140 THE rOTATO DISEASE. 



sclmeiJer, a theory tliat has been proved by a series of observations 

 recently published in a German brochure (De Bary, Kartoffel, 

 &c.), according to this theory the symptoms of the disease would 

 be always produced immediately by the invasion of the parasite. 



It is necessary to recollect that the epidemic of which we speak 

 is characterised by symptoms clearly noted ; that it is not a ques- 

 tion of any malady whatever, but of a single disease quite special. 

 This malady ordinarily appears in the middle or towards the end of 

 the summer by spots of a blackish-brown, that appear upon the 

 haulms and fruits of the potato. The organs fade, take entirely 

 the signalised colour, and at last they dry up and rot. The plants 

 thus destroyed can bear healthy tubers, but it is too frequent that 

 these are altered in a particular manner. Their surface offers 

 wrinkled depressions of a variable disposition and extent. In cut- 

 ting the tubers, one sees the parenchyma that touches the skin of 

 the depressed parts coloured of a dark brown to a depth of some 

 miillemetres. The brown tissue appears to be more dry and more 

 compact than the normal parenchyma. When the malady has 

 made some progress the brown discolouration extends itself upon 

 the entire peripheric parenchyma, and here and there to a more 

 considerable depth, the entire surface of the tuber becomes wrinkled, 

 and of a dirty-brown colour. The parenchyma of the interior of 

 the tuber remains at first healthy and normal, but it finishes by 

 undergoing either the dry or wet rottenness, and the tuber is 

 covered with mouldiness, many times described. When one sows 

 the spores of Peronospora infestans upon the heathy leaves of the 

 potato, in taking the precautions already indicated, the germs enter 

 through the epidermis, the mycelium expands itself in the tissue of 

 the sown spot, and, at the end of a few days, there produces fruit. 

 The tissue invaded by the parasite preserves at first its greyish green, 

 later it becomes a little yellowish ; when the conidia have attained 

 their maturity, the tissue becomes of a dirty-green, softens, then 

 takes a blackish colour, and either dries up or rots. The blackish 

 spot is thus formed. The tubes of the mycelium, that are con- 

 tained there, die with the indicated alteration of the parenchyma ; 

 but those that, in the periphery of the spot, touch the healthy 

 parenchyma, extend themselves in it, to make it undergo the same 

 alterations as those just described. It is thus that the mycelium 

 takes a centrifugal development, and that this development deter- 

 mines a similar extension of the black spots. When one examines 

 the haulms taken from any field whatever, one always finds the 



