282 VEGETABLE PHYSIOLOGY 



distinct cuticle. It is evident that the great quantity of 

 water which is given off by the leaves of a sunflower, to 

 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 

 products 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 construc- 

 tion 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 con- 

 siderable amount of work, which in turn demands the 

 expenditure of energy. Many organs carry out their growth 

 under conditions of pressure : roots, for instance, in their 

 penetration through the soil. 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, 



