TEMPERATURE AND ITS CONDITIONS 327 
the processes of adjustment only take place with difficulty. 
The trunk of a tree is during the day often cooler than 
the air and warmer than the latter durmg the evening 
and night. The mean annual temperature of such a tree 
trunk is, however, about equal to that of the air. Less 
bulky parts than the trunk, the leaves for instance, are 
very often much cooler than the air. This is made evident 
by the frequency with which dew or even hoar-frost may 
be detected on their surfaces. A thermometer placed upon 
grass often gives a much lower reading than one suspended 
in the air a little above the ground. This is, no doubt, due 
to the loss of heat by radiation from the leaves. Roots 
are often cooler than the air, losing heat by conduction to 
the soil, and by the evaporation which takes place into 
their intercellular spaces. 
Aquatic plants are less subject to these disturbances 
than terrestrial ones. The range of temperature of the 
water surrounding them is smaller, and as they are practic- 
ally m contact with water within and without, the internal 
changes of temperature incident to their metabolism are 
much more readily equalised. 
In discussing the changes of temperature in the body 
of a plant we have to deal at the outset with the supplies 
of heat which it receives. We have already examined 
them from the point of view of the absorption of energy 
from without, but we may pursue still further here the 
question of the warming or cooling of the plant itself during 
such absorption. 
The chief source from which heat is derived is the 
radiant energy of the sun. When bright sunshine falls 
upon a leaf about a quarter of its radiant energy is absorbed. 
A much larger relative amount is taken up when the 
light is less bright ; in a strong diffuse light, such as that 
from a clear northern sky, the absorption amounts to about 
96 per cent. of the incident energy. We cannot at present 
discriminate with any accuracy between the influence of 
the heat rays and that of those of the other parts of the 
