Nature and Cause of the Potato Disease. 
325 
visible disease, both externally and internally. Comparing, there- 
fore, the tubers of former years with those of the last season, we have 
an excess of water in the last year's growth of 10 per cent. ; and 
as the diseased tubers contained more water than the sound ones, 
excess of water seems a feature in the case closely connected with 
the disease itself. The tubers of last season, when first taken 
Plate 4. 
Transverse section of Diseased Tuber. — ^Power of object-glass used, 320 diameters. 
1, Bark. 2. Cuticle. 3. Reservoir of empty cells. 4. Channels leading to the gems or eyes. 
5. Cells diseased. 6. Cells not diseased. 
from the ground, rapidly passed through the various stages of the 
disease, but after drying their surfaces by exposure to a current of 
air, their putrescent form was arrested and another phasis pre- 
sented itself. Thus, instead of the tuber becoming pulpy and 
rotten, and emitting a cheese-like, and afterwards a foetid odour, 
it would harden and appear outwardly sound ; but when exa- 
mined internally a dark reddish brown coloured matter, of a 
leathery texture, presented itself. In this state the disease silently 
spread through the tuber without emitting any offensive smell, 
and the tuber appeared as if affected with the dry-rot. The 
black colour of the worst stages of the disease may be produced 
by immersing for some time cut slices of potatoes in solutions of 
any neutral salt, and afterwards exposing them to the action of 
the atmosphere. Acids, alcohol, ether, tannin, and alum (sulph. 
