THE POTATO TUBER. 
69 
So far we have seen that both on histological and 
physiological grounds the skin of the tuber is an eminently 
protective layer. Its complete or partial removal leads 
to loss of weight, which is considerably greater under com- 
parable conditions of experiment than those afforded by 
intact tubers. Among the functions of the skin then are 
the prevention of this loss, and the protection of the storage 
tissue and its contents. 
The formation of cork over the newly exposed surfaces 
of the freshly cut tuber, as we have seen, is a phenomenon 
of considerable biological importance. The fact has been 
demonstrated that its speedier formation may be artificially 
produced by the use of certain chemical substances, the 
most efficient of which is air-slaked lime. 
The Comparative Impermeability of the Skin of the 
Tuber to Water and Certain Salts and Acids 
in Aqueous Solution. 
Earlier in this paper reference has been made to the 
practice of steeping tubers in various antiseptic solutions, 
such as formalin, corrosive sublimate, etc., as a means of 
destroying micro-organisms, chiefly fungi (and their spores), 
a number of which comparatively modern investigations in 
the domain of plant pathology have undoubtedly shown 
to be the causal agents of certain diseases of the potato 
tuber. The customary procedure is to prepare a solution 
in water, usually of one or other of the two compounds men- 
tioned, which possess highly poisonous properties, and to 
steep tubers to be used for seed purposes in these solutions. 
The strength of solution, the duration of immersion, the 
subsequent method of handling the treated material, are 
matters governed more or less by arbitrary convention. 
No investigation, so far as the writer is aware, having the 
specific object of ascertaining whether these substances 
are capable of penetrating through the skin of the tuber 
and thus entering its storage tissue has been made. 
The question naturally arises whether the skin is freely 
penetrable to water, and to salts or acids in solution. 
We may at once turn to the consideration of the question 
whether water readily passes through the skin of the tuber, 
and after having disposed of this point we may then be in 
a better position to study the behaviour of the tuber on 
steeping in water containing substances — salts or acids in 
solution. We may then compare and contrast the effects 
of removal of the skin in relation to the entry of substances 
in solution into the starchy tissue of the tuber. Fin all y, 
we may turn to the changed permeability of the. skin due to 
the ravages of the fungus of Potato Blight. 
