Vegetable Organography and Physiology. 57 
manner in which this gas was produced, whether it was genera- 
ted by the plant or taken from the atmospheric air and afterwards 
discharged, remained an unsettled question until the experiments 
of Sennebier, De Candolle, and other vegetable physiologists, pro- 
ved that in this process are involved some of the most important 
laws of vegetable life. By the experiments of these philosophers 
it was proved that during the night oxygen is absorbed by the 
leaves of the plant, which combining with the carbon previously 
brought up from the earth in the ascending sap, is converted into 
carbonic acid. On the return of solar light, a decomposition takes 
place; the oxygen is evolved, and the carbon is retained for the 
nourishment of the plant. “It is evident,” says Roget, “ that the 
object of the whole process is to obtain carbon, in that precise state 
of disintegration to which it is reduced at the moment of its sep- 
aration from carbonic acid, by the action of solar light, on the 
green substance of the leaves; for it is in this precise state alone 
that it is available in promoting the nourishment of the plant, and 
hot in the crude condition in which it exists when it is pumped 
up from the earth eh with the water which conveys it into the 
interior of the plan 
The amount of ae given off in the day, is much great- 
er than the quantity which is absorbed during the night. Much 
of the carbonic acid of the atmosphere is also decomposed by — 
the leaves of plants; the carbon is retained, and the oxygen is 
evolved. Hence it is that the growth of healthy plants exercises’ 
a purifying influence upon the surrounding atmosphere, and beau- 
tifully adapts it to the respiration of animals. 
Besides this decomposition of carbonic acid, various other chem- 
ical changes are wrought upon the sap, and the materials contain- 
ed in the sap, in their circulation through the leaves. A part of 
the water is decomposed, and by the various combination of its 
elements with the different mineral substances which it held in 
solution, those vegetable products are formed which are suited for 
the further growth and development of the plant. Thus will be 
Seen the beautiful and perfect analogy which exists between the 
circulation in vegetables and animals. The ascending sap, like 
the venous blood in its circulation through the lungs, having been a 
me Se 
* Animal and Veaulile Physiol. Vol. I, a 31. 
Vol. xxxvit, No. 1.—Oct.—Dec. 1839 8 
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