a double proportion of carbonic acid, a
plant absorbs, under the same condition, twice the quantity of
carbon. Boussingault observed, that the leaves of the vine, inclosed
in a vessel, withdrew all the carbonic acid from a current of air
which was passed through it, however great its velocity. (Dumas
Lecon, p.23.) If, therefore, we supply double the quantity of
carbonic acid to one plant, the extent of the surface of which is
only half that of another living in ordinary atmospheric air, the
former will obtain and appropriate as much carbon as the latter.
Hence results the effects of humus, and all decaying organic
substances, upon vegetation. If we suppose all the conditions for
the absorption of carbonic acid present, a young plant will increase
in mass, in a limited time, only in proportion to its absorbing
surface; but if we create in the soil a new source of carbonic acid,
by decaying vegetable substances, and the roots absorb in the same
time three times as much carbonic acid from the soil as the leaves
derive from the atmosphere, the plant will increase in weight
fourfold. This fourfold increase extends to the leaves, buds,
stalks, &c., and in the increased extent of the surface, the plant
acquires an increased power of absorbing nourishment from the air,
which continues in action far beyond the time when its derivation of
carbonic acid through the roots ceases. Humus, as a source of
carbonic acid in cultivated lands, is not only useful as a means of
increasing the quantity of carbon--an effect which in most cases may
be very indifferent for agricultural purposes--but the mass of the
plant having increased rapidly in a short time, space is obtained
for the assimilation of the elements of the soil necessary for the
formation of new leaves and branches.

Water evaporates incessantly from the surface of the young plant;
its quantity is in direct proportion to the temperature and the
extent of the surface. The numerous radical fibrillae replace, like
so many pumps, the evaporated water; and so long as the soil is
moist, or penetrated with water, the indispensable elements of the
soil, dissolved in the water, are supplied to the plant. The water
absorbed by the plant evaporating in an aeriform state leaves the
saline and other mineral constituents within it. The relative
proportion of these elements taken up by a plant, is greater, the
more extensive the surface and more abundant the supply of water;
where these are