PLANT FOOD AND PLANT GROWTH 33 



to the animal the energy or heat' equivalent to that of ordinary 

 combustion in the furnace, of the same materials; and the carbon 

 dioxid is then thrown off through the lungs into the air, again to 

 become the source of carbon and oxygen for plants. Thus, the 

 fixation of carbon by the plants on the one side, and, on the other, 

 all forms of combustion, including the visible flame, the consump- 

 tion and oxidation of food by animals, or the oxidation of organic 

 matter in the soil, completes the endless carbon cycle. 



But for this carbon cycle, plant growth and crop production 

 would soon cease. A simple computation reveals facts not com- 

 monly appreciated : 



A column of air one inch square and the height of the atmosphere 

 weighs 15 pounds, which is equivalent to 2160 pounds per square 

 foot, or less than 95 million pounds per acre. In ten thousand 

 pounds of average air there is less than four pounds of carbon 

 dioxid (CO 2 ) or about one pound of carbon. Consequently, there is 

 less than 10,000 pounds of carbon in the air above one acre of land. 

 In loo bushels of corn (5600 pounds), there are 2500 pounds of 

 carbon. (See Table 2, or compute from the per cent of carbon in 

 starch and fiber, C 6 H 10 O 5 .) Thus, the total supply of carbon over 

 an acre of land is only equal to the needs of four such corn crops as 

 are commonly produced on the best-treated corn-belt land in the 

 best seasons, the grain only being considered, or to only two crops, 

 considering both grain and stalks. If, however, only one fourth 

 of the earth's surface is land, if only one fourth of the land is 

 cropped, and if only one fourth of 100 bushels is the average crop, 

 then the supply of carbon is sufficient, not for two years only, but 

 for 128 years, which, however, still emphasizes the fact that the 

 carbon cycle makes possible the continuation of plant life on the 

 earth. 



A maintenance ration for animals is a supply of food sufficient 

 only to support the animal body in health, to provide food materials 

 for repairing the daily waste, and to furnish energy sufficient to 

 keep the body warm and to maintain the circulation of the blood 

 and other necessary activities. Plants also have some vital pro- 

 cesses to provide for, and, to a limited extent, plants are consumers 

 of energy day and night. Food materials are stored by the plant, 

 chiefly to be utilized in subsequent plant development. Thus 



