_Jungermanniaceae Anacrogynae._--The great range of form in the sexual
plant is well illustrated by the nine genera of this group [v.04 p.0649]
which occur in Britain. One thalloid form has already been described in
_Pellia_ (fig. 2). _Sphaerocarpus_, which occurs rarely in stubble fields,
is in many respects one of the simplest of the liverworts. The small
thallus bears the antheridia and archegonia, each of which is surrounded by
a tubular involucre, on the upper surface of distinct individuals. The
sporogonium has a small foot, but the sterile cells among the spores do not
develop into elaters. The same is true of the capsule of _Riella_. The
plants of this genus, none of the species of which are British, grow in
shallow water rooted in the mud, and are unlike all other liverworts in
appearance. The usually erect thallus has a broad wing-like outgrowth from
the dorsal surface and two rows of rather large scales below. No provision
for the opening of the capsule exists in either of these genera. In
_Aneura_ the form of the plant may be complicated by a division of labour
between root-like, stem-like and assimilating branches of the thallus. The
sexual organs are borne on short lateral branches, while in the related
genus _Metzgeria_, which occurs on rocks and tree trunks, the small sexual
branches spring from the lower surface of the midrib of the narrow thallus.
In these two genera the elaters are attached to a sterile group of cells
projecting into the upper end of the capsule, and on dehiscence remain
connected with the tips of the valves. _Pallavicinia_ and some related
genera have a definite midrib and broad wings formed of one layer of cells,
and are of interest owing to the presence of a special water-conducting
strand in the midrib. This consists of elongated lignified cells with
pitted walls. _Blasia pusilla_, which occurs commonly by ditches and
streams, affords a transition to the foliose types. Its thallus (fig. 7)
has thin marginal lobes of limited growth, which are comparable to the more
definite leaves of other anacrogynous forms. The ventral surface bears flat
scales in addition to the concave scales which, as mentioned above, are
inhabited by _Nostoc_. This interesting liverwort produces two kinds of
gemmae, and in the localities in which it grows is largely reproduced by
their means. In _Fossombronia_, of which there are a number of British
species, the plant consists of a flattened stem creeping on muddy soil and
bearing two rows of large obliquely-placed leaves. The sexual organs are
borne on the upper surface of the midrib, and the sporogonium is surrounded
by a bell-shaped involucre which grows up after fertilization. _Treubia_,
which grows on rotting wood in the mountain forests of Java, is similarly
differentiated into stem and leaf, and is the largest liverwort known,
reaching a length of thirty centimetres. Lastly _Haplomitrium_, a rare
British genus, forms with the exotic _Calobryum_, an isolated group which
is most naturally placed among the anacrogynous forms although the
archegonia are in terminal groups. The erect branches bear three rows of
leaves, and spring from a creeping axis from which root-like branches
destitute of rhizoids extend into the substratum.
[Illustration: FIG. 8.--_Chiloscyphus polyanthos._ The plant bears three
mature sporogonia which show the elongation of the seta. One of the
sporogonia has opened. B, The "perianth" with the small perichaetial leaves
below it. (After Goebel.)]
_Jungermanniaceae Acrogynae._--The plant consists of leafy shoots, the
origin of which can be understood in the light of the foliose forms
described above. The great majority of existing liverworts belong to this
group, the general plan of construction of which is throughout very
similar. In Britain thirty-nine genera with numerous species are found.
With few exceptions the stem grows by means of a pyramidal apical cell
cutting off three rows of segments. Each segment gives rise to a leaf, but
usually the leaves of the ventral row (amphigastria) are smaller and
differently shaped from those of the two lateral rows; in a number of
genera they are wanting altogether. Sometimes the leaves retain their
transverse insertion on the stem, and the two lobes of which they consist
are developed equally. More often they come to be obliquely inserted, the
anterior edge of each leaf lying under or over the edge of the leaf in
front. The two lobes are often unequally developed. In _Scapania_ the upper
lobe is the smaller, while in _Radula_, _Poretta_ and the _Lejeuneae_ this
is the case with the lower lobe. The folding of one lobe against another
assists in the retention of water. Pitcher-like structures have arisen in
different ways in a number of genera, and are especially common in
epiphytic forms (_Frullania_, _Lepidolaena_, _Pleurozia_). In some forms
the leaves are finely divided, and along with the hair-like paraphyllia
form a loose weft around the stem (_Trichocolea_). The rhizoids spring from
the lower surface of the stem, and sometimes from the bases of the leaves.
The branches arise below and by the side of the leaves.
[Illustration: FIG. 9.--_Cephalozia bicuspidata._ Longitudinal section of
the summit of a shoot bearing a nearly mature sporogonium, sg, still
enclosed in the calyptra; ar', archegonia which have remained unfertilized;
st, stem; b, leaf; p, perianth. (After Hofmeister.)]
The sexual organs may occur on the same or on distinct individuals. The
antheridia are protected by leaves which are often modified in shape. The
archegonia are borne at the apex of the main stem or of a lateral branch. A
single archegonium may arise from the apical cell (_Lejeunea_); more
commonly a number of others are formed from the surrounding segments. The
leaves below the archegonial group are frequently modified in size and
shape, but the chief protection is afforded by a tubular perianth, which
corresponds to a coherent whorl of leaves and grows up independently of
fertilization. The perianth serves also to enclose and protect the
sporogonium during its development. In a number of forms belonging to
different groups the end of the stem on which the sporogonium is borne
grows downwards so as to form a hollow tubular sac enclosing the
sporogonium; in other cases this marsupial sac is formed by the base of the
sporogonium boring into the thickened end of the stem. The sac usually
penetrates into the soil and bears rhizoids on its outer surface. _Kantia_,
_Calypogeia_ and _Saccogyna_ are British forms, which have their sporogonia
protected in this way. The sporogonium is very similar throughout the group
(figs. 8, 9). At maturity the seta elongates rapidly, and the wall of the
capsule splits more or less completely into four valves, allowing the
elaters and spores to escape. In the Jubuloideae, which in other respects
form a well-marked group, the seta is short and the elaters extend from the
upper part of the capsule to the base; at dehiscence they remain fixed to
the valves into which the capsule splits. The germinating spore usually
forms a short filament, but in other cases a flat plate of cells growing by
a two-sided apical cell is first formed (_Radula_, _Lejeunea_). In one or
two tropical forms the pro-embryonic stage is prolonged, and leafy shoots
only arise in connexion with the sexual organs. In _Protocephalozia_, which
grows on bare earth in South America, this pro-embryo is filamentous, while
in _Lejeunea Metzgeriopsis_, which grows on the leaves of living plants, it
is a flat branched thallus closely applied to the substratum. Other cases
of the plant being, with the exception of the sexual branches, apparently
thalloid, are on the other hand to be explained as due to the reduction of
the leaves and flattening of the stem of a shoot (_Pteropsiella_,
_Zoopsis_).
The Acrogynous Jungermanniaceae fall into a number of natural groups, which
cannot, however, be followed out here. They occur in very various
situations, on the ground, on rocks and stones, on tree trunks, and, in the
damp tropics, on leaves. Usually they form larger or smaller tufts of a
green colour, but some forms have a reddish tint.
[Illustration: FIG. 10.--_Anthoceros laevis._ sp, Sporogonium; c,
columella.