, pretty large root, a series of thin longitudinal
sections should be made either holding the root directly in the
fingers or placing it between pieces of pith. In order to avoid
drying of the sections, as is indeed true in cutting any delicate
tissue, it is a good plan to wet the blade of the razor. If the
section has passed through the apex, it will show the structure
shown in Figure 68, _D_. The apical cell (_a_) is large and
distinct, irregularly triangular in outline. It is really a
triangular pyramid (tetrahedron) with the base upward, which is
shown by making a series of cross-sections through the root tip, and
comparing them with the longitudinal sections. The cross-section of
the apical cell (Fig. _L_) appears also triangular, showing all its
faces to be triangles. Regular series of segments are cut off in
succession from each of the four faces of the apical cell. These
segments undergo regular divisions also, so that very early a
differentiation of the tissues is evident, and the three tissue
systems (epidermal, ground, and fibro-vascular) may be traced
almost to the apex of the root (68, _D_). From the outer series of
segments is derived the peculiar structure (root cap) covering the
delicate growing point and protecting it from injury.

The apices of the stem and leaves, being otherwise protected,
develop segments only from the sides of the apical cell, the outer
face never having segments cut off from it.

CHAPTER XIII.

CLASSIFICATION OF THE PTERIDOPHYTES.

There are three well-marked classes of the Pteridophytes: the ferns
(_Filicinae_); horse-tails (_Equisetinae_); and the club mosses
(_Lycopodinae_).

CLASS I.--FERNS (_Filicinae_).

The ferns constitute by far the greater number of pteridophytes, and
their general structure corresponds with that of the maiden-hair fern
described. There are three orders, of which two, the true ferns
(_Filices_) and the adder-tongues (_Ophioglossaceae_), are represented
in the United States. A third order, intermediate in some respects
between these two, and called the ringless ferns (_Marattiaceae_), has
no representatives within our territory.

The classification is at present based largely upon the characters of
the sporophyte, the sexual plants being still very imperfectly known
in many forms.

The adder-tongues (_Ophioglossaceae_) are mostly plants of rather small
size, ranging from about ten to fifty c