MR. BERKELEY ON THE POTATO MURRAIN. 23 



Some have attempted to account for it by the notion that in 

 consequence of long culture, and from the practice of raising 

 plants from cutting?, not from seed, the species is exhausted and 

 incapable of producing healthy offspring. But this depends upon 

 mere tlieory, it being more than doubtful whether any species or 

 variety does really become exhausted by long culture or by tlie 

 practice of using cuttings, however popular the notion may be ; 

 and were the notion well grounded, whether its degeneracy 

 would produce, and tliat so suddenly and universally, the effects 

 attributed to it ; and the fact that the newest seedling varieties 

 have suffered* quite as much as the old ones at once overthrows 

 this theory. Others again have attributed it to over-manuring 

 and the use of guano. But tliis is contradicted by the fact that 

 all soils were subject to the disease, however poor or rich, and 

 whatever the nature of the manure might be with which the land 

 had been prepared. 



It has also been conceived that the disease is a sort of epidemic 

 analogous to cholera, arising from some pestilent emanation or 

 specific poison in the atmosphere affecting the health of the plant. 

 Others again affirm that it is due to microscopic insects wafted 

 by the air. M. Gruby goes beyond the rest, and sees three 

 causes at work at the same time — a parasitic insect, a specific 

 disease, and a parasitic fungus. The^e conjectures however, it 

 is obvious, are not supported by facts, and cannot be adopted 

 provided other more tangible causes can be assigned with any 

 degree of probability. 



We come now to the theory which has been so much can- 

 vassed, and which is now peculiar almost to Dr. Morren, unless 

 M. Payen is to be reckoned also as its advocate. Of this opi- 

 nion, notwithstanding the opposition, and in some instances the 

 ridicule almost, with which it has been assailed, I must, as said 

 above, profess myself at present. I do not mean to say that there 

 are no difficulties in the case, or that weighty objections may not 

 be raised, but I tliink that these difficulties have been exaggerated, 

 Avhile in other instances the exact question has not been understood. 

 It is agreed that the disease commences in the leaves ; and in those 

 instances where the mould has not been observed, it is probable 

 that the decayed foliage was examined too late. In a few hours 

 the fungus has run its course, and in a week or so the greater 

 portion of a field is laid \\aste so as to make it difficult to get 

 specimens for examination. The decay is the consequence of the 

 presence of the mould, and not the mould of the decay. It is not 

 the habit of the allied species to prey on decayed or decaying 



* See especially Gard. Chron. 1845, p. 768, where this part of the subject 

 is admirably discussed. 



