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 (CsHi005)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. 
