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 

 them are small quantities of various other substances. 

 Chief among the sccondar}' 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 

 an}' carbonaceous substance burns, carbon dioxid 

 is form(>d. Leaves have the jxiwer of absorbing 

 the gas carl)on dioxid from the air and separating 

 the car]:)on 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 l)}' the plant of oils, fats, starches, sugars, pro- 

 tein, and all the other ]3roducts of plant growth. 



This important process known as carbon assimila- 

 tion is made {possible I33' the aid of countless small 

 openings which exist chiefl}' on the surfaces of leaves 

 and knownas "stomata." The stomata are dehcately 

 balanced valves, exceedingly sensitive to external 

 influences. Their a]3]3earance under a high power mi- 

 croscope is shown in Figures 41 and 42. They are 

 more numerous on the lower side than on the upper 

 side of plants. In fact, there is often flve 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 



