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 hy the internal 
forces they exhibit, 
