rd order (_Marchantiaceae_) includes the most conspicuous
members of the whole class. Some of them, like the common liverwort
(_Marchantia_), shown in Figure 57, _F_, _K_, and the giant liverwort
(Fig. 57, _D_), are large and common forms, growing on the ground in
shady places, the former being often found also in greenhouses. They
are fastened to the ground by numerous fine, silky hairs, and the
tissues are well differentiated, the upper surface of the plant having
a well-marked epidermis, with peculiar breathing pores, large enough
to be seen with the naked eye (Fig. 57, _E_, _J_, _K_) Each of these
is situated in the centre of a little area (Fig. 57, _E_), and beneath
it is a large air space, into which the chlorophyll-bearing cells
(_cl._) of the plant project (_J_).

The sexual organs are often produced in these forms upon special
branches (_G_), or the antheridia may be sunk in discs on the upper
side of the stem (_D_, _an._).

[Illustration: FIG. 57.--Forms of liverworts. _A_, _Riccia_, natural
size. _B_, _Anthoceros_ (horned liverwort), natural size. _sp._
sporogonia. _C_, _Lunularia_, natural size, _x_, buds. _D_, giant
liverwort (_Conocephalus_), natural size. _an._ antheridial disc. _E_,
small piece of the epidermis, showing the breathing pores, x 2. _F_,
common liverwort (_Marchantia_), x 2. _x_, cups containing buds. _G_,
archegonial branch of common liverwort, natural size. _H_, two young
buds from the common liverwort, x 150. _I_, a full-grown bud, x 25.
_J_, vertical section through the body of _Marchantia_, cutting
through a breathing pore (_s_), x 50. _K_, surface view of a breathing
pore, x 150. _L_, a leafy liverwort (_Jungermannia_). _sp._
sporogonium, x 2.]

Some forms, like _Marchantia_ and _Lunularia_ (Fig. 57, _C_), produce
little cups (_x_), circular in the first, semicircular in the second,
in which special buds (_H_, _I_) are formed that fall off and produce
new plants.

The highest of the liverworts (_Jungermanniaceae_) are, for the most
part, leafy forms like _Madotheca_, and represented by a great many
common forms, growing usually on tree trunks, etc. They are much like
_Madotheca_ in general appearance, but usually very small and
inconspicuous, so as to be easily overlooked, especially as their
color is apt to be brownish, and not unlike that of the bark on which
they grow (Fig. 57, _L_).

CLASS II.--THE TRUE MOSSES.

The true mosses (_Musci_) resemble in many respects the high