POTATO DISEASE. 281 



The theory put forth by Mr. Smee, an English experimenter, 

 maintains that the disease is produced by an insect. This 

 hypothesis has been advocated by several writers who have stated 

 many important facts to Corroborate their views. Mr. Smee 

 found, however, that when he placed the insect that infested 

 the unsound tuber upon one that was sound, they would not 

 remain upon it, but left it. Hence it is to be inferred that they 

 will not feed upon the potato unless it be iii a corrupted state. 

 It is also worthy of remark, that the different advocates of 

 this theory have attributed it to several different species of 

 insect. This fact lessens the probability that their suppositions 

 are correct. Indeed, the same reasoning that has been used to 

 controvert the fungus theory, renders it probable that the 

 insects do not produce the disease, but they are attracted, if not 

 generated, by the corrupt matter which is consequent upon it. 



Mr. Bradford's hypothesis, which, if correct, explains the 

 cause, rather than the nature of the disease, supposes it to be 

 the consequence of the " running out" of the plant from the 

 effects of old age. The tubers, he considers, not as the literal 

 offspring of their predecessors, but as continuations of the 

 individual from which they were originally derived ; as grafts 

 are but continuations of the parent stock, and not its literal 

 offspring. Hence, as scions are supposed to suffer the effects of 

 old age at the time when the original tree from which they 

 were taken would suffer these effects, the tubers of the potato 

 are supposed to be liable to a similar decay after an indefinite 

 period of time. 



There is some fallacy in this idea. Nature's laws respecting 

 animal and vegetable propagation are far from being identical ; 

 and they differ in this important respect — that while animals 

 can be propagated only by the union of the sexes, plants may 

 be propagated both in this way and in another way which is 

 independent of sex. The assumption, that a graft or a scion is 

 a continuation of the tree from which it was taken, is true only 

 in a qualified sense : it is really but a continuation of the mul- 

 titude of plants sustained by the tree in one vast community, 

 each individual of which dies annually. A tree is a family of 

 individual and annual plants, deriving their nourishment from 

 one common source, and supported in their position by the 

 branches, which serve also as conveyances of nutriment to the 



36 



