112 FU^'CT10N OF LEAVESi 



of the sap as it enters the leaves. That it is changed in its as- 

 cent is certain ; of this we have often convinced ourselves 

 by making an incision in t!ie .spring of tlie 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 wiili the elevation of the incision. 

 Sugar makers (from the Maple) know that the higher they 

 lap the trees the sweeter is the sap. 



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

 chemical changes, enters the leaves. 



138. The first action of the leaves is to get rid of the superflu* 

 ous 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 kast, over 

 the quantity. Tlie construction of the stoniates plainly in* 

 dicates this. It is influenced by the same causes which gov- 

 ern common evaporation. Under the direct influence of the 

 sun's rays it is most rapid ; in the diffused light of day it is 

 less, and in tr.e dark it almost ceases. 



The quantity of fluid given out hy plants is in some cases 

 very great. We may convince ourselves of this by holding 

 a glass near the under surfice 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 off the plate. Hales found 

 the evaporation of a sunflower to l;e one pound and four oun« 

 ces, and o cabbage one pound and three ounces in a single 

 day, and estimates the evaporation of plants to he seventeen 

 times greater than that ot animals. 



139. The next and most inportani function of leaves is the 

 decomposition of carbonic acid. It is only by the performance 

 of this function by the leaves, thfit the solid parts of vegeta- 

 bles are deposited. Any cause which arrests t;.is operation 

 immediately renders the plant sickly and its peculiar secre- 

 tions cease to be deposited. Light is ahsolutely essential to 

 the performance of this function of the leaves. In the dark 

 no carbon is deposited and no oxygen is liberated. 



DeCandolle 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 qsiantity of carbonic 

 acid, and are removed at the end of twelve hours, we shall 

 find that the first has diitiinished neither the quantity of oxy- 

 gen nor carbonic acid; and that in the second, on the con- 

 trary, the quantity of carbonic acid has diminished, while 

 the quantity of fiee oxygen has increased in the same pro- 

 portion." This experiment shows beyond doubt the function 



