328 
Nature and Cause of the Potato Disease. 
mands certain conditions to enable its organism to healthily per- 
form their functions. The potato is a striking instance of this; 
its roots have a great absorbing power, and its leaves a low 
exhaling one ; and if the structure of a vegetable ever addressed 
us. in plain language as to what the plant needed in its cultivation, 
that structure is the organism of the potato plant. Its thousands 
of absorbents teach us that they have a power to gather water 
from an almost arid soil, and its products that such a situation 
best suits them; we may therefore conclude that the potato 
plant is constructed for a dry soil rather than a wet one. 
1 1 . Nature and Cause of the Disease. 
The nature of the disease is clearly that of putrefaction ; and so 
satisfactory are the proofs on this head, that I apprehend no 
doubt can arise on the subject. The results of the microscopical 
and chemical examinations are in unison with each other ; the 
former shows the destruction of the vessels, the latter that putre- 
faction arises from their decomposition. The vessels are azotised 
matter, and we have thus clear evidence that the vessels are 
broken up by putrefaction. So far, then, as the nature of the dis- 
ease is concerned, all is clear ; and could the cause be as easily 
arrived at, the subject would be simplified and brought within a 
small compass. But it is otherwise, for the cause is complicated 
and cannot therefore be so summarily dealt with. This compli- 
cation arises partly from the nature of the apparent cause, and 
partly from the difficulty in proving a cause. For all our proofs 
of a cause are necessarily imperfect, ending as they always do in 
an effect of some other cause. But notwithstanding this difficulty, 
I apprehend it will be sufficient to treat the proximate cause as 
the real one, disregarding all other influences as to how that cause 
became active ; and I shall therefore treat the apparent as the 
active power or influence, regardless of other considerations. 
From the evidence which precedes this, I shall now draw 
such inferences as will, 1 trust, throw some light upon the cause 
of the disease we are considering of. These inferences I shall 
arrange numerically, so that we may consider them in a regular 
order, and enlarge upon them, if necessary, hereafter. In ac- 
cordance with this plan, I observe — 
1st. That the season was unusually cold and wet, and marked 
by a continued absence of sun. 
2ndly. That the disease was more fatal on heavy wet lands 
than on light dry lands; and generally more so on wet soils than 
on dry soils. 
3rdly. That exhalation and evapcnation was at a low point 
during the numths of July and August. 
4thly. 'J'hat fungi are the effect and not the cause of disease. 
