THE POTATO TUBER. 75 
termination of these experiments furnished the following 
results : — 
Grams boracic acid per toocc of solution. 
At commencement 1.61 0.63 
At termination 1.60 0.65 
The experimental evidence afforded by these steeping tests, 
in which tubers have been steeped either in a solution of 
salt or acid, shows that in none ot these cases does the salt 
of acid used penetrate the tissue of the tuber in quantity. 
Removal of the skin, on the contrary, as shown by the 
brine steeping experiments, and damage to the skin as the 
result of fungous invasion, as will be shortly shown, leads 
to the ready entry of sulphuric acid through the skin. 
There exists, therefore, a mass of presumptive evidence 
for the view that one of the properties of the skin of the 
tuber is its impermeability to these various substances in 
solution. 
No definite evidence, however, has yet been advanced 
with regard to the localisation of the points of entry of the 
dissolved substances - salts or acids- into the steeped tuber. 
Ordinary quantitative chemical determinations, as we have 
seen, measure with tolerable precision the march of the 
diffusion processes which occur when intact tubers are 
steeped in solutions of certain salts or acids, but furnish 
little information as to whether entry of these substances 
takes place through both skin and eyes or eyes alone. Before 
dealing with the steeping of tubers in sulphuric acid, and the 
localisation of the points of entry of the acid into the tuber, 
reference may be made to certain important structural 
differences between the skin, which forms the major portion 
of the external covering of the tuber, and the external layer 
in the region of the eye. This may be more readily followed 
by reference to Fig. 2, which diagrammatically represents a 
section through the " eye ol a tuber. 
The layer indicated by the letters cc, and shaded by 
the vertical lines, is the corky skin of the tuber, a is the 
apex or growing point of the bud ; bb< , b«, b<>< are leaves. 
The figure shows that in the section under examination the 
skin terminates on either side of the bud abbi . I he point 
to be noted is that the external layer covering the apex, a, 
anti its surrounding leaves, bb 1 , consists simply of one 
layer of epidermal cells, the walls of which are not im- 
pregnated with s liber in ; that is to say, they are not of a 
corky nature. The enlarged drawings (Figs. 4, 5 and 6) 
illustrate this point. Fig. 6 representing the bud abb ■ , 
Figures 4 and 5 showing the structure of the epidermal layer 
of the leaves b, b> . There is then, structurally, a marked 
difference between the external layer of the eye of the 
