THE LEAF 145 
The cotyledons are thick and solid, while the 
foliage leaves are more or less thin and membranous. 
In most plants the cotyledons grow smaller as the 
store of reserve is used up, while foliage leaves, on the 
other hand, grow larger. This, however, is not true of 
the pumpkin and several other plants, in which the seed 
leaves grow larger and become green. 
SUMMARY. 
The leaf takes carbon dioxide from the air and by 
means of its chlorophyll utilizes the energy of the 
sunlight to carry out photosynthesis. The arrange- 
ment of leaves on the stem and the formation of leaf 
mosaics indicate the light relation. 
Structure.—The upper epidermis is protective and 
has but few stomata, the lower has many stomata that 
provide for ingress and egress of gases and exit of 
water. The mesophyll has many air spaces and econ- 
tains chlorophyll. 
Photosynthesis is the process by which, in the 
presence of chlorophyll and sunlight, carbon dioxide is 
decomposed and the carbon built up with the elements 
of water from the soil to form organic compounds. The 
carbohydrate is stored temporarily in the leaf as 
starch. Amides are formed by the addition of nitrogen. 
The carbohydrate circulates in the plant in the form of 
sugar, and may form starch, inulin, or cellulose. The 
amides, with sulphur and phosphorus, form proteins. 
Not all the material formed goes to build the plant, 
some is oxidised to supply energy. 
Without iron chlorophyll is not formed and 
without potash there is no photosynthesis. Light is 
proved to be necessary by covering parts of leaves, 
and chlorophyll by testing variegated leaves. The 
oxygen given off may be collected. To prove that the 
carbon comes from the carbon dioxide of the air grow 
a plant in an atmosphere devoid of that gas. 
