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.

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.

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.