FUNCTIONS OF LEAVES 



33 



10. These simple experiments teach us the function 

 of the leaves. In the first place it is through them 



almost entirely that the 

 water taken up in such large 

 quantities by the roots is 

 sent out again into the air. 

 This large quantity of water 

 is necessary, as we saw, 

 to supply the mineral 

 substances which can only 

 be taken up by the roots 

 in a very dilute solution. 



The second and more 

 important function is the 

 making of the plant's food, 

 that is to say, assimilation 

 of carbon and perhaps also 



the formation of proteids. 



FIG. 4 



These processes take 

 place only in the green parts, and only in sunlight. 



And thirdly, through the stomas of the leaves also 

 passes out some of the carbon dioxide produced by 

 respiration of the living substance. 



11. In order that leaves may do their work pro- 

 perly they must have plenty of air and light. The 

 more of these they get the more carbon can be 

 assimilated from the air, and the more proteid food 

 substance be made. We see therefore why it is that 

 the branches of most trees and bushes spread so 

 widely, and divide so repeatedly ; and why nearly all 

 the leaves are on the smallest branches, and towards 

 their ends, on the outside : so that a well-branched 

 3 



