. The
individuality of plants and trees in this respect is most remarkable.
The cells of each seem to know what particular elements they want from
the soil, as of course they do.

The vital activity of the tree goes on at three points--in the leaves,
in the rootlets, and in the cambium layer. The activity of the leaf and
rootlet furnishes the starchy deposit which forms this generative
layer--the milky, mucilaginous girdle of matter between the outer bark
and the wood through which the tree grows and increases in size.
Generation and regeneration take place through this layer. I have called
it the girdle of perpetual youth. It never grows old. It is annually
renewed. The heart of the old apple-tree may decay and disappear, indeed
the tree may be reduced to a mere shell and many of its branches may die
and fall, but the few apples which it still bears attest the fact that
its cambium layer, at least over a part of its surface, is still
youthful and doing its work. It is this layer that the yellow-bellied
woodpecker, known as the sapsucker, drills into and devours, thus
drawing directly upon the vitality of the tree. But his ravages are
rarely serious. Only in two instances have I seen dead branches on an
apple-tree that appeared to be the result of his drilling.

What we call the heart of a tree is in no sense the heart; it has no
vital function, but only the mechanical one of strength and support. It
adds to the tree's inertia and power to resist storms. The trunk of a
tree is like a community where only one generation at a time is engaged
in active business, the great mass of the population being retired and
adding solidity and permanence to the social organism. The rootlets of a
plant or a tree are like the laborers in the field that produce for us
the raw material of our food, while the leaves are like our many devices
for rendering it edible and nourishing. The rootlets continue their
activity in the fall, after the leaves have fallen, and thus gorge the
tree with fluid against the needs of the spring. In the growing tree or
vine the sap, charged with nourishment, flows down from the top to the
roots. In the spring it evidently flows upward, seeking the air through
the leaves. Or rather, we may say that the crude sap always flows
upward, while the nutritive sap flows downward, thus giving the tree a
kind of double circulation.

A tree may be no more beautiful and wonderful when we have come to a
knowledge of all