band (_x_) of
large empty cells, with somewhat angular outlines, and distinctly
separated walls; and an outer portion (_y_) filling up the space
between these central cells and the bundle sheath. The central
tissue (_x_) is called the woody tissue (xylem); the outer, the bast
(phloem). The latter is composed of smaller cells of variable form,
and with softer walls than the wood cells.

A longitudinal section of either the stem or leaf stalk shows that
all the cells are decidedly elongated, especially those of the
fibro-vascular bundle. The xylem (Fig. 68, _C_, _x_) is made up
principally of large empty cells, with pointed ends, whose walls are
marked with closely set, narrow, transverse pits, giving them the
appearance of little ladders, whence they are called "scalariform,"
or ladder-shaped markings. These empty cells are known as
"tracheids," and tissue composed of such empty cells, "tracheary
tissue." Besides the tracheids, there are a few small cells with
oblique ends, and with some granular contents.

The phloem is composed of cells similar to the latter, but there may
also be found, especially in the stem, other larger ones (Fig. 67,
_J_), whose walls are marked with shallow depressions, whose bottoms
are finely pitted. These are the so-called "sieve tubes."

For microscopical examination, either fresh or alcoholic material
may be used, the sections being mounted in water. Potash will be
found useful in rendering opaque sections transparent.

The leaves, when young, are coiled up (Fig. 67, _C_), owing to growth
in the earlier stages being greater on the lower than on the upper
side. As the leaf unfolds, the stalk straightens, and the upper
portion (blade) becomes flat.

The general structure of the leaf stalk may be understood by making a
series of cross-sections at different heights, and examining them with
a hand lens. The arrangement is essentially the same as in the stem.
The epidermis and immediately underlying ground tissue are
dark-colored, but the inner ground tissue is light-colored, and much
softer than the corresponding part of the stem; and some of the outer
cells show a greenish color, due to the presence of chlorophyll.

The section of the fibro-vascular bundle differs at different heights.
Near the base of the stalk (Fig. _D_ i) it is horseshoe-shaped; but,
if examined higher up, it is found to divide (II, III), one part going
to each of the main