This embraces only forms with a thallus, which is more or less
distinctly dichotomously branched, in some, one or two rows of thin
leaves are situated on its under surface. On the upper surface of the
thallus are found large air-chambers.
Order 1. =Ricciaceæ.= The sporogonia are, with the exception of a few
genera, situated singly on the surface of the thallus, and consist only
of a capsule without foot or stalk. They always remain enclosed by the
wall of the archegonium (calyptra), and open only by its dissolution.
Elaters are not developed. Some genera are found floating like
Duckweed.--_Riccia glauca_ grows on damp clay soil. _R. fluitans_ and
_R. natans_ float in stagnant waters.
[Illustration: FIG. 197.--_Marchantia polymorpha. A_ Female
plant (nat. size): _a_ and _b_ are archegoniophores in various
stages of development; _s_ cupules with gemmæ (see page 183). _B_
An archegoniophore seen from below, the short-stalked sporangia
are seen placed in 8–10 double rows. _C_ Male plant, with a
young and an older antheridiophore. _D_ Antheridiophore halved
vertically to show the antheridia (_h_); _m_ the aperture of the
pits in which they are sunk--the older ones to the left, the
younger to the right.]
Order 2. =Corsiniaceæ.= (Not native). Intermediate forms between the
preceding and the following order. In internal and external structure
mainly resembling the Marchantiaceæ. _Corsinia_; _Boschia_.
Order 3. =Marchantiaceæ=, are large, fleshy forms. The surface of the
thallus is divided into small rhombic areas, in the centre of each of
which is found a large, peculiarly constructed stoma (Fig. 197 _A_);
beneath each of these a large air-cavity is to be found. From the floor
of the air-cavity a number of alga-like cells project into it; these
contain chlorophyll and are therefore the assimilating cells. The
antheridia and archegonia are each found aggregated on specially formed
branches (somewhat resembling Mushrooms) projecting from the surface of
the thallus. The antheridia are developed on the upper surface (Fig.
197 _C_, _D_) and the archegonia on the lower (Fig. 197 _A_, _B_),
near the centrally-placed stalk.
_Marchantia polymorpha_ is diœcious (Fig. 197), and very common on damp
places. _Lunularia_ (South Europe), frequently found on flower-pots in
conservatories; _Preissia_, _Fegatella_, _Reboulia_, _Targionia_.