_Pellia epiphylla_ (fig. 2) can be found at any season growing in large
patches on the damp soil of woods, banks, &c. The broad flat thallus is
green and may be a couple of inches long. It is sparingly branched, the
branching being apparently dichotomous; the growing point is situated in a
depression at the anterior end of each branch. The wing-like lateral
portions of the thallus gradually thin out from the midrib; from the
projecting lower surface of this numerous rhizoids spring. These are
elongated superficial cells, and serve to fix the thallus to the soil and
obtain water and salts from it. No leaf-like appendages are borne on the
thallus, but short glandular hairs occur behind the apex. The plant is
composed throughout of very similar living cells, the more superficial ones
containing numerous chlorophyll grains, while starch is stored in the
internal cells of the midrib. The cells contain a number of oil-bodies the
function of which is imperfectly understood. The growth of the thallus
proceeds by the regular segmentation of a single apical cell. The sexual
organs are borne on the upper surface, and both antheridia and archegonia
occur on the same branch (fig. 3, A). The antheridia (an) are scattered
over the middle region of the thallus, and each is surrounded by a tubular
upgrowth from the surface. The archegonia (ar) are developed in a group
behind the apex, and the latter continues to grow for a time after their
formation, so that they come to be seated in a depression of the upper
surface. They are further protected by the growth of the hinder margin of
the depression to form a scale-like involucre (in). Fertilization takes
place about June, and the sporogonium is fully developed by the winter. The
embryo developed from the fertilized ovum consists at first of a number of
tiers of cells. Its terminal tier gives rise to the capsule, the first
divisions in the four cells of the tier marking off the wall of the capsule
from the cells destined to produce the spores. In fig. 4, C, which
represents a longitudinal section of a young embryo of _Pellia_, these
archesporial cells are shaded. The tiers below give rise to the seta and
foot. The mature sporogonium (fig. 3, B) consists of the foot embedded in
the tissue of the thallus, the seta, which remains short until just before
the shedding of the spores, and the spherical capsule. It remains for long
enclosed within the calyptra formed by the further development of the
archegonial wall and surmounted by the neck of the archegonium. The
calyptra is ultimately burst through, and in early spring the seta
elongates rapidly, raising the dark-coloured capsule (fig. 2). In the young
condition the wall of the capsule, which consists of two layers of cells,
encloses a mass of similar cells developed from the archesporium. Some of
these become spore-mother-cells and give rise by cell division to four
spores, while others remain undivided and become the elaters. The latter
are elongated spindle-shaped cells with thick brown spiral bands on the
inside of their thin walls. They radiate out from a small plug of sterile
cells projecting into the base of the capsule, and some are attached to
this, while others lie free among the spores. The latter are large, and at
first are unicellular; but in _Pellia_, which in this respect is
exceptional, they commence their further development within the capsule,
and thus consist of several cells when shed. [v.04 p.0647] The cells of the
capsule wall have incomplete, brown, thickened rings on their walls, and
the capsule opens by splitting into four valves, which bend away from one
another, allowing the loose spores to be readily dispersed by the wind,
assisted by the hygroscopic movements of the elaters. On falling upon damp
soil the spores germinate, growing into a thallus, which gradually attains
its full size and bears sexual organs.
[Illustration: FIG. 4.--Semi-diagrammatic figures of young embryos of
Liverworts in longitudinal section. The cells which will produce the
sporogenous tissue are shaded. (After Kienitz-Gerloff and Leitgeb.)
A, _Riccia_.
B, _Marchantia polymorpha_.
C, _Pellia epiphylla_.
D, _Anthoceros laevis_.
E, _Cephalozia bicuspidata_.