon takes all the 

 characters that are found in the adult state. Besides, when 

 cultivated for some time, conidiiphorous branches can be seen 

 growing, identical with those to which it owes its origin. Such 

 cultivation is so readily accomplished that it can be made upon 

 cut leaves preserved fresh in a moist atmosphere. 



In the species of Peronospora that inhabit perennial plants, or 

 annual plants that last through the winter, the mycelium hidden 

 in the tissues of the foster-plant lasts with it. In the spring it 

 recommences vegetation, and emits its branches into the newly- 

 formed organs of its host, there to fructify. The Peronospora 

 of the potato is thus perennial by means of its mycelium con- 

 tained in the browned tissue of the diseased tubers. When in 

 the spring a diseased potato begins to grow, the mycelium rises 

 in the stalk, and soon betrays itself by blackish spots. The 

 parasites can fructify abundantly on these little stalks, and in 

 consequence propagate themselves in the new season by the 

 conidia coming from the vivacious mycelium. 



The diseased tubers of the potato always contain the myce- 



