4-4 A TEXTBOOK OF GENERAL BOTANY 
As photosynthesis is necessary for the manufacture of all the 
food used by plants, it is indispensable for their existence. 
Some of the oxygen liberated in photosynthesis is used by the 
plants in respiration, but most of it diffuses out of the cells into 
the intercellular spaces and then through the stomata into the 
external atmosphere. 
When photosynthesis is active, sugar is formed much faster 
than it is used by the leaf or conveyed to other parts of the 
plant. Under these conditions much of the sugar in the leaf is 
transformed into starch, which is insoluble in water at ordinary 
temperatures. At night, starch is changed back into sugar and 
is conducted away from the leaf. 
Starch and sugars belong to a class of compounds composed 
of carbon, hydrogen, and oxygen, and called carbohydrates. This 
name was originally given to vegetable compounds the mole- — 
cules of which contain six, or a multiple of six, atoms of carbon, 
and in which the hydrogen and oxygen occur in the same propor- 
tion as in water. While the more important carbohydrates have 
these characteristics, the meaning of the term has been extended 
to include some related compounds which do not fit the above 
definition. The most important carbohydrates in plants are sugars, 
starch, and cellulose. Starch is one of the principal forms in 
which food is stored in plants. It occurs as small grains which 
differ in appearance according to the species (Fig. 34). Many of 
them show striations, owing to the deposition of successive layers. 
The amount of sugar formed by photosynthesis depends upon 
the intensity of the light, more sugar being formed in bright 
than in diffused light. Many plants will not erow under the 
shade of others, because when in dense shade they cannot manu- 
facture sufficient sugar to keep them alive. For this reason it is 
not advisable to grow crop plants too close together, as they 
then shade each other, with the result that the plants are not as 
vigorous as they should be. 
On the same plant the leaves which are fully exposed to the 
sun are thicker than those grown in the shade. This difference 
in thickness is of advantage, because when the light is strong 
