TEMPERATUKE AND ITS CONDITIONS 327 



the processes of adjustment only take place with difficulty. 

 The trunk of n tree is during the day often cooler than 

 the air and warmer than the latter during 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. Boots 

 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 in 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 



