276 VEGETABLE PHYSIOLOGY 
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 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 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 
power of active movement or locomotion. Zoospores of 
