144 THE POTATO DISEASE, 



the coiiiJia are easily found at a considerable depth. These facts 

 prove, then, that the conidia are carried to the tubers bj the water 

 ^Thich penetrates the soil, that this liquid determines the dovclop- 

 nient of the spores, and the germs in the soil even, and that these 

 invade the tubers, there to produce the known alterations. 



One can thus suppose that the mycelium enclosed in the leaves 

 can arrive in the tubers in descending through the tissues of the 

 stalk. This is a supposition which appears to me possible enough, 

 but which I have not been able exactly to verify. If it be thus, 

 there will be a second way by which the jDarasite can reach the 

 tubers. However it may be, the first way, of which the existence 

 is directly proved, appears to me perfectly to explain the phe- 

 nomena in question. 



One will easily understand, by what has been stated, why fre- 

 quently the leaves of a field are entirely destroyed by the parasite? 

 while the greater part of the tubers remain unhealthy. However 

 great the number of the conidia fallen upon the soil may be, they 

 cannot penetrate when there is no water to carry them down ; they 

 can encounter on their way numerous difficulties, at last they can 

 reach the tubers without the quantity of water contained in the 

 soil being sufficient to determine the development, and introduc- 

 tion of the germs. The want of water can equally arrest the 

 growth of the mycelium if this were capable of descending through 

 the stalks into the tubers. The observation mentioned is not then 

 in contradiction to the theory advanced, but quite the contrary, I 

 believe that it receives its explanation by it. It is the same, it 

 appears to me, for all the observations that have been made in 

 the culture on a large scale, and I dare say that these obser- 

 vations will necessarily agree with a theory that is founded upon 

 conclusive experiments. 



I remind the reader here that the first appearance of the 

 parasite in the season of cultivation has been explained in one of 

 the preceeding paragraphs by the faculty that the mycelium con- 

 tained in the diseased tubers possesses of conserving its life dur- 

 ing the winter. Indeed, potatoes are frequently found of which 

 one part of the parenchyma is infested by the Peronosjwra, while 

 the rest remains healthy so long as the tubers are preserved in 

 a dry place. It is by such tubers that the parasite is preserved, 

 and probably by this means was introduced into this country. 



As regards the Mucedines that infest the diseased potatoes, 



