318 
Nature and Cause of the Potato Disease. 
eggs of the CEstrus oris in the head ; and the forest-fly (Hippo- 
bosca equina) is frequently troublesome to cattle. It will be 
seen that these instances bear no relation to fung:!, neither are 
they parallel nor in any way similar to the potato disease : they 
merely show that insects have committed ravages, and that a 
certain loss was sustained in consequence. We have no evidence 
that in these attacks the vessels only of the plant and animals 
suffered, and that in consequence they perished by putrefaction. 
On the contrary, the attacks appear to have been generally of an 
external character, and mostly local in their action, being princi- 
pally confined to a small sphere, and frequently not extending 
beyond a few individuals. 
8. The Commencement and Course of the Disease in the Stem and 
Tubers : its observed Action. 
The commencement of the disease has been a subject of some 
dispute — one party pointing to the leaf, another to the stem, and 
a third to the tuber ; and cases have been named in support of 
each. The most numerous testimony, however, points to the 
stem as the part first observed, and some few to the leaves. I 
shall not, however, enter deeply into the merits of these opinions, 
as it is of little moment where the disease was first observed, 
provided we find out the cause ; but, for reasons which I shall 
hereafter state, I view the stem as the part where it commenced, 
notwithstanding any apparent proofs to the contrary. The first 
e.xternal appearance of disease, in the stem, was marked by a 
deepening of colour, accompanied by an enlargement of the 
affected part, and succeeded by a weeping or exudation. As the 
disease advanced, the part of the stem affected became soft and 
pulpy, and the colour changed to a yellowish brown. The stem 
was generally, if not universally, first affected at a joint, or the 
part from whence a lateral branch springs; and the leaves above 
the diseased part blackened, drooped, and died. 
Now from the striking appearance of the leaves, a casual ob- 
server would conclude they were the parts first affected, and con- 
sidering the body they present as compared with the stem, and 
also tlie significant change of colour and drooping they undergo, 
it is no unreasonable view of the matter. But reasonable as this 
view may appear for a glance, a more searching examination of 
the subject shows it to be distorted : for we shall be enabled to 
trace the disease through its stages to its seat, and thus identify 
the part from which it sprang in the stem ; and if we succeed in 
this, it will be seen that the condition of the leaves is referable to 
the stem, and that their change of colour and drooping is the 
effect of lessened vitality caused by a gradual cessation of com- 
munication between the upper and lower parts of the plant, by a 
