CULTIVATION. 265 



long for quotation here, except his observations on the develop- 

 ment of the threads of Peronospora infestans on the cut surface 

 of the tubers of diseased potatoes. When a diseased potato is 

 cut and sheltered from dessication, the surface of the slice covers 

 itself with the mycelium and conidiiferous branches of Perono- 

 spora, and it can easily be proved that these organs originate 

 from the intercellulary tabes of the brown tissue. The mycelium 

 that is developed upon these slices is ordinarily very vigorous ; 

 it often constitutes a cottony mass of a thickness of many milli- 

 metres, and it gives out conidiiferous branches, often partitioned, 

 and larger and more branched than those observed on the leaves. 

 The appearance of these fertile branches ordinarily takes place 

 at the end of from twenty-four to forty-eight hours ; sometimes, 

 nevertheless, one must wait for many days. These phenomena 

 are observed in all the diseased tubercles without exception, so 

 long as they have not succumbed to putrefaction, which arrests 

 the development of the parasite and kills it. 



Young plants of the species liable to attack may be inoculated 

 with the conidia of the species of Peronospora usually developed 

 on that particular host, in the same manner that young cruci- 

 ferous plants, watered with an infusion of the spores of Cysiopus 

 candidus, will soon exhibit evidence of attack from the white 

 rust. 



It is to the cultivation and close investigation of the growth 

 and metamorphoses of the minute fungi that we must look for 

 the most important additions which have yet to be made to our 

 knowledge of the life- history of these most complex and interest- 

 ing organisms. 



