Ask the pupils to separate the bark into its three layers and to try
the strength of each. The two outer will easily break, but the inner is
generally tough and flexible. It is this inner bark, which makes the
Poplar and Willow branches so hard to break. These strong, woody fibres
of the inner bark give us many of our textile fabrics. Flax and Hemp come
from the inner bark of their respective plants (_Linum usitatissimum_ and
_Cannabis sativa_), and Russia matting is made from the bark of the Linden
(_Tilia Americana_).

We have 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,