THE ENERGY OF THE PLANT 279 
the process of adjustment is often very slow, the tissues 
being generally very poor conductors of heat. Still it 
seems not improbable that a certain amount of energy is 
devoted to the attainment of the range which is most suit- 
able for the vital processes. Though the dominating factor 
in the determination of the plant’s temperature is to be 
looked for in the environment, the development of heat 
during germination and while the flower-bud is opening is 
an indication that it is not the only one. 
A fuller consideration of the relations of the plant to 
heat must be deferred to a subsequent chapter. 
The evolution of light by plants is a comparatively rare 
phenomenon, being probably confined to certain Fungi, 
though it has been attributed also to a few species of Alge. 
It must call, however, for a certain expenditure of energy 
in such cases as have been authenticated. 
If we turn now to consider the sources of the plant’s 
energy, it is evident that they must be in the first instance 
of external origin. The radiant energy of the sun indeed 
is the only possible source which can supply it to normal 
green plants. The question of the absorption of this energy 
has already been incidentally alluded to when we discussed 
the chlorophyll apparatus, but it may now be examined 
more closely. 
The rays which emanate from the sun are generally 
alluded to as falling into three categories, those of the 
visible spectrum, those of the infra-red, and those of the 
ultra-violet. The second of these are frequently spoken of 
as heat rays, and the last as chemical. 
The greatest absorption of energy appears to take place 
in consequence of the peculiarities of chlorophyll. As we 
have seen, this substance, whether in the plant or when in 
solution in various media, absorbs a large number of rays 
in the red and in the blue and violet regions of the 
spectrum, together with a few others in the yellow and the 
green. 
The solar spectrum after the light has passed through 
