eathing pores (_stomata_, sing. _stoma_), not
unlike those on the capsules of some of the bryophytes. Each
breathing pore consists of two special crescent-shaped epidermal
cells (guard cells), enclosing a central opening or pore
communicating with an air space below. They arise from cells of the
young epidermis that divide by a longitudinal wall, that separates
in the middle, leaving the space between.

[Illustration: FIG. 69.--_A_, mother cell of the sporangium of the
maiden-hair fern, x 300. _B_, young sporangium, surface view, x 150:
i, from the side; ii, from above. _C-E_, successive stages in the
development of the sporangium seen in optical section, x 150. _F_,
nearly ripe sporangium, x 50: i, from in front; ii, from the side.
_an._ ring. _st._ point of opening. _G_, group of four spores, x 150.
_H_, a single spore, x 300.]

By holding a leaflet between two pieces of pith, and using a very
sharp razor, cross-sections can be made. Such a section is shown in
Fig. 68, _A_. The epidermis (_e_) bounds the upper and lower
surfaces, and if a vein (_f.b._) is cut across its structure is
found to be like that of the fibro-vascular bundle of the leaf
stalk, but much simplified.

The ground tissue of the leaf is composed of very loose, thin-walled
cells, containing numerous chloroplasts. Between them are large and
numerous intercellular spaces, filled with air, and communicating
with the breathing pores. These are the principal assimilating cells
of the plant; _i.e._ they are principally concerned in the
absorption and decomposition of carbonic acid from the atmosphere,
and the manufacture of starch.

The spore cases, or sporangia (Fig. 69), are at first little papillae
(_A_), arising from the epidermal cells, from which they are early
cut off by a cross-wall. In the upper cell several walls next arise,
forming a short stalk, composed of three rows of cells, and an upper
nearly spherical cell--the sporangium proper. The latter now divides
by four walls (_B_, _C_, i-iv), into a central tetrahedral cell, and
four outer ones. The central cell, whose contents are much denser
than the outer ones, divides again by walls parallel to those first
formed, so that the young sporangium now consists of a central cell,
surrounded by two outer layers of cells. From the central cell a
group of cells is formed by further divisions (_D_), which finally
become entirely separate