It is not practicable for the pupils to
compare the number of these rings with the bud-rings, and so find out for
themselves that the age of the branch can be determined from the wood, for
in young stems the successive layers are not generally distinct. But, in
all the specimens, the sap is found just between the wood and the bark,
and here, where the supply of food is, is where the growth is taking
place. Each year new wood and new bark are formed in this _cambium-layer_,
as it is called, new wood on its inner, new bark on its outer face. Trees
which thus form a new ring of wood every year are called _exogenous_, or
outside-growing.
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.
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