THE POTATO TUBER. 
77 
( b ) Blight infested mature tubers. 
Parasitic fungi, and notably that causative of Potato 
Blight ( Phytophtliora infestans) are particularly partly destruc 
tive to the skin of the tuber. In a tuber which has been 
attacked by Potato Blight the fungous organism remains in the 
form which has by convention been termed a hibernating 
mycelium ; this consists of minute thread-like filaments 
which are usually resident in the more or less disorganised 
tissue immediately beneath depressions in the skin of the 
diseased tuber. These depressions, it is to be noted, are 
not always diagnostic of Potato Blight, and this is especially 
the case when mixed infection is present, or the appearances 
are complicated by the interaction of other factors, and 
also in cases of very slight infection. When Blight infected 
tubers are steeped in solutions of acid or salt the disorganised 
condition of the skin is revealed. The skin is apparently 
intact so far as it is possible to determine this point by the 
unaided vision, or the lower magnifying powers of the 
microscope. The steeping of such a tuber in a solution of 
sulphuric acid, or, better still, a solution of the acid deeply 
coloured by the addition of methyl orange, leads to results 
which strikingly demonstrate the altered character of those 
portions of the skin beneath which the seat of attack of the 
fungus is located. Instead of the entry of acid being con- 
fined, as in the case of blight free tubers, to the eyes, it now 
diffuses and with great rapidity through those parts of 
the skin beneath which fungal attack is in evidence (Fig. 8 ). 
Without actual demolition of the skin as a result possibly 
of the action of certain secretions of the attacking fungus, 
certain components of the walls which constitute the cells 
of the skin are profoundly altered or possibly destroyed. 
Whatever the actual nature of the changes may be, the nett 
result is clear : the previously impermeable skin is now 
rendered freely permeable to the various salts and acids 
which have been investigated. 
Experiments with blight infected material showed that 
when mature tubers were steeped in 10% sulphuric acid 
for ten hours the vitality of both the tuber buds and the 
infesting fungus was destroyed. Repetition of these ex- 
periments, under rigid conditions of control, were invariably 
attended with results which were in no sense equivocal. 
The infected tubers after treatment invariably yielded nega- 
tive results w r hile untreated, but otherwise similar control 
tubers invariably gave positive results. That is to say, when 
treated and untreated, but otherwise similar tubers were 
placed under conditions favourable to the development of 
the fungus ; in the former case no fructification was pro- 
duced, while in the latter fructification invariably took place. 
