FUNCTION OF LEAVES. 115 



sition it nssumes thron<^li tlic day, tlms l)avinrr no propensity 

 to devcIopiULMit in any particiilar direction by llie action of 

 the li^ht. 



142. Another function of the leaves is that of absorbing and 

 tjivina out oxygen. Tliat plants yiehl oxygen to tiie atmos- 

 phere iiAs long been known, and that they absorb oxygen du- 

 ring the night is not a very recent discovery. Our preceding 

 remarks render it unnecessary to add much to our observations 

 on plants giving out o\ygen in respiration, as it results most- 

 Iv from tlie decomposition of carbonic acid ; but we arc un- 

 able to account for the constitution of various vegetable pro- 

 ducts without supposing the decomi»osition of water, Irom 

 whicli the vegetable derives the hydrogen used in the forma- 

 tion of these substances. Oil lor instance, is composed of 1 

 atom of oxygen, II of hydrogen, and 10 of carbon. Now 

 no substance, that enters as food into the |)Iant can yield this 

 great amount of hydrogen but water. This decomp<jsition of 

 water, no doubt, in a great measure, takes place in the leaves, 

 for volatile oils are very comn)only found in these organs, and 

 when tiiey are not found in the leaves, they often exist in 

 cavitiis with apparently no secreting l)odies for their for- 

 mation in the situations in which we find them. In many 

 cases, at least, it seems proi)able, from these considerations, 

 that tln'y are generated in the leaves, or in parts p<'rforming 

 the »ime functions. The bark, in certain states, is capable of 

 performing the same operations as the leaves. 



143. The absorption of oxygen takes place in the night. We 

 may convince ourselves of this by confining a plant during 

 Dight in atmospheric air, and the oxygen will be perceptibly 

 diminis^bed. This oxygen unites chomically with subs'ances 

 contained within the leaves, and proba!)ly with carbonaceous 

 food not in the form of carbonic.acid, and it would seem that 

 only in this state is it fitted for assimilation. The oxygen is 

 immediately given back to the atmosphere on the return of 

 day. The quantity of oxygen absorbed by |)lan(s is too small 

 to have any effect on the health of animals by its abstraction. 

 * 144 Besides absorbing and giving off oxygen, plants con- 

 stantly, in healthy action, give ofi* a small quantity of carbonic 

 acid, both day and night, it has been supposed that this was the 

 true and only effect of vegetable respiration on the atmosj)here, 

 and that the absorbing and giving off oxygen and decompo- 

 sing carbonic acid and water, belonged to digestion, 'ibis 

 we are disposed to believe is the true state of the case. From 

 this it would result that the respiration of vegetables vitiates 

 the atmosphere as does that of animals; but the effects of their di- 



