TEMPERATURE AND ITS CONDITIONS 321 
ture 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 
practically in contact with water within and without, the 
internal changes of temperature incident to their meta- 
bolism 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 spectrum. No doubt the relative 
proportions vary considerably during the year. 
This radiant energy falling upon the leaf comes into 
21 
