e of this that
the upper surface is of a deeper green than the lower.

Such, then, is the laboratory of the leaf, the place where certain
inorganic, lifeless substances such as water, lime, sulphur, potash,
and phosphorus are transformed and converted into living and organic
vegetable matter, and from which this is sent forth to build up every
part of the tree from deepest root to topmost sprig. It is in the
leaves also that all the food of man and all other animals is
prepared, for if any do not feed upon vegetable substances directly
but upon flesh, that flesh nevertheless has been made only as
vegetable food has been eaten to form it. It is, as the Bible says,
"The tree of the field is man's life."

But let us consider a little further the work of the leaves. The tree
is made up almost wholly of oxygen, hydrogen, and carbon. It is easy
to see where the oxygen and hydrogen are obtained, for they are the
two elements which compose water, and that, we have seen, the roots
are absorbing from the ground all the while and sending through the
body of the tree into the leaves. But where does the carbon come from?
A little examination will show.

The atmosphere is composed of several gases, mainly of oxygen and
nitrogen. Besides these, however, it contains a small portion of
carbonic acid, that is, carbon chemically united with oxygen. The
carbonic acid is of no use to us directly, and in any but very minute
quantities is harmful; but the carbon in it, if it can be separated
from the oxygen, is just what the tree and every plant wants. And now
the work of separating the carbon from the oxygen is precisely that
which is done in the wonderful laboratory of the leaf. Under the magic
touch of the sun, the carbonic acid of the atmosphere which has
entered the leaf through the breathing pores or stomates and is
circulating through the air-passages and cells, is decomposed, that
is, taken to pieces; the oxygen is poured out into the air along with
the watery vapor of the crude sap, while the carbon is combined with
the elements of water and other substances which we have mentioned, to
form the elaborated sap or plant-material which is now ready to be
carried from the leaves to all parts of the plant or tree, to nourish
it and continue its growth. Such is the important and wonderful work
of the leaf, the tender, delicate leaf, which we crumple so easily in
our fingers. It builds up, atom by atom, the tree and the great
forests wh