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 



