250 THE CULTURE OF FARM CROPS. 



moisture in dry weather, and expand and open it when moist. 

 It is the action of these contracting cells which cause leaves 

 to curl in very dry weather. These air pores are very nu- 

 merous on the under side of the leaves, varying in number 

 from 1000 to 170,000 to the square inch of surface. A leaf 

 of corn will therefore have many millions of these breath- 

 ing pores upon its surface; and an apple leaf, which is not 

 so well provided with these organs, has only about 100,000 

 of them to each leaf 



It is through these pores that the leaves perform their 

 most important office, viz: that of elaborating the crude sap 

 into organic matter which is deposited throughout the plant, 

 and from which its substance is formed. The water in ex- 

 cess of the needs of the plant is exhaled through these pores, . 

 and thus the solid matter of the sap is deposited. 



It is also by these pores that air enters into the leaves, 

 and mingling with the sap produces such chemical changes. 

 in it as fit it for its purpose. The air also enters and brings 

 in with it the carbonic acid which is mixed with it, in the 

 proportion of one twenty -five hundredth part of its bulk. 

 This carbonic acid is supposed to furnish a large part of the 

 carbon of which plants consist, but as the water taken in 

 by the roots also contains carbonic acid in solution, there is 

 no doubt that a large part of the carbon of plants is derived 

 in this way through the soil. Water and carbonic acid 

 taken in by the leaves (or by the roots) are the raw mater- 

 ials of which the fabric of plants are mostly made up; and 

 to change these dead mineral matters into living organic 

 matter is the principal function of the leaves. 



This function is performed in the green leaves alone, and 

 only in the light of the sun. The sun beam is the giver of 

 life to the dead matter, and the grand chemical agency of 

 all plant growth. The proof of this is one of the simplest 

 rational propositions. 



First. The green part of leaves exhale oxygen only in 

 sunshine or bright daylight. 



Second. To give out oxygen is all that is required to 

 change water and carbonic acid into cellulose or plant food. 



