While the general course of the life-history of all liverworts resembles
that of _Pellia_, the three great groups into which they are divided differ
from one another in the characters of both generations. Each group exhibits
a series leading from more simple to more highly organized forms, and the
differentiation has proceeded on distinct and to some extent divergent
lines in the three groups. The Marchantiales are a series of thalloid
forms, in which the structure of the thallus is specialized to enable them
to live in more exposed situations. The lowest members of the series
(_Riccia_) possess the simplest sporogonia known, consisting of a wall of
one layer of cells enclosing the spores. In the higher forms a sterile foot
and seta is present, and sterile cells or elaters occur with the spores.
The lower members of the Jungermanniales are also thalloid, but the thallus
never has the complicated structure characteristic of the Marchantiales,
and progress is in the direction of the differentiation of the plant into
stem and leaf. Indications of how this may have come about are afforded by
the lower group of the Anacrogynous Jungermanniaceae, and throughout the
Acrogynous Jungermanniacae the plant has well-marked stem and leaves. The
sporogonium even in the simplest forms has a sterile foot, but in this
series also the origin of elaters from sterile cells can be traced. The
Anthocerotales are a small and very distinct group, in which the
gametophyte is a thallus, while the sporogonium possesses a sterile
columella and is capable of long-continued growth and spore production. The
mode of development of the sporogonium presents important differences in
the three series that may be briefly referred to here. In fig. 4 young
sporogonia of a number of liverworts are shown in longitudinal section, and
the archesporial cells from which the spores and elaters will arise are
shaded. In _Riccia_ (fig. 4, A) the whole mass of cells derived from the
ovum forms a spherical capsule, the only sterile tissue being the single
layer of peripheral cells forming the wall. In other Marchantiales (fig. 4,
B) the lower half of the embryo separated by the first transverse wall (1,
I) forms the sterile foot and seta, while in the upper half (ka) the
peripheral layer forms the wall of the capsule, enclosing the archesporial
cells from which spores and elaters arise. In the Jungermanniales (fig. 4,
C, E, F) the embryo is formed of a number of tiers of cells, and the
archesporium is defined by the first divisions parallel to the surface in
the cells of one or more of the upper tiers; a number of tiers go to form
the seta and foot, while the lowest segment (a) usually forms a small
appendage of the latter. In the Anthocerotales (fig. 4, D) the lowest tiers
form the foot, and the terminal tier the capsule. The first periclinal
divisions in the cells of the terminal tier separate a central group of
cells which form the sterile columella (col). The archesporium arises by
the next divisions in the outer layer of cells, and thus extends over the
summit of the columella. In none of the liverworts does the sporogonium
develop by means of an apical cell, as is the rule in mosses.
Leaving details of form and structure to be considered under the several
groups, some general features of the Hepaticae may be looked at here in
relation to the conditions under which the plants live. The organization of
the gametophyte stands in the closest relation to the factors of light and
moisture in the environment. With hardly an exception the liverworts are
dorsiventral, and usually one side is turned to the substratum and the
other exposed to the light. In thalloid forms a thinner marginal expansion
or a definite wing increasing the surface exposed to the light can be
distinguished from a thicker midrib serving for storage and conduction. The
leaves and stem of the foliose forms effect the same division of labour in
another way. The relation of the plant to its water supply varies within
the group. In the Marchantiales the chief supply is obtained from the soil
by the rhizoids, and its loss in transpiration is regulated and controlled.
In most liverworts, on the other hand, water is absorbed directly by the
whole general surface, and the rhizoids are of subordinate importance. Many
forms only succeed in a constantly humid atmosphere, while others sustain
drying for a period, though their powers of assimilation and growth are
suspended in the dry state. The cell-walls are capable of imbibing water
rapidly, and their thickness stands in relation to this rather than to the
prevention of loss of water from the plant. The large surface presented by
the leafy forms facilitates the retention and absorption of water. The
importance of prolonging the moistened condition as long as possible is
further shown by special adaptations to retain water either between the
appressed lobes of the leaves or in special pitcher-like sacs. In thalloid
forms fimbriate or lobed margins or outgrowths from the surface lead to the
same result. Sometimes adaptations to protect the plant during seasons of
drought, such as the rolling up of the thallus in many xerophytic
Marchantiales, can be recognized, but more often a prolonged dry season is
survived in some resting state. The formation of subterranean tubers, which
persist when the rest of the plant is killed by drought, is an interesting
adaptation to this end, and is found in all three groups (_e.g._ in species
of _Riccia_, _Fossombronia_ and _Anthoceros_). No examples of total
saprophytism or of parasitism are known, but two interesting cases of a
symbiosis with other organisms which is probably a mutually beneficial one,
though the nature of the physiological relation between the organisms is
not clearly established, may be mentioned. Fungal hyphae occur in the
rhizoids and in the cells of the lower region of the thallus of many
liverworts, as in the endotrophic mycorhiza of higher plants. Colonies of
_Nostoc_ are constantly found in the Anthocerotaceae and in _Blasia_. In
the latter they are protected by special concave scales, while in the
Anthocerotaceae they occupy some of the mucilage slits between the cells of
the lower surface of the thallus.
Other adaptations concern the protection of the sexual organs and
sporogonia, and the retention of water in the neighbourhood of the
archegonia to enable the spermatozoid to reach the ovum. In thalloid forms
the sexual organs are often sunk in depressions, while in the foliose forms
protection is afforded by the surrounding leaves. In addition special
involucres around the archegonia have arisen independently in several
series. The characters of the sporogonium have as their object the
nutrition and effective distribution of the spores, and only exceptionally,
as in the Anthocerotaceae, are concerned with independent assimilation. In
most forms the capsule is raised above the general surface at the time of
opening, usually by the rapid growth of the seta, but in the Marchantiaceae
by the sporogonia being raised on a special archegoniophore. The elaters
serve as lines of conduction of plastic material to the developing spores,
and later usually assist in their dispersal. The spores, with few
exceptions, are unicellular when shed, and may develop at once or after a
resting period. In their germination a short filament of a few cells is
usually developed, and the apical cell of the plant is established in the
terminal cell. In other cases a small plate or mass of cells is formed.
With one or two exceptions, however, this preliminary [v.04 p.0648] phase,
which may be compared with the protonema of mosses, is of short duration.
The power of vegetative propagation is widely spread. When artificially
divided small fragments of the gametophyte are found to be capable of
growing into new individuals. Apart from the separation of branches by the
decay of older portions, special gemmae are found in many species. In
_Aneura_ the contents of superficial cells, after becoming surrounded by a
new wall and dividing, escape as bi-cellular gemmae. Usually the gemmae
arise by the outgrowth of superficial cells, and become free by breaking
away from their stalk. When separated they may be single cells or consist
of two or numerous cells. In _Blasia_ and _Marchantia_ the gemmae are
formed within tubular or cup-shaped receptacles, out of which they are
forced by the swelling of mucilage secreted by special hairs.
[Illustration: FIG. 5.--_Marchantia polymorpha_. (After Sachs.)
A. Portion of thallus (t) bearing two stalked antheridiophores (hu).
B. Longitudinal section through a young antheridiophore. The antheridia (a)
are seated in depressions of the upper surface (o); b, scales; h, rhizoids.