d from each other. The outer cells of the
spore case divide only by walls, at right angles to their outer
surface, so that the wall is never more than two cells thick. Later,
the inner of these two layers becomes disorganized, so that the
central mass of cells floats free in the cavity of the sporangium,
which is now surrounded by but a single layer of cells (_E_).

Each of the central cells divides into four spores, precisely as in
the bryophytes. The young spores (_G_, _H_) are nearly colorless and
are tetrahedral (like a three-sided pyramid) in form. As they ripen,
chlorophyll is formed in them, and some oil. The wall becomes
differentiated into three layers, the outer opaque and brown, the
two inner more delicate and colorless.

Running around the outside of the ripe spore case is a single row of
cells (_an._), differing from the others in shape, and having their
inner walls thickened. Near the bottom, two (sometimes four) of
these cells are wider than the others, and their walls are more
strongly thickened. It is at this place (_st._) that the spore case
opens. When the ripe sporangium becomes dry, the ring of thickened
cells (_an._) contracts more strongly than the others, and acts like
a spring pulling the sporangium open and shaking out the spores,
which germinate readily under favorable conditions, and form after a
time the sexual plants (prothallia).

The roots of the sporophyte arise in large numbers, the youngest being
always nearest the growing point of the stem or larger roots (Fig. 67,
_C_). The growing roots are pointed at the end which is also
light-colored, the older parts becoming dark brown. A cross-section of
the older portions shows a dark-brown ground tissue with a central,
light-colored, circular, fibro-vascular bundle (Fig. 68, _F_). Growing
from its outer surface are numerous brown root hairs (_r_).

When magnified the walls of all the outer cells (epidermis and
ground tissue) are found to be dark-colored but not very thick, and
the cells are usually filled with starch. There is a bundle sheath
of much-flattened cells separating the fibro-vascular bundle from
the ground tissue. The bundle (Fig. 68, _G_) shows a band of
tracheary tissue in the centre surrounded by colorless cells, all
about alike.

All of the organs of the fern grow from a definite apical cell, but
it is difficult to study except in the root.

Selecting a fresh