RELATION OF CARBON DIOXIDE TO PLANT GROWTH 197 



Efficiency of the Food-Making Machine. The green leaf 

 under ordinary conditions absorbs and reflects about 50 per cent, 

 of the sunlight which falls on it, but in diffused light it may al> 

 sorb or reflect as much as 95 per cent. Only a small part of 

 this (variously estimated from .5 to 3 per cent.) is used for 

 photosynthesis. A considerable part is transformed to heat, and 

 the temperature of the interior of the leaf may be a little higher 

 than the surrounding air ; in exceptional cases it may be 10 to 

 15 degrees higher. It has been estimated that a square meter of 

 leaf surface will produce about one gram of organic matter in 

 one hour. This work requires the CO 2 from 2.5 cubic meters 

 of atmosphere. 



Sugar and Starch Transformation and Migration. It will 

 be readily seen that grape sugar is formed only in the presence 

 of chlorophyll and sunlight, mostly in the leaves of the plant. 

 Other sugars are formed from the grape sugar. Sugar is soluble 

 and can be readily transferred to other parts of the plant. Prob- 

 ably some of the sugar is transferred immediately, but if it is 

 formed more rapidly than it can be removed, the excess is trans- 

 formed into starch. In the presence of subdued sunlight and 

 during the night, the excess starch is transformed into sugar 

 again and removed to other parts of the plant, where it is either 

 used for growth or stored as sugar or starch for future use. 

 Sugar is found in great abundance in ripe fruits and in other 

 parts of the plant, such as the roots of the sugar beet and the 

 stem of the sugar cane (page 135). Starch is found in much 

 greater abundance in fruits, seeds, fleshy roots and fleshy stems. 



Formation of Other Foods. The formation of carbohy- 

 drates by photosynthesis is only one step in the making of foods. 

 We have already learned that plants contain carbohydrates, fats 



