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 misettled 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 

 not in the crude condition in which it exists when it is pumped 

 up from the earth along with the water which conveys it into the 

 interior of the plant."* 



The amount of oxygen given off during 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 



* Animal and Vegetable Physiol. Vol, II, p. 31. 

 Vol. XXXVIII, No. 1.— Oct.-Dec. 1839. 8 



