FOODS IN PLANTS 47 
gases between the exterior and the interior of the living body 
— between the lungs and the air in the case of the higher ani- 
mals and between the leaves and the air in the case of the 
higher plants. This interchange is, however, no longer re- 
garded as the fundamental thing in respiration, since respira- 
tion takes place in active, living protoplasm in all parts of the 
plant. It consists of decomposition of protoplasm or of some 
of its parts, or (as is supposed by some botanists) it may con- 
sist of decomposition of food materials that have not yet become 
assimilated so as to become protoplasm. Through respiration 
complex plant substances are broken down, and the energy 
released by this decomposition is the energy by means of 
which plants carry on their work. Energy in the form of heat 
is also one of the results of respiration. Respiration may occur 
in the absence of free oxygen, but it is more complete, and 
thus releases more energy, when oxygen is present. When 
respiration is complete, it results in the formation of various 
compounds, of which carbon dioxide and water are conspicu- 
ous. Carbon dioxide and water may be carried away from the 
plant through the leaf or other parts, and the oxygen supply 
may enter in the same way. It is evident, however, that the 
transfer of these gases is merely an incident associated with 
the real respiration, which consists in the decomposition of 
complex substances and the release of energy therefrom. It is 
also evident that, so far as respiration is concerned, plants and 
animals behave in the same way. It should be noted that in 
photosynthesis green plants utilize carbon dioxide, though they, 
like other plants and animals, may produce carbon dioxide as 
one of the products of respiration. 
47. Importance of the food cycle in plants. The importance 
of the food cycle of green plants can scarcely be overestimated. 
The human race depends, directly or indirectly, upon photo- 
synthesis and the storage of food by plants. Men have selected 
and improved those plants which produce the most desirable 
food in the largest quantities. They have studied, and must 
continue to study, the conditions of soil and the climatic 
