ngle row, in others forming very conspicuous groups of some size.
The green tissue of the leaf is much more compact than in the fern
we examined, and the cells are more nearly round and the
intercellular spaces smaller. The chloroplasts are numerous and
nearly round in shape.

Scattered through the green tissue are several resin passages (_r_),
each surrounded by a circle of colorless, thick-walled cells, like
those under the epidermis. At intervals in the latter are
openings--breathing pores--(Fig. 76, _J_), below each of which is an
intercellular space (_i_). They are in structure like those of the
ferns, but the walls of the guard cells are much thickened like the
other epidermal cells.

Each leaf is traversed by two fibro-vascular bundles of entirely
different structure from those of the ferns. Each is divided into
two nearly equal parts, the wood (_x_) lying toward the inner, flat
side of the leaf, the bast (_T_) toward the outer, convex side. This
type of bundle, called "collateral," is the common form found in the
stems and leaves of seed plants. The cells of the wood or xylem are
rather larger than those of the bast or phloem, and have thicker
walls than any of the phloem cells, except the outermost ones which
are thick-walled fibres like those under the epidermis. Lying
between the bundles are comparatively large colorless cells, and
surrounding the whole central area is a single line of cells that
separates it sharply from the surrounding green tissue.

In longitudinal sections, the cells, except of the mesophyll (green
tissue) are much elongated. The mesophyll cells, however, are short
and the intercellular spaces much more evident than in the
cross-section. The colorless cells have frequently rounded
depressions or pits upon their walls, and in the fibro-vascular
bundle the difference between the two portions becomes more obvious.
The wood is distinguished by the presence of vessels with close,
spiral or ring-shaped thickenings, while in the phloem are found
sieve tubes, not unlike those in the ferns.

The fibro-vascular bundles of the stem of the seedling plant show a
structure quite similar to that of the leaf, but very soon a
difference is manifested. Between the two parts of the bundle the
cells continue to divide and add constantly to the size of the
bundle, and at the same time the bundles become connected by a line
of sim