THE ENERGY 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 
