FUNCTIONS OF LEAVES. 105 



the lower branches decay, and thus forming tall, straight, cylin- 

 drical trunks, with the branches near their summits, Trees in 

 open fields never grow as tall as in forests, but they have a 

 greater number of branches, and nearer to the earth, and the 

 reason is plain from the foregoing remarks. They receive the 

 direct rays of the sun at every different position it assumes 

 through the day, thus having no propensity to development in 

 any particular direction by the action of the light. 



185. We are unable to account for the constitution of vari- 

 ous vegetable products, without supposing the decomposition of 

 water, from which the vegetable derives the hydrogen used in 

 the formation of those substances. Oil, for instance, is com- 

 posed of 1 atom of oxygen, 1 1 of hydrogen, and 1 of carbon. 

 No substance that enters as food into the plant can yield this 

 great amount of hydrogen but water. This decomposition of 

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

 for volatile oils are very commonly found in these organs, and 

 when they are not found in the leaves, they often exist in cavi- 

 ties, with apparently no secreting bodies for their formation, in 

 the situations in which we find them. In many cases, at least, 

 it seems probable, from these considerations, that they are 

 generated in the leaves, or in parts performing the same func- 

 tions. The bark, in certain states, is capable of performing the 

 same operations as the leaves. 



186. The absorption of oxygen takes place in the night. 

 "We may convince ourselves of this by confining a plant during 

 night in atmospheric air, and the oxygen will be perceptibly 

 diminished. This oxygen unites chemically with substances 

 contained within the leaves, and probably 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 plants is too small 

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



187. Besides absorbing and giving off oxygen, plants con- 

 stantly, in healthy action, give off 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 atmos- 

 phere, arid that the absorbing and giving off oxygen and de- 

 composing carbonic acid and water belonged to digestion. 

 From this it would result that the respiration of vegetables 

 vitiates the atmosphere as does that of animals ; but the effects 



Why expanding in the open fields ?— 185. What proves that water is de- 

 composed ?— 186." What takes place at night?— 187. What other function 

 have leaves? What have Borne supposed? 



