76 
F. STOWARD, D.SC. : 
tuber and that of the skin ; this differentiation extends also 
to the permeability of the two to substances in solution. 
Sulphuric acid, it may at once be stated, under the 
conditions of experiment selected, diffuses into the tuber 
exclusively through the eyes, provided the skin of the 
object is intact, perfectly sound, and unruptured. The 
point is capable of precise demonstration. 
If a freshly cut tuber, whether untreated or after 
steeping in water, be tested by placing a drop of a solution 
of methyl orange on the freshly exposed surface laid bare 
by cutting, the tissue reacts alkaline to this substance ; 
that is to say, the coloration produced is faint yellow. If a 
drop of the indicator (methyl orange) is added to a solution 
of sulphuric acid, the colorless acid solution acquires a 
pink colour. If now a tuber which has been steeped for 
five hours or longer in a 10% solution of sulphuric acid 
rendered intensely reddish pink by the addition of methyl 
orange, be examined, we may readily localise the points 
where the acid has entered by slicing the tuber, so that 
the plane of section includes an eye. Examination shows, 
assuming that the skin of the tuber was perfectly sound, 
that the eye (Fig. 7) and the immediately subjacent tissue 
for a few millimetres is more or less definitely mapped out 
by a faint but easily perceptible pinkish coloration. The 
areas in which this pink coloration is visible mark out those 
portions of the tissue into which the acid has penetrated, 
for since the tissue itself reacts alkaline to the indicator, 
i.e., gives a yellow colour, the occurrence of the pink colour- 
ation indicates the presence of the abosrbed acid. That the 
pink colouration is due to penetration of the acid may also- 
be controlled by examining a tuber which has been steeped 
in a solution of methyl orange of similar strength to that 
used in the acid solution. If the tuber be sectioned and the 
plane of the section traverses an eye, no pink colouration 
will be visbile either there or elsewhere. As a matter of fact, 
the pink colouration, using methyl orange as indicator, is 
only given when acid and indicator have both entered the 
tissue. Examination of the sectioned tuber at points other 
than the eyes fails to reveal any pink colouration and the 
conclusion is therefore unavoidable that entry of the acid 
is confined to the eyes. We are therefore justified in con- 
cluding that the skin of the tuber is impermeable to sulphuric 
acid, and there are also good grounds for supposing that 
this also holds for boracic acid, and the various salts so far 
examined. The absorption of the various chemical sub- 
stances then occurs almost, if not wholly, through the eyes 
of the tuber. 
