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® Physical Structure of Plants. 59 
Palmer, Daubeny, and Draper, also sustain this position. The 
uniform absorption of a portion of oxygen by the plant, is proved 
by the researches of Gough, Achard, Scheele, Cruickshank, Saus- 
sure and others. Gough. showed that an atmosphere without 
oxygen was incapable of producing the green color of leaves by 
changing the yellow color of such as were raised in darkness, 
(Manch. Mem., iv, p. 501.) -'This singular fact is now explained 
by the doce of the nature of chlorophyl, which M. renen 
shows to be an oxydized product. 
30. The absorptions taking place under the influence of light 
owe, their continuance during the growth of the plant to its ac- 
The gas entering, is destined to equilibrate the internal 
ieee and give it a composition analogous to common air, 
but this it is incapable of accomplishing because of the decom- 
posing action of light. Hence a constant current of carbonic 
acid sets into the plant, the oxygen of which is partly employed 
in producing chemical results, and a deficiency of both carbonic 
acid and oxygen is consequently presented by the plant atmos- 
phere, these two bodies being withdrawn by the organic changes 
faster than they penetrate. 
‘31. It is only so long as the carbonic acid and oxygen are ap- 
propriated by chemical forces that the internal plant gas differs 
materially from common air. As soon as decomposition by the 
Sun’s light is arrested, the absorption of carbonic acid from the at- 
mosphere ceases, and that portion which rises along the roots, is, 
according to Ingenhousz, in part thrown into the air. Oxygen is 
still absorbed for the production of special compounds. After a 
time, supposing the chemical affinities satisfied, the plant atmos- 
phere has obtained af equilibrium with the air, and all move- 
Ments cease. In Messrs. Calvert and Ferrand’s experiments, the 
increase of carbonic acid during night in the plant gas is a con- 
Stant feature, the oxygen also became equal to, the proportion in 
air. Under these circumstances if the fluid absorbed from the 
earth be rich in carbonic acid, a portion will be evolved as soon 
as it has been conveyed by the sap into the plant atmosphere in 
excess. This condition may not be satisfied in one case, and no 
carbonic acid liberated, as found by Mr. Pepys, whilst in ssnsthier, 
from the nature of the soil, the gas may be abundantly thrown 
Sut, as proved by Saussure, Ingenhousz, and others. 
32. W Dacrdianiedie’ justified in regarding a plant as a porous 
system, the normal or internal’ atmosphere of which has a com- 
