_Marchantiales._--The plants of this group are most abundant in warm sunny
localities, and grow for the most part on soil or rocks often in exposed
situations. Nine genera are represented in Britain. _Targionia_ is found on
exposed rocks, but the other forms are less strikingly xerophytic;
_Marchantia polymorpha_ and _Lunularia_ spread largely by the gemmae formed
in the special gemma-cups on the thallus, and occur commonly in
greenhouses. The large thallus of _Conocephalus_ covers stones by the
waterside, while _Dumortiera_ is a hygrophyte confined to damp and shady
situations. Among the Ricciaceae, most of which grow on soil,
_Ricciocarpus_ and _Riccia natans_ occur floating on still water. The
dorsiventral thallus is constructed on the same plan throughout the group,
and shows a lower region composed of cells containing little chlorophyll
and an upper stratum specialized for assimilation and transpiration. The
lower region usually forms a more or less clearly marked midrib, and
consists of parenchymatous cells, some of which may contain oil-bodies or
be differentiated as mucilage cells or sclerenchyma fibres. Behind the
apex, which has a number of initial cells, a series of amphigastria or
ventral scales is formed. These consist of a single layer of cells, and
their terminal appendages often fold over the apex and protect it. Usually
they stand in two rows, but sometimes accessory rows occur, and in _Riccia_
only a single median row is present. The thallus bears two sorts of
rhizoids, wider ones with smooth walls which grow directly down into the
soil, and longer, narrower ones, with peg-like thickenings of the wall
projecting into the cell-cavity. The peg-rhizoids, which are peculiar to
the group, converge under shelter of the amphigastria to the midrib,
beneath which they form a wick-like strand. Through this water is conducted
by capillarity as well as in the cell cavities. The upper stratum of the
thallus is constructed to regulate the giving off of the water thus
absorbed. It consists of a series of air-chambers (fig. 6, B) formed by
certain lines of the superficial cells growing up from the surface, and as
the thallus increases in area continuing to divide so as to roof in the
chamber. The layer forming the roof is called the "epidermis," and the
small opening left leading into the chamber is bounded by a special ring of
cells and forms the "stoma" or air-pore. In most species of _Riccia_ the
air-chambers are only narrow passages, but in the other Marchantiales they
are more extended. In the simplest cases the sides and base of the chambers
perform the work of assimilation (_e.g._ _Corsinia_). Usually the surface
is extended by the development of partitions in the chambers (_Reboulia_),
or by the growth from the floor of the chamber of short filaments of
chlorophyllous cells (_Targionia_. _Marchantia_, fig. 6). The stomata may
be simply surrounded by one or more series of narrower cells, or, as in the
thallus of _Marchantia_ and on the archegoniophores of other forms, may
become barrel-shaped structures by the division of the ring of cells
bounding the pore. In some cases the lowermost circle of cells can be
approximated so as to close the pore. In _Dumortiera_ the air-chambers are
absent, their formation being only indicated at the apex.
[Illustration: FIG. 6.--_Marchantia polymorpha._ A, Stoma in surface view.
B, Air-chamber with the filaments of assimilating cells and stoma in
vertical section.