The Muscineae agree in many of their conditions with the Thallophytes, in others again with the next higher division, the vascular Cryptogams (Pteridophytes), to which the Ferns, Equisetums, and Lycopodiums belong. In many Muscineae of the division of Liverworts (Hepa- ticae) the vegetative body has still the form of a thallus (fig. 1), which is ribbon-shaped and grows in close contact with the substratum. There are, however, several gradual stages of transition from this ribbon-shaped thallus to a leaved stem, such as many Liverworts (Hepaticae) and all Leaf-mosses (Λfusci frondosi) possess. Yet the structure of the leaves and of the stem, even in the latter, is very simple. The leaves present for the most part a cellsurface of one layer, and that (if we leave out of sight those in the middle part of the leaf, which in Leaf-mosses form the “mid-rib”) is ordinarily composed of cells of uniform character. There is thus no epidermis as in higher plants, and the leaf itself attains but very trifling dimensions. The stem is also of simple structure; it possesses no “vascular bundles,” but consists only of simple cells, among which there is no differentiation, except that those which lie outside often have thicker walls, and thus form a firmer rind-layer, while the interior ones are more elongated and serve for the storage and transmission of the plastic substances (albumen, hydrocarbons, <fcc.). The roots also by which the stems are attached to the ground are of very simple organization. They are either hair-like tubules as in many Liverworts, or rows of cells as in the Leaf-mosses. To distinguish them from the roots of higher plants they have been termed “rhizoids.” In many cases the vegetative body scarcely attains one twenty-fifth of an inch in length, in others, however, it rises into a much-branched form of from about 4 to about 12 inches or more. The duration of its life reaches in some small forms (Ephemerum, Phascum, <fcc.) only a few weeks or months; in most cases, however, it is virtually unlimited, since the vegetative body continues to grow at its point while the older parts below are dying away. From this cause too the branches are isolated from each other, and become independent plants. In a peat-moor, for instance, the Bog-moss plants on the upper surface are the points and branches of the very same plants whose under-parts have long ago died away, and have principally contributed to the formation of the peat. This isolation of the branches through the dying away of the older parts which keep them together is at the same time a means of multiplication. Mosses also possess many other arrangements for asexual multiplication, as through gemmae, “innovations,” <fcc. In the Liverworts, for instance, almost every cell is capable of giving origin to a new plant.

The most extensive propagation, however, of the Mosses is that by means of spores, small roundish cells which are formed in a peculiar capsule, the sporogonium. The formation of these sporogonia is especially characteristic of the Muscineae. They originate, as a consequence of fertilization, from a cell—the ovum-cell—which is generated in the female sexual organ, the archegonium. These archegonia (fig. 6) have, when ready for fertilization, the form of a flask. They consist of an inferior and somewhat swollen portion, which contains the ovum-cell, and of a superior portion drawn out to some length, the neck. The neck forms a canal through which the male fertilizing bodies, the spermatozoïde (fig. 5, Z>), enter to blend with the ovum-cell. In the immature condition the archegonium is closed, its neck portion is filled up with a string of cells and covered at the top by a layer of cells which closes it like a lid (fig. 6). At a later period, however, the membranes of the string of cells which fills up the neck (the canal-cells of the neck) are converted into a jelly, which on contact with water swells up greatly, forces open the apex of the neck portion, and thus presents an open access to the ovum-cell. The mucilage which arises from the swelling up of the canal-cells of the neck is also so far of importance with regard to fertilization that by it the freely-moving male fertilizing bodies are stopped when they reach the neck of the archegonium. The male fertilizing bodies are here, as in many Algae, in the vascular Cryptogams, and in most animals, termed spermatozoids. They are spirally-coiled filaments, thickened at the posterior extremity, finely pointed at the anterior, and bearing at this thin end two long fine cilia by means of which they can move in water. The spermatozoïde are formed in the male sexual organs, the antheridia (fig. 5, C ’), which are bodies with long or short stalks, and of spherical, oval, or club-shaped form, surrounded by a wall-stratum consisting of a simple layer of cells, and which possess in their interior a tissue formed of numerous small cells, in each of which one spermatozoid takes its origin. When the wall-layer of the antheridium is torn asunder the spermatozoïde are set free at its apex, and move in water by means of their cilia, like Infusoria. Thus water is always necessary to fertilization; it is only in water that the neck of the archegonium opens, and that the spermatozoids can move. Since either the Mosses grow in moist localities, or those which spring up in dry situations always form little turfs so as to suck up every drop of rain like a sponge, their fertilization is always secured, provided the organs of both sexes are present.

