THE POTATO TUBER. 
65 
It respires, but its energies, in this respect, are reduced to 
their lowest level of activity. When we mutilate a tuber, 
as' happens in peeling it, two things result ; on the one 
hand, respiration, i.e., the taking in of oxygen and giving 
out of carbon dioxide, is increased, and this is accompanied 
by the formation of wound cork over the cut surface, and 
on the other, loss of moisture through dessication of the 
residual starchy tissue. The total loss immediately following 
mutilation is thus of a dual kind. If, for example, a tuber 
is injured by being cut up, as in the preparation of what 
arc known as “ cut sets for seeding purposes, the respira- 
tory exchanges, as shown by Richards’ researches, are 
notably increased. 
The output of respiratory carbon-dioxide in unit 
time is from five to eight times greater than that yielded 
by intact tubers. In several of Richards’ experiments the 
tubers were merely bisected by slicing them lengthwise. 
In .the above experiments with peeled tubers mutilation 
was much more extensive. It is therefore very probable 
that the losses in these experiments to be attributed to 
enhanced respiratory activity are much greater than those 
recorded by that author. The peeling or slicing of a tuber 
involves the wounding of the tissue and the increased 
respiration of wounded tissues, as shown by Richards’ 
work, is referred to in the literature of plant physiology as 
a “wound reaction.” A wound reaction is virtually the 
response of plant tissue to stimuli induced by injury. 
Other examples of wound reactions are to be seen in the 
healing phenomena which normally ensue after pruning 
operations. 
To return to the tuber : immediately after injury the 
parenchymatous tissue, and as already ' stated the tran- 
sitional cells also, make an effort to repair the damage 
caused by the slicing or peeling of the tuber, and thus check 
that rapid and considerable dessication of the storage tissue 
which ensues when the skin of the tuber is removed or 
mutilated. The formation of wound cork over the exposed 
surface, and the accompanying increased rate of respiration, 
are the outward and measurable expressions of this repara- 
tive effort. In the case of the mature tuber it is probable 
that this increased respiration is due chiefly to wounding 
alone, and not so much to renewed cell division, as occurs 
in tubers which may be still capable of growth. 
In the common practice of preparing “ cut sets ” the 
phenomenon must evidently be of common occurrence. It 
will not, therefore, be foreign to the subject in hand, seeing 
that in many respects the functions of wound cork in the 
case of cut sets are very similar to those of the skin of the 
