4*2 THE LEAF. 



double offices of absorbing and emitting both liquids and 

 gases. The effects which the leaves of plants produce on 

 the air they absorb, or inhale, has been the subject of very 

 laborious experiments. The chemical changes which have 

 been ascertained to be thus produced are too important not to 

 be noticed in this place. " The most important chemical 

 phenomenon," says Professor Lindley, " connected with the 

 growth of plants, is the property possessed by their leaves, 

 or green parts, of absorbing and parting with carbonic acid in 

 the dark, and parting with their oxygen under the influence 

 of the sun." 



" No plant can long exist [live] in which an alternate ab- 

 sorption and expulsion of oxygen does not take place, except 

 the Fungi [mushrooms.] The expulsion of oxygen is deter- 

 mined by the quantity of light to which the plant is exposed. 

 Light causes the decomposition of the carbonic acid gas, and 

 the accumulation of solid matter [in the plant.] Hence if a 

 plant is exposed to too strong a light, it generally perishes 

 from the excessive expulsion of oxygen. And if it is not 

 exposed to the influence of light, it dies from the accumula- 

 tion of that principle. If there is too great an accumulation 

 of oxygen, an attempt will always be made by the plant to 

 reach the light, for the purpose of parting with the superflu- 

 ity ; as in seeds which in germination shoot from darkness 

 into the light. If this cannot be effected, etiolation [whiten- 

 ing] first takes place, which is caused by the accumulation of 

 oxygen, and the consequent non-deposition of carbon, and 

 death succeeds." Natural System. 



From these principles, or rather from experiments on which 

 these principles have been founded, we learn that if a plant 

 be confined to a close vessel in the dark, containing a certain 

 portion of atmospheric air, the vegetation of the plant will 

 convert the oxygen which the air contains (being 20 parts to 

 every 100 of the whole) into carbonic acid. This change 

 the leaves effect during the absence of light, by first absorb- 

 ing the oxygen, which then combines with a portion of the 

 carbon, contained in the leaf, after which the compound gas 

 is expelled, and thus the oxygen the vessel contained is con- 

 verted into carbonic acid. 



If now the plant is exposed to the direct rays of the sun 

 in the same glass vessel, and containing the carbonic acid. 



What are the most important chemical phenomena produced by the 

 eaves of plants ? Under what circumstances do plants emit and absorb 

 oxygen ? When does a plant convert oxygen into carbonic acid ? 



