The protonema has been already described. The stem is regularly
branched owing to the fact that a branch, or collection of branches,
arises at every fourth leaf. These branches are closely covered with
leaves, some are erect, while others hang down and surround the stem.
No rhizoids are developed. These Mosses are of a whitish-green colour,
and when water is present are always saturated with it like a sponge,
the reason for this being found in the construction of the stem and
leaves. The stems are covered by an external layer of large clear
cells, without chlorophyll, but with annular or spiral thickenings
on the walls, which are also perforated by large holes. By means of
capillary attraction, water is thus raised to the summit of the stem.
Similarly constructed cells are also found in the leaves, but they
are surrounded by a net of very narrow, chlorophyll-containing cells
(Fig. 199 _F_), whose colour is thus to a great extent lost amongst
those which are colourless. This anatomical structure is an essential
condition for the formation of peat. The Bog-Mosses grow by preference
on moors, which they cover with a thick carpet saturated with water.
The lower extremities of the plants perish very rapidly, and gradually
become converted into peat, and the branches thus separated from each
other become independent plants. The sporangia (Fig. 199 _D_, _E_) are
spherical, but with a very short stalk. They open by a _lid_, but have
no _annulus_. The _archegonium_ (Fig. 199 _C_) persists at the _base of
the sporogonium_ as in the Liverworts. Only one genus, _Sphagnum_.