led therefore
fibro-vascular bundles. We have already spoken of them as making the dots
on the leaf-scars, and forming the strengthening fabric of the leaves.[3]

[Footnote 1: See page 46.]

[Footnote 2: If elements of the same kind are untied, they constitute a
tissue to which is given the name of those elements; thus parenchyma cells
form parenchyma tissue or simply parenchyma; cork-cells form cork, etc. A
tissue can therefore be defined as a fabric of united cells which have had
a common origin and obeyed a common law of growth.--Physiological Botany.
p. 102.]

[Footnote 3: See page 58.]

We will now examine our series of branches. The youngest twigs, in spring
or early summer, are covered with a delicate, nearly colorless skin.
Beneath this is a layer of bark, usually green, which gives the color to
the stem, an inner layer of bark, the wood and the pith. The pith is soft,
spongy and somewhat sappy. There is also sap between the bark and the
wood. An older twig has changed its color. There is a layer of brown bark,
which has replaced the colorless skin. In a twig a year old the wood is
thicker and the pith is dryer. Comparing sections of older branches with
these twigs, we find that the pith has shrunk and become quite dry, and
that the wood is in rings. 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