a leaf is a
sector, its proportional part, of a circle. Theoretically the leaf, in
section, should indicate the number of leaves composing its fascicle.
This is absolutely true for fascicles of two leaves only. No fascicle of
five leaves, that I have examined, is equally apportioned among its five
members. It may be divided in various ways, one of which is shown in
fig. 18, where the leaf (a) might be mistaken for one of a fascicle of
3, and the leaf (b) for one of a fascicle of 6. Therefore if absolute
certainty is required, a fascicle of triquetral leaves is best
determined by actual count.

The transverse section of a leaf may be conveniently divided into three
distinct parts--1, the dermal tissues, epiderm, hypoderm and stomata
(fig. 17-a)--2, the green tissue, containing the resin-ducts (fig.
17-b)--3, the stelar tissues, enclosed by the endoderm and containing
the fibro-vascular bundle (fig. 17-c).

THE DERMAL TISSUES OF THE LEAF.

The stomata of Pine leaves are depressed below the surface and interrupt
the continuity of epiderm and hypoderm. They are wanting on the dorsal
surface of the leaves of several Soft Pines, constantly in some species,
irregularly in others. In Hard Pines, however, all surfaces of the leaf
are stomatiferous. In several species of the Soft Pines the longitudinal
lines of stomata are very conspicuous from the white bloom which
modifies materially the general color of the foliage.

Under the action of hydrochloric acid the hypoderm is sharply
differentiated from the epiderm by a distinct reddish tint, but without
the aid of a reagent the two tissues do not always differ in appearance.
The cells of epiderm and hypoderm may be so similar that they appear to
form a single tissue. In most species, however, the epiderm is distinct,
while the cells of the hypoderm are either uniform, with equally thin
or thick walls--or biform, with very thin walls in the outer row of
cells and very thick walls in the inner row or rows of cells--or
multiform, with cell-walls gradually thicker toward the centre of the
leaf. These conditions may be tabulated as follows--

Cells of epiderm and hypoderm similar fig. 19.
Cells of epiderm and hypoderm distinct.
Cells of hypoderm uniform, thin or thick figs. 20, 21.
Cells of hypoderm biform fig. 22.
Cells of hypoderm multiform fig. 23.

The biform hypoderm is not always obviou