1846.] Cause of the Potato Disease. 197 



We do not suppose that all the phenomena accompanying the 

 potato rot, as it is sometimes called, can be satisfactorily ex- 

 plained on the ground that it is owing to the state of the atmo- 

 sphere. But obscurities belong to every other hypothesis which 

 has been offered. We cannot tell why, during the prevalence of 

 an epidemic, many escape its attack. Why, too, many trees 

 of the same species have escaped the blight; why many fields of 

 potatoes remain unaffected. It is quite difficult to be satisfied 

 why it is that potatoes should rot merely because the leaves have 

 been injured in part. It is difficult to determine whether the af- 

 fection of the herbage of the vine is a symptom of disease merely, 

 or one of the causes connected closely with the decay of the tu- 

 ber; and it is difficult to determine, on the other hand, whether 

 the decay of the tuber produces the destruction of the herbage. 

 These are the facts, however, the decay of the latter seems to 

 precede the former, and the latter too is exposed to the withering 

 influence of hot and dry winds. This, when taken in connection 

 with the importance of the leaf to the life of the plant, seems to 

 thi'ow some light upon the subject. Tubers and roots to be 

 healthy and sound require healthy leaves; and whatever injures 

 their integrity, injures them also. This is a well known fact, 

 still it is after all strange that the tuber should rot; that it 

 should go through with regularity all the stages of a specific 

 disease. But here again we must admit that the obscurity is 

 not increased by the foregoing hypothesis. The insect and 

 fungus hypotheses can claim no advantages on this ground. 

 If plants are stripped of their leaves they usually die; they may 

 survive the operation once; they are more likely to survive if 

 the stem is cut directly off, unless it be below all the leat-buds, 

 as at the neck of the plant. Mowing the potatoe vines does not 

 kill the plant, as the stem below contains many leaf-buds; but it 

 is injurious to the crop under any circumstances. The fact that 

 it survives and sets new tubers afterwards is no objection to this 

 view. 



We believe it will be found generally true, especially with 

 some varieties, that if they are highly manured and grow rapidly, 

 and are full of the peculiar juices of the plant, that they will suf- 



