ting rows. Their walls are not very
thick and the cells are usually somewhat flattened in a radial
direction.

Some of the cells are larger than the others, and these are found to
be, when examined in longitudinal section, sieve tubes (Fig. 76,
_E_) with numerous lateral sieve plates quite similar to those found
in the stems of ferns.

[Illustration: FIG. 76.--Scotch pine. _A_, cross-section of a
two-year-old branch, x 3. _p_, pith. _c_, bark. The radiating lines
are medullary rays. _r_, resin ducts. _B_, part of the same, x 150.
_cam._ cambium cells. _x_, tracheids. _C_, cross-section of a
two-year-old branch at the point where the two growth rings join: _I_,
the cells of the first year's growth; _II_, those of the second year.
_m_, a medullary ray, x 150. _D_, longitudinal section of a branch,
showing the form of the tracheids and the bordered pits upon their
walls. _m_, medullary ray, x 150. _E_, part of a sieve tube, x 300.
_F_, cross-section of a tracheid passing through two of the pits in
the wall (_p_), x 300. _G_, longitudinal section of a branch, at right
angles to the medullary rays (_m_). At _y_, the section has passed
through the wall of a tracheid, bearing a row of pits, x 150. _H_,
cross-section of a resin duct, x 150. _I_, cross-section of a leaf,
x 20. _fb._ fibro-vascular bundle. _r_, resin duct. _J_, section of a
breathing pore, x 150. _i_, the air space below it.]

The growing tissue (cambium), separating the phloem from the wood,
is made up of cells quite like those of the phloem, into which they
insensibly merge, except that their walls are much thinner, as is
always the case with rapidly growing cells. These cells (_B_,
_cam._) are arranged in radial rows and divide, mainly by walls, at
right angles to the radii of the stem. If we examine the inner side
of the ring, the change the cells undergo is more marked. They
become of nearly equal diameter in all directions, and the walls
become woody, showing at the same time distinct stratification (_B_,
_x_).

On examining the xylem, where two growth rings are in contact, the
reason of the sharply marked line seen when the stem is examined
with the naked eye is obvious. On the inner side of this line (_I_),
the wood cells are comparatively small and much flattened, while the
walls are quite as heavy as those of the much larger cells (_II_)
lying on the outer side of the line. The small cells show the point