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 



