102 FUNCTIONS OF LEAVES. 



the sap as it enters the leaves. That it is chaiiored in its ascent 

 is certain. Of this we have often convinced ourselves by 

 making an incision in the spring of the year near the root of a 

 Birch, and sap, with very little taste, will be obtained ;'but by 

 making the incision several feet high, the sap is bitter, and the 

 bitterness increases with the elevation of the incision. 



The sap in this partially altered state, whicli is owing to 

 chemical changes, enters the leaves. 



179. The first action of the leaves is to get rid of the super- 

 fluous water, in which its food is dissolved. Whether this is 

 any thing more than simple evaporation, we are not prepared 

 to say, yet we believe the plant has partial control, at leastr, 

 over the quantity. The construction of the stomata plainly in- 

 dicates this. It is influenced by the same causes which govern 

 common evaporation. Under the direct influence of the sun's 

 rays it is most rapid ; in the difl'used light of day it is less ; 

 and in the dark it almost ceases. 



180. The quantity of fluid given out by plants is in some 

 cases very great. We may convince ourselves of this by hold- 

 ing a glass near the under surface of a vigorous leaf of the vine, 

 and it will soon be covered with moisture, and in a little while 

 it will accumulate in drops, and run oft' the plate. Hales found 

 the evaporation of a Sunflower to be one pound and four 

 ounces, and a Cabbage one pound and three ounces in a single 

 day, and estimates the evaporation of plants to be seventeen 

 times greater than that of animals. 



181. The next and most important function of leaves is the 

 decomposition of carbonic acid. It is only by the performance 

 of this function by the leaves that the solid parts of vegetables 

 are deposited. Any cause whicli arrests this operation imme- 

 diately renders the plant sickly, and its peculiar secretions cease 

 to be deposited. Light is absolutely essential to the perform- 

 ance of this function of the leaves. In the dark no carbon is 

 deposited, and no oxygen is liberated. 



De Candolle says : " If two plants are exposed, the one to 

 darkness, and the other to the sun's rays, in a close vessel, and 

 in an atmosphere containing a known quantity of carbonic 

 acid, and are removed at the end of twelve hours, w^e shall find 

 that the first has diminished neither the quantity of oxygen or 

 cart)onic acid, and that in the second, on the contrary, the 

 quantity of carbonic acid has diminished, while the quantity of 



What proves it?— 179. What is the first action of the leaves? What 



Sartially controls it ? When most rapid? — ISO. What of the quantity of 

 uid given out? How proved? — 181. The next function ? What does D« 

 Candolle say ? 



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