ilar growing cells, so that very early we find a ring of
growing cells extending completely around the stem. As the cells in
this ring increase in number, owing to their rapid division, those
on the borders of the ring lose the power of dividing, and gradually
assume the character of the cells on which they border (Fig. 76,
_B_, _cam._). The growth on the inside of the ring is more rapid
than on the outer border, and the ring continues comparatively near
the surface of the stem (Fig. 76, _A_, _cam._). The spaces between
the bundles do not increase materially in breadth, and as the
bundles increase in size become in comparison very small, appearing
in older stems as mere lines between the solid masses of wood that
make up the inner portion of the bundles. These are the primary
medullary rays, and connect the pith in the centre of the stem with
the bark. Later, similar plates of cells are formed, often only a
single cell thick, and appearing when seen in cross-section as a
single row of elongated cells (_C_, _m_).

As the stem increases in diameter the bundles become broader and
broader toward the outside, and taper to a point toward the centre,
appearing wedge-shaped, the inner ends projecting into the pith. The
outer limits of the bundles are not nearly so distinct, and it is
not easy to tell when the phloem of the bundles ends and the ground
tissue of the bark begins.

A careful examination of a cross-section of the bark shows first, if
taken from a branch not more than two or three years old, the
epidermis composed of cells not unlike those of the leaf, but whose
walls are usually browner. Underneath are cells with brownish walls,
and often more or less dry and dead. These cells give the brown
color to the bark, and later both epidermis and outer ground tissue
become entirely dead and disappear. The bulk of the ground tissue is
made up of rather large, loose cells, the outer ones containing a
good deal of chlorophyll. Here and there are large resin ducts
(Fig. 76, _H_), appearing in cross-section as oval openings
surrounded by several concentric rows of cells, the innermost
smaller and with denser contents. These secrete the resin that fills
the duct and oozes out when the stem is cut. All of the cells of the
bark contain more or less starch.

The phloem, when strongly magnified, is seen to be made up of cells
arranged in nearly regular radia