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assumed between the absorption of oxygene and 
the formation of carbonic acid gas in germination, 
and in the respiration of the foetus. Similar argu- 
ments will apply against the pursuit of this analogy, 
between the functions of the leaves of the adult 
plant, and those of the lungs of the adult animal. 
Plants grow vigorously only when supplied with 
light ; and most species die if deprived of it. It 
cannot be supposed that the production of oxygene 
from the leaf, which is known to be connected with 
its natural colour, is the exertion of a diseased 
function, or that it can acquire carbon in the day- 
time, when it is in most vigorous growth, when 
the sap is rising, when all its powers of obtaining 
nourishment are exerted ; merely for the purpose 
of giving it off again in the night, when its leaves 
are closed, when the motion of the sap is imperfect, 
and when it is in a state approaching to that of 
quiescence. Many plants that grow upon rocks, 
or soils, containing no carbonic matter, can only 
be supposed to acquire their charcoal from the car- 
bonic acid gas in the atmosphere ; and the leaf may 
be considered at the same time as an organ of ab- 
sorption, and an organ in which the sap may un- 
dergo different chemical changes. 
When pure water only is absorbed by the roots 
of plants, the fluid, in passing into the leaves, will 
probably have greater power to absorb carbonic 
acid from the atmosphere. When the water is 
saturated with carbonic acid gas, some of this 
substance, even in the sunshine, may be given off 
by the leaves *, but a part of it likewise will be 
p 2 
