332 VEGETABLE PHYSIOLOGY 
form of heat and passes from the plant to the sugary liquid 
in which it lives. 
Terrestrial plants show less direct evidence of the loss 
of heat by conduction. Their roots, however, no doubt 
give up a certain amount to the soil at different times, just 
as at others they absorb heat from the latter. 
When we compare approximately the amount of heat 
absorbed by a green plant with that which is given off by 
it, we find that in all cases there is a certain excess of the 
former. Most plants thus show a certain gain of heat 
from their environment. This does not, however, usually 
manifest itself by a rise of temperature in the tissues. 
‘There is no uniformity in the absorption either. At times 
when there is the greatest balance in favour of absorption 
throughout the whole plant, parts of it may be giving off 
considerable quantities and may be cooler than the average 
temperature of the whole plant. 
The gain of heat which is secured in this way is to be 
largely regarded, as we have already seen, as supplying 
energy to the plant. This is devoted at first to constructive 
processes, and thus much of it is rendered potential, being 
afterwards reconverted into the kinetic form and made to 
reappear, when it once more largely takes the shape of heat, 
and is subsequently devoted to purposes of growth, meta- 
bolism, repair of cell-substance, &c., as we have already 
seen. But we may now lay a certain stress on the fact that, 
at any rate, a part of this liberated heat is devoted to a 
raising of the temperature of the cells which are the seat 
of its liberation. 
We have thus an elementary though very incomplete 
mechanism for the regulation of the temperature of the 
plant. An excess of heat is absorbed: part is at once 
applied to purposes of growth, metabolism, &c.; part is 
retained, and the store is, as it were, economised, being 
liberated later with some reference to the temperature of 
the parts concerned in the vital processes. 
This regulation of heat, however, is very rudimentary 
