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THE POTATO DISEASE. 141 



same deTelopment of the parasite, and the same extension of the 

 spots. Tlie mycelium always occupies at first the green and 

 healthy tissue, that, the fructification of the parasite being finished, 

 becomes softened and browned. One cannot, then, doubt that the 

 s]3ots of the leaves may not be produced by the parasite that has 

 entered them. And as to the rapid propagation of the disease, it 

 explains itself by the great quantity of sporangia that the parasite 

 produces, and by the rapichty of its development. We must re- 

 member that the reproductive organs of Peronospora are already 

 abundantly developed when we observe in a field the first traces of 

 the disease. It is true that, according to the facts already ex- 

 plained, the sporangia and the spores of the parasite require water 

 to take their normal development, but the results of the experi- 

 ments accord very well with what is observed in the cultivation on 

 a large scale, where the progress of the disease is always more 

 rapid when the weather and the aspect of the field are more 

 favourably situated for the aqueous precipitations of the atmos- 

 phere, whilst drought arrests the development of the parasite, and 

 the progress of the disease. The appearance of the fungus on the 

 fruit of the potato and allied plants, especially the tomato, has been 

 known for a long time, and it is known that similar alterations are 

 there produced to what occurs upon the leaves. 



As to the brown spots that are found upon the stalks and the 

 petioles of the diseased haulms, it has often been denied that the 

 parasite is found there, because one finds but rarely the fruit at 

 the surface. Nevertheless, it is always enclosed. The mycelium 

 that crawls among the cellules of the compact tissue is always diffi- 

 cult to meet with. The intercellulary passages appear to be filled 

 with granular matter, that nevertheless, in good preparations, show 

 the proper membrane of the tubes of the mycelium. Their nature 

 can be placed beyond doubt when the spots are strongly moistened. 

 The doubtful tubes can then be seen shooting out their branches • 

 these perforate the cellules, elevate themselves to the surface, and 

 there engender the normal fruit of Peronospora. Besides, one can 

 easily obtain the same results that are observed in the spontaneous 

 state, in sowing the parasite u[)on the stalks of the potatoe ; it is 

 by this sowing that the alterations of the tissue are directly deter- 

 mined by the vegetation of the endophyte. In the altered tissues 

 of the leaves it is principally the contents of the cellules of the 

 parenchyma that undergo the discoloration ; the membranes take 



