CHAPTER XVIII 

 USES OF FOOD IN PLANTS 



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II. RESPIRATION AND PLANT DEVELOPMENT 



There is often a close correlation between the rate of growth of plants 

 and the rate of respiration. If any one of the conditions necessary for 

 respiration is not present, growth soon stops. On the other hand, the 

 rate of respiration may be high or low after growth has ceased in mature 

 plant organs, such as leaves and fruits, because it is still affected by such 

 factors as temperature and the amount of soluble food present. 



The amount of food that accumulates in a green plant is in part 

 dependent upon the rate of respiration. Sugar made by photosynthesis 

 is oxidized back to carbon dioxide and water during respiration, and 

 unless photosynthesis exceeds respiration no food will accumulate in the 

 plant. Several external factors affect the relative rates of these two op- 

 posed processes. Consequently, a knowledge of these processes in rela- 

 tion to environmental factors may help us understand many common 

 plant phenomena and also the bases of certain practices in the cultiva- 

 tion and handling of plants and plant products. 



The food that accumulates in green plants may be used by them, but 

 it is also available to animals and to non-green plants. The amount of 

 food available to the human race depends, therefore, on the amount 

 made by green plants in excess of what they use in their development, 

 and on the amount consumed by non-green plants and by all the other 

 animals. Man may obtain much more food per acre by eating plants 

 directly than by eating animals that feed on plants, because most of the 

 food eaten by an animal is consumed in respiration. The above state- 

 ments will now be considered in more detail. 



Factors that affect respiration. Respiration is dependent upon the pres- 

 ence of available food (especially sugar) and ordinarily upon an external 

 supply of free oxygen. Among other important internal conditions are 

 the presence of certain enzymes in an active state, the acidity and water 

 content of the cells, and the presence of the ions of certain salts that 



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