In this class the protonema is well developed, and resembles a
branched filamentous Alga, from which it can be easily distinguished
by its oblique septa (in _Sphagnum_ it is a cellular expansion). The
Moss-plant, which is developed directly from the protonema, generally
has an erect, thick, cylindrical stem similarly constructed on all
sides. The leaves are arranged spirally, the most frequent divergence
being 2/5 or 3/8 (Fig. 200 _A_). A midrib is often present and also
marginal veins formed by longitudinally elongated cells; at these veins
the leaf is more than one layer in thickness. In _Leucobryum_ the
leaves are generally constructed of more than one layer.
The stem grows by means of a three-sided, pyramidal, apical cell which
gives rise to three rows of segments, each segment forming a leaf. The
lateral branches arise from the lower portions of the segments, the
upper portion of which does not take any part in the construction of
the leaf. From their mode of origin the branches are not axillary, and
differ in this respect from the Flowering-plants.
The ventral portion of the archegonium is very early ruptured _at its
base_ by the growing sporogonium, upon which it remains, and it is thus
raised into the air, forming a “hood,” the calyptra (Figs. 192; 200
_B_). In the Sphagnaceæ the hood is not present; in this order, as in
the Liverworts, the archegonium remains at the base of the sporogonium.
The sporangium opens by circumsessile dehiscence, the upper portion
(_operculum_) being separated along a specially constructed ring of
cells, and falls off like a “lid” (Fig. 200). Only in a few forms
(families 2 and 3) does any variation of this take place. Elaters are
never found, but (with the exception of _Archidium_) there is always
present in the sporangium a central mass of cells, the _columella_,
which take no part in the formation of the spores. The columella, in
some, does not reach quite to the operculum and in these cases the
spore-sac is bell-shaped and covers the columella (_Andreæa_, Fig. 190;
_Sphagnum_, Fig. 199 _D_); but in the majority of Mosses the columella
extends to the lid, so that the space containing the spores becomes a
hollow cylinder.
The _sporangium_ is generally raised on a long stalk; in the great
majority this stalk is formed from the lower half of the oospore and
belongs to the asexual generation--it is then known as the _seta_. In
_Andreæa_ and _Sphagnum_ the seta is very short, and the sporangia are
raised upon a long stalk (_pseudopodium_) developed from the summit
of the sexual generation (Figs. 190, 192). In the latter figure an
archegonium (_a_) is seen attached to the pseudopodium, having been
carried up with this during the course of its development. The summit
of the pseudopodium is enlarged to embrace the foot of the sporogonium
(Figs. 192, 199 _D_).
A. The sporangium is supported on a pseudopodium; the columella
does not extend to the operculum.
[Illustration: FIG. 199.--_Sphagnum acutifolium._--_A_ The
upper portion of a plant: _a_ branches with antheridia; _ch_
branches with terminal archegonia and perichætia; _b_ the
upper stemleaves. _B_ A male branch whose leaves are partly
taken off in order to show the antheridia. _C_ Group of three
archegonia: the central one (_a_) is formed from the apical
cell. _D_ Sporogonium in longitudinal section: the broad foot
(_sg’_) is sunk in the vaginula, _v_; _c_ calyptra; _ar_ neck
of the archegonium; _ps_ pseudopodium. _E_ ripe sporangium with
operculum, and the remains of the archegonium situated on the
pseudopodium which is still surrounded by the perichætium; to the
left is a barren branch. _F_ Portion of a foliage-leaf seen from
above: _l_ perforations; _b_ chlorophyll-containing cells; _s_
spiral thickenings.]