d
outside of it.

2. The GREEN BARK or _Middle Bark_. This consists of cellular tissue
only, and contains the same green matter (_chlorophyll_, 417) as the
leaves. In woody stems, before the season's growth is completed, it
becomes covered by

3. The CORKY LAYER or _Outer Bark_, the cells of which contain no
chlorophyll, and are of the nature of _cork_. Common cork is the thick
corky layer of the bark of the Cork-Oak of Spain. It is this which gives
to the stems or twigs of shrubs and trees the aspect and the color
peculiar to each,--light gray in the Ash, purple in the Red Maple, red
in several Dogwoods, etc.

4. The EPIDERMIS, or skin of the plant, consisting of a layer of
thick-sided empty cells, which may be considered to be the outermost
layer, or in most herbaceous stems the only layer, of cork-cells.

[Illustration: Fig. 481. Magnified view of surface of a bit of young
Maple wood from which the bark has been torn away, showing the
wood-cells and the bark-ends of medullary rays.]

[Illustration: Fig. 482. Section in the opposite direction, from bark
(on the left) to beginning of pith (on the right), and a medullary ray
extending from one to the other.]

432. The green layer of bark seldom grows much after the first season.
Sometimes the corky layer grows and forms new layers, inside of the old,
for years, as in the Cork-Oak, the Sweet Gum-tree, and the White and the
Paper Birch. But it all dies after a while; and the continual
enlargement of the wood within finally stretches it more than it can
bear, and sooner or later cracks and rends it, while the weather acts
powerfully upon its surface; so the older bark perishes and falls away
piecemeal year by year.

433. So on old trunks only the inner bark remains. This is renewed every
year from within and so kept alive, while the older and outer layers
die, are fissured and rent by the distending trunk, weathered and worn,
and thrown off in fragments,--in some trees slowly, so that the bark of
old trunks may acquire great thickness; in others, more rapidly. In
Honeysuckles and Grape-Vines, the layers of liber loosen and die when
only a year or two old. The annual layers of liber are sometimes as
distinct as those of the wood, but often not so.

434. =The Wood= of an exogenous trunk, having the old growths covered
by the new, remains nearly unchanged in age, except from decay. Wherever
there is an annual suspension and renewal of growth, as in temperate
climat