THE ENEKGY OF THE PLANT 281 



Finally we have evidence of the power of plants to 

 avail themselves of the heat rays. The relations existing 

 between the organism and its environment have already 

 been mentioned. Not only can the air rob the plant of 

 heat by radiation, but when its own temperature is high it 

 can communicate heat to it in turn. Leaves have been 

 proved to absorb heat with great avidity, particularly those 

 which are succulent or fleshy, a difference of more than 

 20 C. having been noted between their temperature and 

 that of the ,air. The direct absorption of the rays of heat 

 from the sun has also been noted, apart from the tempera- 

 ture of the air through which the rays were passing. 



The supply of radiant energy is very much in excess of 

 the amount which is needed for the internal work. Indeed 

 its absorption by the leaves would be a source of consider- 

 able danger to the plant were it not for the cooling effect 

 of transpiration, which we have seen dissipates 98 per cent, 

 of it during bright sunshine. No doubt this dissipation is 

 one of the chief benefits secured by transpiration. 



It is evident, however, that in the general economy 

 of the plant something further must be at work in connec- 

 tion with the supply of energy. The absorption of these 

 external forms must take place at the exterior of the plant, 

 while many of the processes of expenditure are carried out 

 in parts which are more or less deep-seated. We are obliged 

 to turn our attention, therefore, in this connection as in 

 that of the construction and utilisation of food, to processes 

 of accumulation, distribution, and economy. 



We may ask ourselves what is the immediate fate of 

 the energy absorbed. It enters the plant in what is known 

 as the kinetic form. A very considerable part of the 

 kinetic energy of the sun's rays, we have already seen, is 

 devoted at once to the evaporation of the water of transpira- 

 tion, but some of it is employed by the chloroplasts to con- 

 struct some form of carbohydrate. The energy so applied 

 can be again set free by the decomposition of this formed 

 material. If the latter were burned its combustion would 



