HEPATICÆ, OR LIVERWORTS.
THE Hepaticæ are the small herbaceous plants, which constitute the three
distinct natural orders called respectively Ricciaceæ, Marchantiaceæ,
and Jungermanniaceæ. They are distinguished primarily by the first
having the sporangia valveless, without elaters; by the second having
dependent valvate or irregularly bursting sporangia; and by the third
having the sporangia valvate and erect. Both the latter, moreover, have
the spores mixed with elaters.
The Ricciaceæ, popularly called Crystalworts, which form the lowest
grade of Hepaticæ, are inconspicuous plants, growing in mud, or floating
on water. They have spreading, horizontal fronds, of a delicate cellular
structure and of indefinite form. Their fructification consists of
valveless spherical sporangia, or spore sacs, imbedded in the frond;
cells are formed within these sporangia, each of which cells is divided
into four parts, which become spores, and, when ripe, the surface of the
sporangia is fractured to give them egress. In some species there are
many air passages in the cavities where the sporangia are produced. The
under-surface of the frond is often beset with scales. The genus Riella
differs from all the other genera in having an upright branchless stem
with a distinct wing or limb forming a continuous spiral round it. In
the male plant, the edge of the frond bears the antheridia containing
spermatozoids; in the female plant the sporangia spring from the stem;
they have a separate envelope, and the spores are echinate.
[Illustration: Fig. 43. Marchantia polymorpha:—_a_, gemmiparous
conceptacles; _b_, lobed receptacles bearing pistillidia.]
The Marchantiaceæ rank as higher forms of Hepaticæ. In the Marchantia
polymorpha (figs. 43, 44), which may be taken as an illustration of the
group, the structure of the horizontal frond is complicated, for,
besides the colourless transparent skin, there are three distinct
layers, the uppermost of which consists of cells filled with green
matter, the lowermost (or base of the frond) being formed of close-set
cells full of very solid matter, while between the two there is a cavity
filled with air and loosely branching filaments, which spring from the
base, and consist of green cells fixed end to end, as in fig. 44 B. The
surface of the plant appears to be smooth and shining, but when
magnified it is found to be marked with numerous narrow elevated green
bands, crossing one another diagonally so as to mark out the surface
into a number of small lozenge-shaped divisions (fig. 44 A). These
elevated bands are merely the tops of very solid walls which descend
perpendicularly to the base of the plant, consequently they divide the
internal air chamber into a number of lozenge-shaped compartments, each
of which communicates with the external atmosphere by means of chimneys
opening in the centre of each compartment on the surface of the plant.
These chimneys are so constructed of four or five superposed rings of
cells (see fig. 44 B), that by the expansion and contraction of the
cells of the lowermost ring, more or less air can be admitted into the
cavity below. This is a very beautiful instance of the contractile vital
energy acting for the production of motion, its object being to supply
air, so essential to the health of all plants. White filaments from the
base at once fix the Marchantia to the earth, and supply it with food.
This is the youthful state of the Marchantia polymorpha, but after a
time green points appear from under little reddish scales on the
surface, and these are developed into stalks an inch or less in height,
which terminate differently, some in lobed shields, others in spoked
whorls, like a carriage wheel without the rim (fig. 43). The lobed
shields are rather concave and covered with little elevations, in each
of which there is a flask-shaped cavity with a long neck opening on the
surface of the shield. In all of these hollows there is a mass of cells
full of an amorphous substance, which is changed into spermatozoids,
having the form of delicate spiral filaments, thicker at one end, and
furnished with two cilia, with which they revolve in a spiral within
their cell. At last they emerge from it, and come through the neck of
the flask-shaped hollow to the surface of the shield.
[Illustration: Fig. 44. Marchantia polymorpha:—A, portion of frond seen
from above, showing lozenge-shaped divisions (_a_), with central stomata
(_b_); B, vertical section, showing the layers of tissue, and one of the
stomata (_g_).]
[Illustration: Fig. 45. Marchantia polymorpha:—archegonia.]
[Illustration: Fig. 46. Elater and spores of Marchantia.]
In the companion female receptacles at an early age, certain objects
called archegonia (fig. 45) are found to be concealed between membranes
which connect the spokes of the whorl at their origin. These archegonia
are shaped like flasks with long necks, and each has a germ cell in its
interior, into which a canal leads down from the extremity of the neck.
