it is carried into all parts of the tree for its nourishment.
Protected and upheld by these expanded woody ribs, the body of the
leaf consists of a mass of pulpy cells arranged somewhat loosely, so
that there are spaces between them through which air can freely pass.
Over this mass of cells there is a skin, or epidermis as it is called,
the green surface of the leaf. In this there are multitudes of minute
openings, or breathing pores, through which air is admitted, and
through which also water or watery vapor passes out into the
surrounding atmosphere. In the leaf of the white lily there are as
many as 60,000 of these openings in every square inch of surface and
in the apple leaf not fewer than 24,000. These breathing pores, called
stomates, are mostly on the under side of the leaf, except in the case
of leaves which float upon the water. There is a beautiful contrivance
also in connection with these pores, by which they are closed when the
air around is dry and the evaporation of the water from the leaves
would be so rapid as to be harmful to the tree, and are opened when
the surrounding atmosphere is moist.

The green color of the leaves is owing to the presence in the cells of
minute green grains or granules, called chlorophyll, which means
leaf-green, and these granules are indispensable to the carrying on of
the important work which takes place in the leaves. They are more
numerous and also packed more closely together near the upper surface
of the leaf than they are near the lower. It is because 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