THE POTATO DISEASE. 143 



tion tlie surface of tlie slice covers itself with tbe mycelium and 

 conidiiferous branclies of Peronospora infestans, and it can easily be 

 proved that these organs derive their origin from the intercelliilary 

 tubes 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 millemetres, and it gives out 

 conidiiferous branches, often septate, and larger and more branched 

 than those observed on the leaves of the potato. The appear- 

 ance 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 tubers, without exception, so long as they have 

 not succumbed to putrefaction, which arrests the development of 

 the parasite and kills it. 



One can easily imagine, after what has been said, that the 

 Peronospora immediately determines the disease of the tubers, as 

 well as that of the leaves, and this supposition is perfectly proved 

 by experiment. When one sows Peronospora upon a healthy tuber 

 one sees the germs of the parasite penetrate into the superficial 

 cellules, spread itself in the peripheric parenchyma, and produce 

 the same alterations which are observed upon the tubers taken 

 from the field. It is indifferent whether the tuber experimented 

 upon be cut or entire, exposed to the air, or placed in a humid 

 soil ; the parasite ordinarily only fructifies upon the cut surfaces. 

 In the parts of the tuber that are protected by the skin the 

 mycelium remains sterile, or at least, only fructifies when a potato 

 furnished with a thin, fine skin, is exposed to excessive humidity ; 

 a condition which increases the A^egetati^n of the parasite. 



How can the mycelium of the parasite reach the tubers in the 

 ordinary culture of the potato ? There is no doubt that that takes 

 place by the aid of sporangia. When healthy tubers are placed in 

 the earth, at a depth of from one to two centimetres, and when one 

 sows the conidiaof Peronospora on the surface of the earth, watered 

 from time to time, one sees at the end of from eight to ten days 

 the tubers attacked by the disease. This commences in the tuber 

 on the side turned towards the soil. It offers all the symptoms 

 that have just been explained. It is not necessary in these experi- 

 ments to wet the earth excessively ; a moderate watering suffices. 

 When the earth that serves for the experiment is examined, or the 

 soil of a field of which the leaves are invaded by the Peronospora, 



