FUNCTION OF LEAVES. 113 



of the leaves in decomposing carbonic acid, and tliat the light 

 of the hun ia neces^.arv for its operation. 



Tlie same author instituted another ingenious experiment 

 to show the absorption of carbonic acid by the roots and its 

 subsequent decomposition. lie tilleil a cistern, and an inveit- 

 cd bell glass, with distilled water, the glass having a sprig 

 of mint floating in it ; in the same cistern was j)laced an- 

 other bell tilass containin«i carbonic acid. The surface of 

 the water was covered wilij a stratum of oil to prevent 

 the access of atmospheric air. The whole was then expc- 

 aed to the direct action of the sun. The carbonic acid di- 

 minished daily, while the glass containing the mint had ac- 

 quired a quantity of oxygen exactly equal to the loss of car- 

 bonic acid. A similar plant placed under simiar circum- 

 stances with the exception of the jar of carbonic acid, disen- 

 gaged no oxygen, and at the end of twelve days, the time 

 the experiment continued, commenced to decay, wiiile the 

 other one was in good health. No firther proof cntainly 

 could be n^quired to establish any f<»ct than iUeae aflord in 

 establishing the function of the leaves in dccom|)osing car- 

 bonic acid, bv the aid of the sun's liixht. 



It is a singular fact that such experiments will not succeed 

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

 these operations lake place in eome degree in plants growing 

 in shaded |)laces, yet when placed under circumstances for ex- 

 periment, they require the direct action of the sun to pt^rforni 

 this funtrtion. 



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

 proved by exposing plants to the artion of ^n atmosphere that 

 contains carbonic acid, and similar ones to an atuiosplirre that 

 contains none ; the foruier will increase in carbon, while the 

 latter will not. The two principles established by thise ex- 

 periments are of much j)ractical importance. 'I'hev teach us 

 that the presence of carbonic acid is absolutelv necessary for 

 the growth of vegetables, and that if we expect the lull and 

 perfect development of plants ihey must have the direct ac- 

 tion of the sun's light. To secrete all the peculiar products 

 in perfection these circumstanrrs uuist conspire, ''"he gar- 

 dener is well aware of these facts in practice. To prevent 

 the deposition of unpleasant product-;, whicii are natural lo 

 the plant, he covers the stem of the celery, and it ileposits 

 little except the simplest tissue, and becomes loaded with 

 nearly insipid fluids; the sterner juices of the plant previously 

 deposited l>oroming diluted, so that it is a healthful and ngrcea- 

 ble food. This plant, uncultivated in the manner it is, vicld^ 



10* 



