386 
On the Potato Disease. 
wither, and, on pulling thorn np, the undergronnd stems were 
riddled throuorh and throiisli, witli wireworms stickin<r in them, 
many fibres being also gnawed off by them, but the tubers were 
not diseased, and are sound now (December 9tl;). It is, there- 
fore, my opinion that no injury the plant may receive from insects 
will of itself produce the gangrene, by which it is linqucstionably 
destroyed. 
I will now endeavour to desci ibe the progress of the disease, having 
had opportunities of observing it this year at an earlier period 
than last. It usually commences in a semi-transparent blotch on 
the underground parts, which is formed by brown, shiny, extra- 
vasated fluid between tlie epidermis and imier bark, sometimes 
only on one side of the stem or roots ; and, if the soil becomes dry 
and the weather fair, it penetrates through the bark rather slowly, 
but progresses rapidly in a damp soil with a close htimid atmos- 
phere. I have marked several of these blotches in their trans- 
parent state as soon as formed with bits of thread, and find that, 
sooner or later, according to the circumstances above mentioned, 
(hey assume a scurvj', opaque ajipearance outside, but, by cutting 
through this, the shiny fluid may generally be seen within. The 
blotches in the tubers change in a similar manner, and the pro- 
gress of the gangrene in them is accelerated or retarded by the 
like circumstances; the blotches on some of m.y potatoes last 
year, which were kept dry and cool, not having penetrated 
more than l-8th of an inch up to the end of March, having, how- 
ever, assumed a dull smiken appearance in place of their former 
glossiness. 
Previous to my having traced the rustiness of the stems to the 
original transparent blotches I entertained some doubt as to its 
connexion with them, from the fact that the inner bark of the 
specimens which I had before examined was ajiparently healthy, 
and the discolouration entirely confined to the epidermis, which 
had become dull, dry, and wrinkled. Now, however. 1 am con- 
vinced that the one is the result of the other — the epidermis 
having lost its transparency by corrosion of the fluid, which is also 
withdrawn by absorption to the inner parts of the bark, leaving 
the outer coats nearly dry. Similar blotches appear about the 
same time, or soon after, on the roots and cords. Sometimes, 
however, the leaves arc blotched first of all. I have found many 
very large plants with nearly every leaf and the stalks likewise 
blotched, the leaves being covered with botrylis, while the roots 
and lower parts of the stems remained quite clear; but I have 
never seen until the 24ih of October diseased roots or cankered 
stems supporting healthy foliage.* Hence it appears that the 
* Oct. 24. On finding many healtliy-Iooking plants in my field (grown 
