Page:An introduction to physiological and systematical botany (1st edition).djvu/245

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deeply into all the various chemical examinations respecting it than suits our purpose. It is only necessary to add a short view of Dr. Darwin's hypothesis which Dr. Thomson has not mentioned, probably on account of its insufficiency. That lively writer thought the watery perspiration of leaves, acted upon by light, gave out oxygen for the use of the plant itself, such oxygen being immediately absorbed by the air-vessels. This is by no means adequate to explain any of the phenomena, but rather contradictory to most of them, and is totally superseded by the observations and experiments of other writers.

There can be no question of the general purpose answered to the vegetable constitution by these functions of their leaves. They confirm Mr. Knight's theory of vegetation, who has proved that very little alburnum or new wood is secreted when light is kept from the leaves. They also help us to understand how essential oils may be produced, which are known, as well as sugar, to be composed of oxygen, hydrogen and carbon in different p oportions. We can now have a general idea how the nutritious sap, acted upon by