Even before the opening of the archegonium-case the oosphere has formed itself in the inferior portion of the archegonium. Originally in this inferior ventricose portion of the archegonium is a large cell, the central cell (fig. 6). From this a small cell is severed in an upward direction, which perishes at a later period, and is called the ventral canal-cell (fig. 6, ό), while the inferior cell becomes round and forms itself into the oosphere. When ready for fertilization the ovum-cell is a spherical portion of protoplasm with a cell-nucleus (fig. 6, e). Fertilization follows on the blending of one or perhaps several spermatozoïde with the oosphere. The first result of fertilization is that the oosphere becomes surrounded by a membrane of cellulose, and then grows very notably, while correspondingly with this growth it becomes divided by cell-walls, and thus becomes a cellular body. From the fertilized oosphere proceeds the moss-fruit, the sporogonium. The embryo which has proceeded from the fertilized oosphere lives at the expense of the vegetative body on which the archegonium is situated, like a parasite on the plant that nourishes it. The ventral portion of the archegonium grows along with the embryo. In Rieda, the lowest form of one subdivision of the Liverworts, the sporogonium remains even during the whole lifetime of the plant shut up within the ventral portion of the archegonium, and the spores formed in the sporogonium are not set free until that part of the plant on which the archegonium is situated withers away. In most other Liverworts also the sporogonium attains almost its entire development within the ventral portion of the archegonium, which grows contemporaneously with it, and it is not until the spores are mature that the stalk of the sporogonium extends itself considerably (fig. 3, (1 ), Λ); then the ventral portion of the archegonium springs in pieces and the spore-capsule issues forth and scatters its spores,— an occurrence which is completed within a few days. The ventral portion of the archegonium, which increases considerably after fertilization and which surrounds the embryo like a sheath, is termed calyptra. In the case of the Liverworts this calyptra is burst open by the sporogonium; in the Mosses, however, the elongated fusiform-embryo tears away the calyptra at its base and raises it up like a cap on its apex, and it is then found situated on the extremity of the sporogonium. In many Leaf-mosses (Polytriclιum, Hypnum crista castrensis) the sporogonium requires more than a year for its development.

The function of the sporogonium is the production of spores. The simplest mode in which this takes place is seen in the species Ricda. Here the sporogonium is a spherical cell-body, consisting of a wall-layer and an interior tissue. The cells of this latter are called the mother-cells of the spores, because each of them by division forms four spores. In the species, however, which stand higher we can distinguish in the sporogonium a foot, which frequently penetrates into the tissue of the vegetative body, and in many Liverworts and almost all Mosses prolongs itself into a stalk supporting the capsule in which the spores are formed (figs. 3, 7, 8). In the case of most Liverworts we find in this capsule, besides the spores, also a number of cells which do not become spores. These either act as “nutrient cells” to the spore-forming cells, which gradually consume the matters stored up in them (Riella), or they form themselves into fusiform-cells with spiral thickenings of parts of the wall—the so-called elaters, which play a part in the dispersion of the spores. The sporogonia of Mosses do not possess elaters. They are rather complex structures, and attain a higher anatomical differentiation than the moss-stem itself. For example, at certain points in their epidermis they possess openings (stomata), which are entirely absent on the stem. In order to ensure the dispersion of the spores, special arrangements are found in the sporogonia of Mosses.

