THE ENEEGY OF THE PLANT 287 



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 men- 

 tioned. Not only can the air rob the plant of heat by 

 radiation, but when its own temperature is high it can com- 

 municate 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 temperature 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 connection 

 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 transpiration, 

 but some of it is employed by the chloroplasts to construct 

 some form of carbohydrate. The energy so applied can 

 be again set free by the decomposition of this manufactured 

 material. If the latter were burned its combustion would 



