d into four orders: I. The peat mosses
(_Sphagnaceae_); II. _Andreaeaceae_; III. _Phascaceae_; IV. The common
mosses (_Bryaceae_).

[Illustration: FIG. 64.--_A_, a peat moss (_Sphagnum_), x 1/2. _B_, a
sporogonium of the same, x 3. _C_, a portion of a leaf, x 150. The
narrow, chlorophyll-bearing cells form meshes, enclosing the large,
colorless empty cells, whose walls are marked with thickened bars, and
contain round openings (_o_).]

The peat mosses (Fig. 64) are large pale-green mosses, growing often
in enormous masses, forming the foundation of peat-bogs. They are of a
peculiar spongy texture, very light when dry, and capable of absorbing
a great amount of water. They branch (Fig. 64, _A_), the branches
being closely crowded at the top, where the stems continue to grow,
dying away below.

[Illustration: FIG. 65.--Forms of mosses. _A_, plant of _Phascum_,
x 3. _B_, fruiting plant of _Atrichum_, x 2. _C_, young capsule of
hairy-cap moss (_Polytrichum_), covered by the large, hairy calyptra.
_D_, capsules of _Bartramia_: i, with; ii, without the calyptra. _E_,
upper part of a male plant of _Atrichum_, showing the flower, x 2.
_F_, a male plant of _Mnium_, x 4. _G_, pine-tree moss (_Clemacium_),
x 1. _H_, _Hypnum_, x 1. _I_, ripe capsules of hairy-cap moss: i,
with; ii, without calyptra.]

The sexual organs are rarely met with, but should be looked for late
in autumn or early spring. The antheridial branches are often
bright-colored, red or yellow, so as to be very conspicuous. The
capsules, which are not often found, are larger than in most of the
common mosses, and quite destitute of a stalk, the apparent stalk
being a prolongation of the axis of the plant in the top of which the
base of the sporogonium is imbedded. The capsule is nearly globular,
opening by a lid at the top (Fig. 64, _B_).

A microscopical examination of the leaves, which are quite destitute
of a midrib, shows them to be composed of a network of narrow
chlorophyll-bearing cells surrounding much larger empty ones whose
walls are marked with transverse thickenings, and perforated here
and there with large, round holes (Fig. 64, _C_). It is to the
presence of these empty cells that the plant owes its peculiar
spongy texture, the growing plants being fairly saturated with
water.

The _Andreaeaceae_ are very small, and not at all common. The capsule
splits into four valves, something like a liverwort.

The _Phascaceae_ are small