The course of development of the Muscineae divides itself into two sharply-defined stages. The vegetative body, whether it be only a thallus as in many Liverworts, or a leafy stem, produces the sexual organs, the antheridia and the archegonia. From the fertilized oosphere of the archegonium arises a special structure, the sporogonium, which does not nourish itself independently, but lives like a parasite upon the moss-plant, and produces the spores only in a sexual way. The two stages—on the one hand the moss-plants that bear sexual organs, and on the other the sporogonium developed as the result of fertilization—are termed generations ∙, accordingly in the Muscineae the course of development consists in a regular alternation of these two stages—in an alternation of generations. The one generation possesses sexual organs, and is thus the sexual one. The other generation, the result of fertilization, possesses no sexual organs and is asexual, but produces the spores. Such alternation of generations is exhibited still more strikingly in Fems. In this case, however, it is the proper leafy fern-plant which is the asexual generation, producing spores, but possessing no sexual organs. It arises out of a fertilized oosphere, and thus corresponds to the sporogonium of the Muscineae. The sexual generation of the Ferns is a small and insignificant expansion, the prothallium, which bears antheridia and archegonia, and thus corresponds to the sexual generation of the Muscineae, which is here represented by the proper moss-plant. The vegetative body of the Mosses which bears the sexual organs is not, however, the immediate product of the germination of the spore, from which there is produced in the first place a simple structure, a pro-embryo (specially developed in the Mosses proper, where it is termed “protonema,” and where it assumes the appearance of a much-branched filamentous confervaceous Alga, fig. 15); it is from this proembryo that the leafy stems arise as lateral buds.

From a systematic point of view the Muscineae are divided into two sections—Liverworts (Hepaticae) and Mosses (Musci frondosi). The two sections are closely related to each other, but in external characters they are separated by rather sharp boundary lines.

I. Liverworts (Hepaticae).

Liverworts stand below the Mosses proper in regard to the number both of species and of individuals. They are also of much more limited occurrence, for they are found, for the most part, only in damp and shady localities, and seldom occupy any large extent of ground. The most extensive growths are those of many Marchantiaceae— as, for example, Fegatella conica, which often forms a continuous covering on moist stones, walls, <fcc.

(1) The Organs of Vegetation.— With a few exceptions, the vegetative body is closely united to the substratum on which it grows, and fastened to it by clasping-roots (rhizoids) composed of a single cell. The side turned towards the substratum is of different structure from the upper or dorsal side. The Liverworts thus belong, with the exception of two species, to the class of dorsi-ventral plants, or those possessing two sides of different structure, a dorsal side and a ventral side, as in vertebrate animals.

The vegetative body of the Liverworts is either a thallus or a leafy plant; and we have consequently to distinguish between thallose and foliose Liverworts. The latter have already been teπned Hepaticae frondosae. There occur, however, transitional forms between these two. Fig. 1 represents an example of a thallose Liverwort, showing the anterior portion of a plant of Metzgeria furcata. On the right hand is shown the upper, on the left the under side. The vegetative body is a ribbon-shaped thallus, on the under side of which the clasping-roots (rhizoids) spring out of the mid-rib. Here the mid-rib is sharply separated from the side portions, but in other cases (as Aneura and Pellid) it passes into them by a very gradual transition. In front, in an indentation of the growing point (s), are found the only organs which are formed upon it. These are hairs secreting mucilage (papillae), and which stand on the under side. The mucilage which they secrete is probably intended for the purpose of protecting the vegetative point against the effects of drought.

