Some forms in this family have a thallus in which leaf-like structures
are found (_Blasia_), while in others (_e.g. Metzgeria_, _Pellia_,
_Aneura_) they are entirely absent. The majority, however, have
round, thick stems, bearing dorsally two rows of leaves, and one row
ventrally. Some of these have the leaves “underlying” (Fig. 195), while
in others (Fig. 198) they are “overlying.” (See Figs. 195, 198, with
explanation).
The sporangia are spherical, stalked, and situated singly on the apex
of the branches, and open by four valves (in _Sphærocarpus_ they
are indehiscent).
[Illustration: FIG. 198.--_Frullania dilatata._ Portion of a
branch seen from the under side: _r_ and _b_ are the anterior
and posterior edges of the same dorsal leaf; _a_ ventral leaves
(amphigastria). The dorsal leaves are “overlying,” _i.e._ the
anterior edge of the leaf overlaps the posterior edge of the
preceding one.]
All the species in this family were formerly reckoned as belonging to
one genus, _Jungermannia_, but now they are divided into several,
arranged as follows:--
I. ANACROGYNÆ. The archegonia are situated on the upper side of the
thallus or stem, _placed laterally_, and covered by an “involucre,”
formed by the calyptra together with the tissue of the stem or thallus.
a. ANELATEREÆ. Without any elaters: _Sphærocarpus_, _Riella_.
b. ELATEREÆ. α. Thalloid: _Aneura pinguis_, in damp situations;
_Metzgeria furcata_, on trees; _Pellia epiphylla_, in damp situations;
_Blasia pusilla_, on damp clay soil, in the shade (scales are present
on the thallus). β. Foliose and not dorsiventral: _Haplomitrium
hookeri_.
II. ACROGYNÆ. The apex of the stem or of certain branches is adapted
for the formation of female shoots. The archegonia are most frequently
aggregated on the apex of the shoots, and are encircled by their leaves
(perichætium). Between these and the archegonia, enclosing the latter,
a peculiar cup-shaped organ (the involucre) is formed. This group only
includes leaf-bearing genera: _Frullania_, _Radula_, _Madotheca_,
_Ptilidium_, _Calypogeia_, _Lepidozia_, _Mastigobryum_, _Lophocolea_,
_Jungermannia_, _Scapania_, _Plagiochila_.