39 
Heat by Wounded Plants . 
a dead potato. Thus the temperature of a living uninjured 
potato being 0-17° C. over that of the surrounding objects, the 
temperature on the cut surface of a wounded potato will at 
its maximum be from 0*20° C to 0*25° C higher than that of 
the uninjured plant. But it must be remembered that 
in these conclusions no allowance has been made either for 
the conduction of heat by the tissue of the potato itself or for 
radiation into the air, which must, of course, amount to some- 
thing and must increase as the temperature of the tissue 
increases. In relation to this point and also the other regard- 
ing the comparison of the temperature with the respiration, 
it is exceedingly important to have some idea of how far the 
surrounding tissue of the potato is sympathetically affected 
in the increased warmth following injury. 
It will be seen from Experiments 11 and 12 that the 
influence of the wound does not extend very far back from 
the point of irritation. There is in most cases a very 
rapid falling off at a distance of only a few millimeters 
from the wound ; and at a distance of two centimeters 
there is no perceptible difference. It is evident, then, that 
the real heat-producing processes which follow injury must 
be comparatively local, for the conduction of heat by the 
tissue of the potato naturally increases the distance at 
which a temperature-difference is distinguishable. In order 
to obtain a clearer understanding of this question, experi- 
ments were made to determine an approximate idea of the 
conductivity of potato-tissue for heat. Two potatoes in 
separate vessels were half buried in sand which was in one at 
the temperature of the air, namely 24-2° C.; while in the other 
it was kept during the time of the experiment nearly ten 
degrees higher. The needles were thrust into the two 
potatoes at a distance of 30 mm. from the surface of the sand, 
and in fifteen minutes the galvanometer indicated an increase 
in temperature of the warmed potato amounting to 0*25° C. 
By another experiment it was also found that two potatoes 
which showed by the galvanometer a difference in temperature 
of almost exactly 2° C., took five and a half hours to become 
