nt. To attain a
maximum in height in the short period of their existence, the
nourishment contained in the atmosphere is not sufficient. If the
end of cultivation is to be obtained, we must create in the soil an
artificial atmosphere of carbonic acid and ammonia; and this surplus
of nourishment, which the leaves cannot appropriate from the air,
must be taken up by the corresponding organs, i.e. the roots, from
the soil. But the ammonia, together with the carbonic acid, are
alone insufficient to become part of a plant destined to the
nourishment of animals. In the absence of the alkalies, the
phosphates and other earthy salts, no vegetable fibrine, no
vegetable caseine, can be formed. The phosphoric acid of the
phosphate of lime, indispensable to the cerealia and other
vegetables in the formation of their seeds, is separated as an
excrement, in great quantities, by the rind and barks of ligneous
plants.

How different are the evergreen plants, the cacti, the mosses, the
ferns, and the pines, from our annual grasses, the cerealia and
leguminous vegetables! The former, at every time of the day during
winter and summer, obtain carbon through their leaves by absorbing
carbonic acid which is not furnished by the barren soil on which
they grow; water is also absorbed and retained by their coriaceous
or fleshy leaves with great force. They lose very little by
evaporation, compared with other plants. On the other hand, how very
small is the quantity of mineral substances which they withdraw from
the soil during their almost constant growth in one year, in
comparison with the quantity which one crop of wheat of an equal
weight receives in three months!

It is by means of moisture that plants receive the necessary
alkalies and salts from the soil. In dry summers a phenomenon is
observed, which, when the importance of mineral elements to the life
of a plant was unknown, could not be explained. The leaves of plants
first developed and perfected, and therefore nearer the surface of
the soil, shrivel up and become yellow, lose their vitality, and
fall off while the plant is in an active state of growth, without
any visible cause. This phenomenon is not seen in moist years, nor
in evergreen plants, and but rarely in plants which have long and
deep roots, nor is it seen in perennials in autumn and winter.

The cause of this premature decay is now obvious. The
perfectly-developed leaves absorb continually carbonic acid and
ammonia