536 On the Nature and Causes of the Dccaij in Potatoes. 
gluten; so that, out of 26 parts of dry matter in the potato, 16 
parts are of starch. 
V^arious proximate analyses have been made of the potato, the 
composition of which, of course, varies according to its variety. 
The two following analyses exhibit the composition of two varieties 
examined : — * 
Henry. 
G. Phillips. 
Pulp . 
6-8 
3-2 
Starch . 
13-3 
. 15-9 
Albumen 
0-9 
2-1 
Sugar 
3-3 
. 0-7 
Fat 
0-i 
(Gum) 1-3 
Acids and Salts 
1-4 
1-3 
Water . 
74-2 
. 75-5 
100-0 
100-0 
So much for the healthy potato. Now we come to the con- 
sideration of that which is diseased. I have already told you that 
the tuber is merely an enlargement of the underground stem ; so 
that, of course, it is connected by a subterranean unenlarged stem 
with that which is above ground. Now, at the junction of this 
subterranean stem with the tuber the disease generally commences ; 
it extends from this point, following the course of the vascular 
bundles through the cellular tissue to the other eyes. The cha- 
racteristic of the disease is, that it attacks and destroys the cell- 
walls of the tuber ; the walls of the cells become irregular, 
present a ragged, granular appearance ; finally, they yield entirely 
to the destructive influence, the cellular structure disappears, and 
the starch globules which were within them float in a semi-fluid 
mass of putrid matter. During the progress of this decay the 
walls of the cells acquire a reddish colour, then a brown, and 
Fig. II. Representing the destruction of the cells and progress of the disease.f 
* Several analyses give more minute details, and indicate the presence 
of citric acid and other substances ; but these are unnecessary for our 
present purpose. 
t For this drawing I am indebted to Mr. Henfrey, botanist to the 
Geological Survey of the United Kingdom. 
