540 On the Nature and Caiisest of the Decay in Potatoes, 
must be looked upon as the cause, and the fungi as the effect of 
that cause. 
Contending then that there is no a prioy'i ground for rejecting 
the idea that the nature of the disease is simply a chemical one, 
let us see how the phenomena of the disease can be explained on 
this supposition. I told you that all the products capable of 
being abstracted from tlie diseased potato, show that it is in a 
state of decay. I must again remind you that decay is really a 
slow union of organic matter with the oxygen of the air — a very 
gradual burning of the body without flame. If the disease be 
really a decay, it should therefore commence and extend where 
air or oxygen is most freely presented to it. 
In considering the general anatomy of the potato, you saw 
that the vascular tissue of the stem did not increase in the 
tuber. The enlargement of the stem so as to produce the tuber 
was owing to the great development of the cellular tissue, while 
the vascular bundles of the stems did not increase in number, but 
merely elongated themselves and diverged throughout the tuber. 
Now what is the use of these vessels ? There is now scarcely 
any doubt that the spiral vessels of plants are destined to convey 
air to the different parts of its tissue. The point at which the 
disease commences is just at the junction of the tuber with the 
stem, or exactly at that point where the air-containing vessels are 
Fig. III. Diagram representing a porlion of a vascular bundle of Potato. 
a Spiral vessel?, h h Cells of the bundiea. c c Plarcli cells, 
