322 
Nature and Cause of the Potato Disease. 
plant be cut from its stem while full of sap, and the changes that 
ensue noticed, all the phases of the disease will present themselves. 
First, we have the deepening of colour, attended with the libe- 
ration of carbonic acid, then the dull heavy hue preceding putres- 
cence ; then the dark brown hue that appears with putrescence, 
and finally, the disorganisation of the mass itself by putrefaction. 
The character of the leaves then observable in the disease is due 
to incipient putrefaction, for it presents no peculiarity beyond 
what takes place in all vegetable matter while undergoing that 
change when similarly circumstanced to the leaves of the potato- 
plant. 
From a consideration of the commencement of the disease in the 
stem, I shall now trace it through various parts of the plant, and 
notice what takes place by its action. If we examine a diseased 
plant, we never find the roots affected with rottenness, although 
the stem and tubers may be putrescent. Continuing our exami- 
nation from the root upwards, we find nothing remarkable until 
we arrive at the base of the footstalk. At this part we sometimes 
find the botritis located in the cellular arrangement of the me- 
dulla, and vegetating luxuriantly. Continuing our course up the 
stem, having found the botritis at the base, we still trace it, and 
as we approach the first branch the colour of the medulla deep- 
ens, and some disorganisation appears in the waUs of the cells; 
passing onwards until we reach the joint, putrescence presents 
itself, and the cells of the medulla are broken up. Here we find 
the eel animalcula, together with abundance of boleti and radi- 
ated fungi ; still pursuing our way, we find, after passing the joint, 
traces of boleti, but as we approach the foliage we frequently lose 
them altogether. Taking our course now from the stem along 
the .stolon to the tuber, we find no funofi but that of the radiated 
kind ; passing to the tuber, we trace the disease through the pas- 
sage of the stolon, along the channels of the germs, through the 
empty cells, beneath the cuticle, the vessels and walls of the cells, 
and lastly, in the vessels which are in the germs themselves. 
This is an epitome of the course of the disease, and what may 
be traced in all cases, the only variation being that of intensity. 
It is peculiarly a disease of the vessels and not of the products, 
for the starch granules may be found perfect in the midst of 
putresence. From the circumstance of the botritis appearing at 
the base of the stem, and increasing in force until we reach the 
joint affected, and also from its frequently disappearing as we 
approach the foliage, it appears that the leaves are not the first 
point attacked, but rather that the disease ascends with the sap 
irom the roots. Tliis view is strengthened by the fact that we 
can trace no starch below the stolons after the j)lant has formed 
leaves ; and from this fact we may conclude that the downward 
