324 VEGETABLE PHYSIOLOGY 



is readily transmitted from the one to the other. Similar 

 transmission of heat from the soil to the roots can take 

 place and no doubt has a considerable effect in promoting 

 the well-being of the latter, which as we have seen con- 

 tinually lose heat by the evaporation of water from their 

 cells into their intercellular spaces. Here, however, as in 

 the last case, the conduction of heat varies in direction 

 according to the relative temperatures of soil and root. 



The absorption of heat from the air in contact with the 

 general surface must play a part very frequently in the 

 heat interchanges. As in the other cases mentioned, 

 however, the ' direction of this interchange is not 

 constant. 



While we can thus recognise these ultimate sources of 

 heat supply, we find, no less evident, certain ways in 

 which heat is given off by the plant in greater or less 

 amount. 



Of these losses the first and most important is the ex- 

 penditure which is necessary in order to evaporate the water 

 of transpiration. There can be no doubt that the amount 

 of transpiration is very largely determined by the amount 

 of the sun's rays which the plant receives. Not only are 

 its stomata open widely in bright light, so that the vapour 

 can readily diffuse into the air, but the actual evaporation 

 from the cells into the intercellular passages is enormously 

 accelerated during the absorption of the radiant energy. 

 The amount of the latter which is taken up by a leaf has 

 been computed to be nearly fifty times the amount which 

 can be utilised in the process of photosynthesis ; if the 

 heat were allowed to accumulate in the leaf unchecked, 

 it has been calculated that its temperature would rise during 

 bright sunshine at the rate of more than 12 C. per minute, 

 with of course, very rapidly fatal results. What is not used 

 for photosynthesis is employed in the evaporation of the 

 water of transpiration, the leaf being thus kept cool. It 

 is noteworthy that whether the leaf is brightly or only 

 moderately illuminated the same relative proportions of the 



