found, in comparing the bark of specimens of branches of various
ages, that, in the youngest stems, the whole is covered with a skin, or
_epidermis_, which is soon replaced by a brown outer layer of bark, called
the _corky layer_; the latter gives the distinctive color to the tree.
While this grows, it increases by a living layer of cork-cambium on its
inner face, but it usually dies after a few years. In some trees it goes
on growing for many years. It forms the layers of bark in the Paper Birch
and the cork of commerce is taken from the Cork Oak of Spain. The green
bark is of cellular tissue, with some green coloring matter like that of
the leaves; it is at first the outer layer, but soon becomes covered with
cork. It does not usually grow after the first year. Scraping the bark of
an old tree, we find the bark homogeneous. The outer layers have perished
and been cast off. As the tree grows from within, the bark is stretched
and, if not replaced, cracks and falls away piecemeal. So, in most old
trees, the bark consists of successive layers of the inner woody bark.

Stems can be well studied from pieces of wood from the woodpile. The ends
of the log will show the concentric rings. These can be traced as long,
wavy lines in vertical sections of the log, especially if the surface is
smooth. If the pupils can whittle off different planes for themselves,
they will form a good idea of the formation of the wood. In many of
the specimens there will be knots, and the nature of these will be an
interesting subject for questions. If the knot is near the centre of the
log, lead back their thoughts to the time when the tree was as small as
the annular ring on which the centre of the knot lies. Draw a line on this
ring to represent the tree at this period of its growth. What could the
knot have been? It has concentric circles like the tree itself. It was a
branch which decayed, or was cut off. Year after year, new rings of wood
formed themselves round this broken branch, till it was covered from
sight, and every year left it more deeply buried in the trunk.

Extremely interesting material for the study of wood will be found in thin
sections prepared for veneers. Packages of such sections will be of great
use to the teacher.[1] They show well the reason of the formation of a
dividing line between the wood of successive seasons. In a cross section
of Oak or Chestnut the wood is first very open and porous and then close.
This is owing