162 FUNDAMENTALS OF FRUIT PRODUCTION 



applied in everyday life that it is taken as a matter of course and 

 consequently overlooked. 



The principle of limiting factors is particularly important for an 

 understanding of the process of carbon assimilation and it has a direct 

 practical application in the use of fertihzers. These two subjects are 

 discussed in the following pages. 



CARBON ASSIMILATION 



The synthesis of organic compounds in plants depends on the assimi- 

 lation of an element which occurs in and is characteristic of all organic 

 compounds, namely, carbon. This element is provided by the carbon 

 dioxide of the air, which, together with water absorbed by the roots, 

 furnishes the materials for the synthesis of carbohydrates. These 

 compounds contain more potential energy than those from which they 

 are formed; this energy is supplied by the sun, whose radiant energy is 

 transformed into the potential energy of carbohydrates by means of the 

 green pigments of the leaf, the chlorophylls. The reaction or reactions 

 by which water and carbon dioxide in the presence of light and through 

 the agency of chlorophyll form carbohydrates and oxygen depend on two 

 other factors, namely, enzymes and temperature which affects the rate of 

 all chemical reactions. 



Factors Involved 



The rate of carbon assimilation depends on six factors: 



1. The supply of carbon dioxide. 



2. The supply of water. 



3. The intensity, duration and quality of light. 



4. The amount of chlorophyll. 



5. Temperature. 



6. The amount of enzymes. 



Carbon Dioxide. — The carbon dioxide content of the atmosphere is 

 practically constant, varying little from 3 parts in each 10,000 of air. 

 Carbon dioxide enters the leaf mainly through the stomata, though the 

 epidermis with its cuticle is slightly permeable to it. Hence the diffu- 

 sion of carbon dioxide into the leaf depends on about the same factors as 

 the outward passage of water vapor, namely, the number of stomata, the 

 rate at which carbon dioxide is utilized within the leaf and the condition 

 of the air outside the leaf, whether it be moving or still. 



The amount of carbon dioxide assimilated has been shown to depend 

 on the number of stomata on the upper and the lower surfaces of the leaf. 

 For example, Table 58 shows the relation in leaves illuminated on the 

 upper surface. In leaves with stomata confined to one surface the cor- 

 relation of assimilation to number of stomata holds, but with leaves 

 bearing stomata on both surfaces there is more intake of carbon dioxide 



