The Potato Disease. 519 
cause, when such numbers of fields this summer have yielded a 
most luxuriant crop of flowers and foliage with very little, if any, 
indications of the disease. 
Another distinguished writer attributed the disease to fungi 
forming within and without the tubers. It is agreed that the 
diseased tubers are infested with various fungi, but this is true of 
most decaying vegetable matter, and fungi are more probably the 
consequence than the cause of the disease. 
Another cause assigned by some individuals and feared by 
more, is that the Potato, being all of them varieties of the same 
plant, and propagated by tubers so as to be considered the con- 
tinuation of an individual, has lived through its allotted period 
and is disappearing under the infirmities and decrepitude of old 
age. This is the force of the prediction ascribed to Cobbett, and 
resembles many of the absurd notions of that generally wise man. 
Though the oak and many vegetables attain to a certain age and 
then die, it is yet to be proved that propagations by tubers, which 
are the mere reservoirs of the most nutritious matter for the use 
of the new plants to be developed from the eyes or buds, are 
governed by any such law. Besides, new forms of the Potato 
have been propagated from the seeds in the balls of this plant. 
As probable is it that wheat, produced from its seeds, shall in like 
manner be exterminated. Add to this that it has not been the 
course of Divine Beneficence to destroy the food of man utterly 
or without a better substitute. 
From the influence of light on growing vegetables, others have 
asserted that there is a deficiency in the quantity of light sent 
from the sun to the earth, and that the disease originates in this 
supposed fact. The difficulty with this view is, that the fact is 
not known to exist. It is far more probable that light is sent to 
the earth in its usual amount. Why should not other plants be 
affected by this supposed cause of the disease? 
A later notion is urged by Mr. Smee, a British surgeon of emi- 
nence and the author of an excellent work on Electro-Metallurgy. 
Mr. Smee ascribes the disease to an insect which infests and 
infects the leaves of the plant, and destroying the leaves produces 
a diseased state of the fluids which affects and often destroys the 
stem and the tubers. The solid portion of the sap is extracted 
by the insect, and the remaining sap, being " too fluid," does not 
perform its proper office, and the leaf and stem decays, while a 
gangrene, dry or humid, for Mr. Smee finds both in the roots, 
destroys the tubers. 
This insect is named by Mr. Smee, Jiphis vastator, or Aphis 
the Destroyer, from its deadly ravages, and he believes it to be 
the Aphis rapae of Curtis, which was so named because it infested 
the white turnip. The Aphides form a large family of small in- 
