The sexual organs are always situated on the morphologically upper surface
of the thallus. In _Riccia_ they are scattered singly and protected by the
air-chamber layer. The scattered position of the antheridia is also found
in some of the higher forms, but usually they are grouped on special
antheridiophores which in _Marchantia_ are stalked, disk-shaped
branch-systems (fig. 5). The individual antheridia are sunk in depressions
from which the spermatozoids are in some cases forcibly ejected. The
archegonial groups in _Corsinia_ are sunk in a depression of the upper
surface, while in _Targionia_ they are displaced to the lower side of the
anterior end of a branch. In all the other forms they are borne on special
archegoniophores which have the form of a disk-shaped head borne on a
stalk. The archegoniophore may be an upgrowth from the dorsal surface of
the thallus (_e.g._ _Plagiochasma_), or the apex of the branch may take
part in its formation. When the disk, around which archegonia are developed
at intervals, is simply raised on a stalk-like continuation of the branch,
a single groove protecting a strand of peg-rhizoids is found on the ventral
face of the stalk (_Reboulia_). In the highest forms (_e.g._ _Marchantia_)
the archegoniophore corresponds to the repeatedly branched continuation of
the thallus, and the archegonia arise in relation to the growing points
which are displaced to the lower surface of the disk. In this case two
grooves are found in the stalk. The archegonia are protected by being sunk
in depressions of the disk or by a special two-lipped involucre. In
_Marchantia_ and _Fimbriaria_ an additional investment termed in
descriptive works the perianth, grows up around each fertilized archegonium
(fig. 1, 3, d). The simple sporogonium found in the Ricciaceae (fig. 4, A)
has been described above; as the spores develop, the wall of the spherical
capsule is absorbed and the spores lie free in the calyptra, by the decay
of which they are set free. In _Corsinia_ the capsule has a well-developed
foot, but the sterile cells found among the spore-mother-cells do not
become elaters, but remain thin-walled and simply contribute to the
nutrition of the spores. In all other forms elaters with spirally thickened
walls are found. The seta is short, the capsule being usually raised upon
the archegoniophore. Dehiscence takes place either by the upper portion of
the capsule splitting into short teeth or falling away as a whole or in
fragments as a sort of operculum. The spores on germination form a short
germ-tube, in the terminal cell of which the apical cell is established,
but the direction of growth of the young thallus is usually not in the same
straight line as the germ-tube. The Marchantiales are divided into a number
of groups which represent distinct lines of advance from forms like the
Ricciaceae, but the details of their classification cannot be entered upon
here. The general nature of the progression exhibited by the group as a
whole will, however, be evident from the above account.
_Jungermanniales._--This large series of liverworts, which presents great
variety in the organization of the sexual generation, is divided into two
main groups according to whether the formation of archegonia terminates the
growth of the branch or does not utilize the apex. The latter condition is
characteristic of the more primitive group of the Anacrogynous
Jungermanniaceae, in which the branch continues its growth after the
formation of archegonia so that they (and later the sporogonia) stand on
the dorsal surface of the thallus or leafy plant. In the Acrogynous
Jungermanniaceae the plant is throughout foliose, and the archegonia occupy
the ends of the main shoot or of its branches. The antheridia are usually
globular and long-stalked. The capsule opens by splitting into four halves.
[Illustration: FIG. 7.--_Blasia pusilla._ The margin of the thallus bears
leaf-life lobes. r, Rhizoids; s, sporogonium.