THE MANUFACTURE OF FOOD BY LEAVES 255 



bring up the water from the roots and carry away the manufac- 

 tured products to all parts of the plant. 



The formation of starch, although common in leaves, does not 

 depend upon the presence of light except in so far as light is 

 necessary in providing sugar; for starch is formed abundantly in 

 many roots, tubers, and other structures where light is excluded. 

 Starch, as its formula (C 6 Hi O 5 )n shows, is very similar to sugar of 

 which it is considered a storage form. Consequently its abun- 

 dance in leaves where sugar is being formed is to be expected. 

 Sugar is changed to starch not only to make room for more sugar, 

 but also to prevent injuries that may result from its accumulation. 

 According to the laws of osmosis, as the sugar content of the cell 

 sap of the chlorenchyma cells increases, their internal pressure 

 increases. Consequently when the chloroplasts are very active, 

 the changing of the sugar into starch, which is insoluble in the 

 cell sap, is necessary to prevent the internal pressure of the 

 chlorenchyma cells from becoming so high that there is danger 

 of bursting. 



The transformation of sugar into starch not only prevents the 

 accumulation of the sugar from interfering with the process of 

 photosynthesis, but also enables the plant to have in storage 

 food which can be drawn upon when conditions are unfavorable 

 for photosynthesis. Thus at night when photosynthesis is inac- 

 tive, the starch in the leaves is changed to sugar and carried to those 

 regions where it is needed for growth, and in this way the plant 

 is able to maintain its growth at night as well as in the daytime. 



However, starch is not stored in all parts of the plant so 

 temporarily as in foliage leaves. In some organs, such as 

 seeds, fleshy roots, tubers, and stems of trees, starch is stored to 

 remain as a food supply for next season's growth. Since the 

 starch stored in all parts of the plant is transformed sugar which 

 is made mostly in the leaves, the dependence of such structures 

 as seeds, roots, and tubers upon leaves is obvious; for it is only 

 as the leaves supply the sugar that these storage structures can 

 form starch. 



The amount of starch formed in foliage leaves is closely 

 related to the rate of photosynthesis. In general, the more 

 active the process of photosynthesis, the greater the amount of 

 starch formed. For this reason the amount of starch present in 

 leaves can be used in determining the rate of photosynthesis. 



