THE ENEEGY OF THE PLANT 289 



which allusion has been made in an earlier chapter, cannot 

 be evaporated without the expenditure of a considerable 

 amount of energy, which presumably takes the form of 

 heat. It has been computed recently that 98 per cent, of 

 the energy of the rays of light which are absorbed by the 

 chlorophyll is expended in causing this transpiration. 



The great accumulation of material which is so marked 

 a feature of the life of a plant is the result of work which 

 has been carried out in the plant on the simple substances 

 which are absorbed. We may distinguish here between 

 such products as are destined for immediate or ultimate 

 consumption, and those which become incorporated into 

 the actual substance of the plant. The accumulation of 

 the latter is permanent, and the energy which is used in 

 their construction is not subsequently made use of in the 

 working of the organism. That it is stored however is 

 evident from the fact that it can be re-converted into heat 

 if the substance is burned. As we shall see later the pro- 

 ducts which are ultimately consumed in the nutritive 

 processes may be regarded as stores of energy as well as 

 of nutritive material. In both cases however their con- 

 struction involves the expenditure of a considerable 

 amount of energy before they assume their recognisable 

 condition. 



Closely allied to these constructive processes we have 

 the phenomena of- repair and of growth. As we have not 

 yet studied the latter process in detail, we may be content 

 with pointing out that there are involved in it many 

 changes of various substances, which call for the execution 

 of considerable amounts of work, which in turn demand 

 the expenditure of energy. Many organs carry out their 

 growth under conditions of pressure ; roots, for instance, 

 often penetrating through stiff soils. . Not only is energy 

 necessary to produce the growth itself, but the pressure 

 upon the growing organs must be counterbalanced by the 

 internal forces they exhibit. 



Many of the humbler plants possess a considerable 



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