of the earth with a rich
vegetation. Under the tropics, and in those parts of our globe where
the most genial conditions of fertility exist,--a suitable soil, a
moist atmosphere, and a high temperature,--vegetation is scarcely
limited by space; and, where the soil is wanting, it is gradually
supplied by the decaying leaves, bark and branches of plants. It is
obvious there is no deficiency of atmospheric nourishment for plants
in those regions, nor are these wanting in our own cultivated
fields: all that plants require for their development is conveyed to
them by the incessant motions of the atmosphere. The air between the
tropics contains no more than that of the arctic zones; and yet how
different is the amount of produce of an equal surface of land in
the two situations!

This is easily explicable. All the plants of tropical climates, the
oil and wax palms, the sugar cane, &c., contain only a small
quantity of the elements of the blood necessary to the nutrition of
animals, as compared with our cultivated plants. The tubers of the
potato in Chili, its native country, where the plant resembles a
shrub, if collected from an acre of land, would scarcely suffice to
maintain an Irish family for a single day (Darwin). The result of
cultivation in those plants which serve as food, is to produce in
them those constituents of the blood. In the absence of the elements
essential to these in the soil, starch, sugar and woody fibre, are
perhaps formed; but no vegetable fibrine, albumen, or caseine. If we
intend to produce on a given surface of soil more of these latter
matters than the plants can obtain from the atmosphere or receive
from the soil of the same surface in its uncultivated and normal
state, we must create an artificial atmosphere, and add the needed
elements to the soil.

The nourishment which must be supplied in a given time to different
plants, in order to admit a free and unimpeded growth, is very
unequal.

On pure sand, on calcareous soil, on naked rocks, only a few genera
of plants prosper, and these are, for the most part, perennial
plants. They require, for their slow growth, only such minute
quantities of mineral substances as the soil can furnish, which may
be totally barren for other species. Annual, and especially summer
plants, grow and attain their perfection in a comparatively short
time; they therefore do not prosper on a soil which is poor in those
mineral substances necessary to their developme