THE LEAF 49 
derived from sunlight and stored by plants in past geological 
ages. Oil is derived from plants or animals or their remains, and 
the energy released in the combustion of oil is also the stored 
energy of sunlight. We thus see that all of the energy obtained 
by combustion and used for industrial purposes is the energy 
of sunlight stored by plants in the process of photosynthesis. 
Plants and animals use their food not only as building 
materials out of which their tissues are constructed, but also 
as a source of energy. This energy, like that obtained by 
burning wood or coal, is liberated by oxidation. 
RESPIRATION 
Definition. The oxidation by plants or animals of compounds 
containing carbon and hydrogen to carbon dioxide and water, 
with the liberation of energy, is known as respiration. 
Importance. The value of the process lies in the fact that it 
releases the energy of sunlight which was stored by the leaves 
in photosynthesis, so that this energy may be used for the 
vital activities of the plant. 
All work requires energy. Moreover, the energy must be in 
a form suitable for the performance of the particular kind of 
work to be done. In the flowing of water in a large waterfall 
a tremendous amount of energy is expended. This energy can 
be used for cooking or illuminating, but in order that this may 
be done the form of the energy must be changed to heat or light. 
In the activities of a plant, work is performed, and, like all 
other work, this requires energy. The green parts of plants 
use the energy of sunlight for photosynthesis, but energy as it 
occurs in light is not in a form that can be used for such vital 
processes as growth, movement, etc. It becomes converted into 
suitable form for these processes, however, when, after being 
stored in photosynthesis, it is released by respiration. 
In this way, not only is the energy of sunlight changed to a 
form in which it can be used for the general vital activities of 
the plant, but, moreover, any excess which is stored in the 
