whose bottoms are covered with growing plants, 
afford to every eye a familiar and most satis- 
factory illustration of the same phenomenon. 
When these little sheets of water are so com- 
pletely frozen over as to have no communication 
with the atmosphere, small globules of gas are 
observed to escape, in rapid succession, from the 
points of the leaves and twigs, and to accumulate 
under the ice till they form large bubbles; and 
these bubbles steadily increase in volume so long 
as the ice remains unbroken, and are found to 
consist of pure oxygen,—the plants which form 
them obviously absorbing the carbonic acid held 
in solution in the water, resolving it into its ele- 
ments, assimilating the carbon, and liberating 
the oxygen. 
Just as these illustrations, and a thousand 
more, evince that plants absorb and decompose 
the carbonic acid of water ; so do other and quite 
as clear illustrations evince that they absorb and 
decompose the carbonic acid of the atmosphere. 
One of these—an experiment of De Saussure—not 
only proves the fact of the absorption of aerial car- 
bonic acid, but at the same time shows the high- 
est proportion of it in atmospheric air which is 
capable of assimilation or of healthy action. “De 
Saussure, having taken a number of young plants 
of pease, of about the weight of twenty grains 
each, he immersed them by the root in glasses 
filled with water. Thus conditioned, he next in- 
troduced them into receivers exposed to the 
direct rays of the sun, and filled some with com- 
mon air, and some with different mixtures of 
common airand carbonic acid gas. In an atmo- 
sphere of common air, exposed during ten days 
to the sun, they were found to have increased 
their weight by eight grains. In an atmosphere 
of pure carbonic acid gas, they faded and withered 
away, without any farther development. In an 
atmosphere containing three-fourths of carbonic 
acid gas, they withered also. 
only one half, they lived seven days; if but one 
quarter, ten days, augmenting their weight by 
five grains; and if containing only one-twelfth of 
carbonic acid gas, they increased their weight by 
eleven grains. This was the maximum of its 
beneficial application.” Many and most accu- 
rate experiments have been conducted with plants 
artificially placed in soil destitute of carbon, and 
observations made upon plants naturally growing 
on sands and rocks almost destitute of humus; 
and they have ascertained, to a demonstration. 
that the carbon constituting the great propor- 
tion of the plants’ substance, and the acquisition 
and fixing of which is the chief process of their 
growth, could not possibly have been obtained 
elsewhere than from the carbonic acid of the at- 
mosphere, partly carried down to the roots in 
water, but principally or almost wholly absorbed, 
in its aeriform condition, by the leaves and the 
green portions of the stem and branches. 
By another and very numerous set of experi- 
ments and observations, plants have been ascer- 
CARBONIC ACID. 
But if containing | 
tained to absorb carbonic acid and assimilate its 
carbon, only during the presence of light,—to be 
capable of absorbing any proportion artificially 
added to the natural quantity in the atmosphere, 
only during the play of sunshine,—to assimilate 
the carbon of such as is taken up by the roots, 
only when it ascends with the sap to the leaf,— 
and to be incapable of absorbing any from the 
atmosphere during the darkness of night, but, on 
the contrary, to give out, during that time, a 
small proportion into the atmosphere. An opinion 
very generally prevails, and was not long ago all 
but universal, that the assimilation of the carbon 
of carbonic acid, in any circumstances, is effected 
only during the play of sunshine, and cannot be 
effected in diffused light; but this is altogether 
anerror. ‘“Hxactly the same constituents,” re- 
marks Liebig, “are generated in a number of 
plants, whether the direct rays of the sun fall on 
them, or whether they grow in the shade. They 
require light, and indeed sun-light, but it is not 
necessary that the direct rays of the sun should 
reach them. Their functions certainly proceed | 
with greater intensity and rapidity in sunshine 
than in the diffused light of day ; but there is no- 
thing more in this than the similar action which 
light exercises on ordinary chemical combina- | 
tions; it merely accelerates, in a greater or less 
degree, the action already subsisting.” 
The absorption of the carbonic acid of water 
by the leaves of aquatic plants growing under 
water, and the absorption of the carbonic acid 
of the atmosphere by the leaves of land plants 
growing in the air, are processes apparently dif- 
ferent, but really the same. The tissues of leaves 
and other parts in the air are saturated with | 
water, either brought up to them in the form of 
ascending sap from the spongioles, or directly 
imbibed by them from the rains of the atmo- | 
sphere or from the nightly depositions of dew; | 
and, being penetrated and moistened with this | 
water in every part of their succulent structure, 
they do not inhale the carbonic acid atmosphere | 
as a gas, but receive it into solution with this 
saturating moisture, and, in consequence, ab- 
sorb it in exactly the same form, and dispose of | 
it by exactly the same process of assimilation, as | 
the leaves of submerged aquatics do to the car- | 
The only actual difference | 
bonic acid of water. 
between the two classes of plants regards, not at 
all their behaviour with carbonic acid when sup- 
plied to them, but merely the outward and ex- 
traneous contrivance for affording them supplies; | 
and this, in very many instances, furnishes a 
beautiful illustration of the all-pervading care 
with which the Divine Being provides for the 
wants and the mutual adaptations of even his 
inorganic and his unanimated creatures. In the 
case of aquatic plants, which grow wholly sub- 
merged and are fixed to one spot, currents of 
water are established by variations of specific 
gravity to bring them a constant supply of their 
principal food; so that as soon as one portion of 
