FUNCTIONS OF LEA.VES. 103 



free oxygen has increased in the same proportion." This ex- 

 periment shows beyond doubt the function o'f the leaves in de- 

 composing carbonic acid, and that the light of the sun is neces- 

 sary for its operation. 



182. The same author instituted another ingenious experi- 

 ment, to show the absorption of carbonic acid by the roots, and 

 its subsequent decomposition. He filled a cistern and an in- 

 verted bell-glass with distilled water, the glass having a sprig 

 of mint floating in it ; in the same cistern was placed another 

 bell-glass containing carbonic acid. The surface of the water 

 was covered with a stratum of oil, to prevent the access of at- 

 mospheric air. The whole was then exposed to the direct 

 action of the sun. The carbonic acid diminished daily, while 

 the glass containing the mint had acquired a quantity of oxy- 

 gen exactly equal to the loss of carbonic acid. A similar plant 

 placed under similar circumstances, with the exception of the 

 jar of carbonic acid, disengaged no oxygen, and at the end of 

 twelve days, the time the experiment continued, commenced to 

 decay, while the other one was in good health. No further 

 proof certainly could be required to establish any fact, than 

 these afford in establishing the function of the leaves in decom- 

 posing carbonic acid by the aid of the sun's light. 



It is a singular fact that such experiments will not succeed 

 unless exposed to the direct action of the sun's rays. Although 

 these operations take place in some degree in plants growing 

 in shaded places, yet when placed under circumstances for ex- 

 periment, they require the direct action of the sun to perform 

 this function. 



183. That the carbon has been deposited in the plant is also 

 proved by exposing plants to the action of an atmosphere that 

 contains carbonic acid, and similar ones to an atmosphere that 

 contains none : the former will increase in carbon, while the 

 latter will not. The two principles established by these experi- 

 ments are of much practical importance. They teach us that 

 the presence of carbonic acid is absolutely necessary for the 

 growth of vegetables, and that if we expect the full and perfect 

 development of plants, they must have the direct action of the 

 sun's light. To secrete all the -peculiar products in perfection, 

 these circumstances must conspire. The gardener is well aware 

 of these facts in practice. To prevent the deposition of unpleas- 

 ant products, which are natural to the plant, he covers the stem 

 of the celery, and it deposits little except the simplest tissue, 



182. What other experiment ? When only Mill such experiments suc- 

 ceed?— 183. Y»'hat other proof? What do these facts teach us? How 

 does the gardener prevent unpleasant products ? 



