172 DRY-FARMING 
plant substance is gathered from the air by the 
leaves and it is evident that the leaves are very 
active agents of plant growth. The atmosphere 
consists chiefly of the gases oxygen and nitrogen in 
the proportion of one to four, but associated with 
thera are small quantities of various other substances. 
Chief among the secondary constituents of the at- 
mosphere is the gas carbon dioxid, which is formed 
when carbon burns, that is, when carbon unites with 
the oxygen of the air. Whenever coal or wood or 
any carbonaceous substance burns, carbon dioxid 
is formed. Leaves have the power of absorbing 
the gas carbon dioxid from the-air and separating 
the carbon from the oxygen. The oxygen is returned 
to the atmosphere while the carbon is retained to be 
used as the fundamental substance in the construc- 
tion by the plant of oils, fats, starches, sugars, pro- 
tein, and ail the other products of plant growth. 
This important process known as carbon assimila- 
tion is made possible by the aid of countless small 
openings which exist chiefly on the surfaces of leaves 
and knownas “‘stomata.’”’ The stomata are delicately 
balanced valves, exceedingly sensitive to external 
influences. Their appearance under a high power mi- 
croscope is shown in I’igures 41 and 42. They are 
more numerous on the lower side than on the upper 
side of plants. In fact, there is often five times more 
on the under side than on the upper side of a leaf. It 
has been estimated that 150,000 stomata or more are 
