HEPATICÆ, or Liverworts.
MUSCI FRONDOSI. True Mosses or Leafy-Mosses.
Class 1. =Hepaticæ= (=Liverworts=).
The protonema is only slightly developed. The remaining part of the
vegetative body is either a prostrate, often dichotomously-branched
thallus, pressed to the substratum (thalloid Liverworts), with
or without scales on the under side (Figs. 194, 197); or a thin,
prostrate, creeping stem, with distinctly-developed leaves, which are
borne in two or three rows (Figs. 195, 198), viz., two on the upper
and, in most cases, one on the under side. The leaves situated on the
ventral side (amphigastria) are differently shaped from the others
(Fig. 198 _a_), and are sometimes entirely absent. In contradistinction
to the Leafy-Mosses, stress must be laid on the _well-marked
dorsiventrality_ of the vegetative organs; _i.e._ the very distinct
contrast between the dorsal side exposed to the light and the ventral
side turned to the ground. Veins are never found in the leaves.
The _ventral part of the archegonium_ (calyptra) continues to grow for
some time, and encloses the growing embryo, but when the spores are
ripe it is finally ruptured by the sporangium, and remains situated
like a sheath (_vaginula_) around its base. The sporangium opens,
longitudinally, by _valves_ or _teeth_ (Fig. 194, 195, 197 _b_), very
rarely by a lid, or sometimes not at all. _A columella is wanting_
(except in _Anthoceros_, Fig. 194); but on the other hand, a few of the
cells lying between the spores are developed into _elaters_ (Fig. 196),
_i.e._ spindle-shaped cells with spirally-twisted thickenings, which
are hygroscopic, and thus serve to distribute the spores. (They are
seen in Fig. 189 _C_, not yet fully developed, as long cells radiating
from the base of the sporangium. They are wanting in _Riccia_).
[Illustration: FIG. 194.--_Anthoceros lævis_ (nat. size): _K_-_K_
capsules.]
[Illustration: FIG. 195.--_Plagiochila asplenioides_: _a_ unripe,
and _b_ an open capsule; _p_ involucre. The ventral edge of each
leaf is higher than its dorsal edge, and covered by the dorsal
edge of the next one.]
[Illustration: FIG. 196.--An elater with two spores.]
Round the entire archegonium, (or group of archegonia, when
several are developed on the same receptacle) a sheath--the
_involucre_--is often formed, which persists, and encloses the
base of the stalk of the sporangium, together with the sheath
of the archegonium (Fig. 195 _p_). In the Marchantiaceæ each
archegonium is enclosed in a loose investment, the perigynium,
which is developed as an outgrowth from the cells of its stalk.
The majority of the Liverworts are found in damp and shady places,
pressed to the substratum; a few are found floating in fresh water.