When this embryo or germ cell is fertilized by the spermatozoids,
instead of producing a new plant resembling its parent, the embryo cell
developes itself into a sporangium containing spores, which are isolated
cells enclosed in firm yellow envelopes and elaters, or ovoidal cells,
each containing a double spiral fibre coiled up in its interior. This
fibre is so elastic, that when the surrounding pressure is withdrawn by
the bursting of the sporangium at maturity, the spires suddenly extend
themselves with such force as to tear open the cell membrane, and jerk
forth the spores which may be adhering to their coils, and thus to
assist in their dispersion. The spores, when they germinate, develope
themselves into little collections of cells, which gradually assume the
form of a flattened frond.[66] It is only when exposed to light and air
that the Marchantias have regular fructification; in shady places they
are reproduced by buds placed in open conceptacles, which are formed out
of green globules that appear in different parts of the frond, and after
a time split open at the summit and expand into singularly graceful cups
or baskets, whose edges are sharply and regularly indented so as to form
a glistening fringe of teeth, while each tooth is adorned with a narrow
fringe. When mature, the basket contains a number of little green round
or oval discs raised on footstalks, and composed of two or more rows of
cells. As soon as these objects, called gonidia, are ripe, they are
detached from their stalks, and being washed out of their basket by the
rain, they quickly grow on the moist earth around; sometimes they
germinate before they leave their nest, and form irregular lobes on the
parent plant. The Marchantia polymorpha, so admirably constructed,
occurs in all temperate climates, and can bear considerable heat
provided it has abundant moisture.
The Marchantiaceæ are divided into three groups, containing fifteen
genera, which are distinguished from each other by the character of
their fructification. They are minutely described in Mr. Berkeley’s
‘Cryptogamic Botany;’ and are widely dispersed both in temperate and
tropical countries, most of the genera being represented in Europe.
The Jungermanniaceæ, or Scale Mosses, in their lowest forms bear the
same resemblance to Lichens that many of the other Liverworts do. These
lichenoid forms are lobed, leaf-like masses, sometimes ribbed and
sometimes not. There are forty genera of this group of plants,
distributed amongst fifteen tribes, and exhibiting great variety of form
and structure; but in external aspect they are so closely connected,
that a graduated series may be traced from the flat-lobed frond to the
higher forms of erect-stemmed foliaceous plants, approaching in size and
structure to some of the smaller mosses.
The higher groups have a distinct upright stem with symmetrical leaves,
which leaves, however, in the lower genera, are merely fleshy fronds
without veins, of numerous and sometimes grotesque forms, while in the
higher, they are generally oval, and disposed in spiral imbricated rows
on each side of the stem and branches, every leaf overlapping part of
that which is adjacent to it like a row of tiles. They are disposed
after two distinctly different plans. In one, the leaves are arranged in
a spiral, which turns from left to right; consequently, the anterior
border of each inferior leaf is covered by the posterior border of that
immediately above it. This constitutes the succubous group of frondose
Jungermannieæ, which embraces five tribes. In the second case, the
spiral of the leaves turns from right to left, and the anterior border
of each inferior leaf covers the posterior border of that immediately
above it. This constitutes the incubous group embracing three tribes of
the frondose Jungermannieæ. Besides the imbricated spiral leaves, there
are small leaves alternating with them, and applied directly on the stem
between them.
The male fruit of the Scale Mosses consists of antheridia, which form
ovoid masses of cells variously disposed; in each of these cells, there
is a filiform spermatozoid coiled in a circle in active motion, which is
maintained after leaving the cell. The female fruit consists of
archegonia, containing embryo cells, which ultimately develope into
sporangia; but the plants arrive at their full development before the
archegonia appear. The same plant may produce more than one crop of
these objects, which are occasionally abortive. The stalk of the
sporangium is first surrounded by the sac of the archegonium, then by an
involucre or circle of bracts, placed in a rosette of leaves. The
sporangia contain cells which ultimately become spores mixed with long
cells containing elaters, that is, elastic spiral filaments. When the
fruit is ripe, the sporangia split into four equal parts, which form a
cross on the top of their stems, and the elaters spring out of their
cells, and disperse the spores.
The plants of this order are also reproduced by gonidia, but in such
numerous genera and species the arrangement of the reproductive bodies
and their minute details are exceedingly varied. They are chiefly
distinguished by the insertion of the fruit, and the form of the
different organs which surround it. They are pretty little plants,
occasionally of a bright green, but oftener inclined to red, purple, and
chocolate colour; a few are fragrant, but they are of no known utility.
They are found in shady woods and moist situations, throughout all
regions of the globe, but are most abundant in damp, tropical forests.
In all the families of the leafy Cryptogamia as well as in the Hepaticæ,
antheridia exist; they differ much in form and structure, but they
collectively agree in developing in the interior of delicately walled
cells, an amorphous substance, coloured yellow by iodine, in place of
which, at the epoch of maturation, spermatozoids appear, thick at one
end and running to a very fine point at the other, and displaying
several spiral convolutions. When rolled up like a watch-spring the
motion is more or less rotary, but if it be coiled in the form of a
corkscrew, the movement is at the same time advancing. The thin end of
the filament always goes first both within the cell and after it comes
out.[67] The whole structure of the Hepaticæ is full of objects of
interest to the microscopic observer.