A peculiar exception appears in the growth of the genus Riella (fig. 2). In this case a membranous wing, about one-fifth of an inch in width, of the finest green and of extreme delicacy, turns regularly on a central rib or axis, so as to form with the axis a sort of screw or winding staircase, having the form of an inverted cone with the apex downwards. The length of the whole plant of Riella helicophylla is about 2 inches. This plant, which is decidedly one of the most remarkable forms in the vegetable kingdom, grows upright in water, and was discovered in Algiers. At its lower end it is attached to the ground by clasping-roots. This is the only thallose Liverwort that grows upright, and also one of the few that grow in water. There are likewise two species of Rieda which swim in water— Riccia natans and Ricda fluitans ; but these also possess forms which live on land, and it is these forms which attain to the formation of sporogonia. It follows hence that these plants also were originally in every case land plants. A few others too, such as Pellia epiphylla and Marchanda polymorpha, have the power of vegetating in water, but this happens only exceptionally, and the plants are then sterile.

Returning to the thallose Liverworts that live on land, we have to remark that the differentiation of their tissues is a very simple one. The cells of the thallus, so far as they are not employed in the formation of clasping-roots or of sexual organs, are all homogeneous. Those only of the mid-rib present a difference in Metzgeria, Blyttia, and others by their lengthened form. From this simple form has arisen a further complication in the development of the vegetative body in two different series. In the Marchantiaceae the vegetative body retains the form of a thallus, but its anatomical organization is of higher structure. In the Jungermanniaceae it is the external organization of the vegetative body which is the richer, and here the transition takes place in it to the foliose Liverworts, which possess a stem with leaves.

We may consider as representative of the Marchantiaceae the very widely - spread Marchanda polymorpha. Its vegetative body is a broad ribbonshaped thallus, which has on its under side clasping-roots (single-celled tubules, which are either simple, or have on their inner side characteristic conical or tubercular thickenings) and two rows of membranous lamellae, the uppermost of which cover the growing point and form.a protection to it. (See the section through Fegatella, an allied form, fig. 3, (1),I..) The upper surface presents a division into small rhomboidal spaces (areolae), and in the middle of each space is an opening, the stoma or breathing hole. A section through the thallus shows a strongly-marked epidermis broken through at definite spots by the stomata or breathing holes, which, however, in this case have a very different structure from that presented in the higher plants. In Marchanda (fig. 3, (5), Sp) they are of barrel-shaped figure, and consist of several rings or tiers of cells placed over one another. Their origin may be traced in fig. 3, (3) and (4). Below the epidermis we find chambers filled with air. These, however, are not empty; from their base sprout forth cellular threads (fig. 3, (5)), the cells of which çontain a large quantity of chlorophyll. It is in these that the assimilation of the carbon takes place. Those cells of the thallus which lie beneath serve only for the storing up and transmission of the materials for construction. The individual chambers are separated from each other by flat layers of cells, and on these rest the rhomboidal areolae of the upper surface which can be seen with the naked eye. In other Marchandées the stomata are of a different structure. In Fegatella, for example, they are not barrel-shaped, but are bounded by several concentric circles of cells, which all lie on the upper surface of the thallus. Early stages of these are represented in fig. 3, (2). The way in which the air-chambers and stomata attain their condition is very remarkable. Small depressions in the cell-layer of the upper surface are formed in large numbers behind the growing point (fig. 3, (1), U). These widen considerably, and become, as it were, roofed over by the neighbouring parts, so that an epidermis (fig. 3, (1), F) grows in this manner over the hollows from the bottom of which at a later period the green cellular threads sprout forth. The strings of mucilage-cells which are found in the tissue of Fegatella, the brown threads which appear in that of Preissia, and the cells filled with a brown oily secretion which are found in Marchanda and other species must also be mentioned.

The second series, which comes next to the simple thallose forms, is that of the foliose or leafy Jungermannieae. Forms intermediate between the thallose and the foliose are represented by the species Blasia and Fossombronia. The former, which not unfrequently grows in the damp soil of forests, possesses a flat, thallus-like stem, which bears on its longitudinal axis two rows of leaves inserted parallel to each other, and has also on the under side two rows of toothed scales, the amphigastria. Fossombronia makes a nearer approach to the proper leafy forms; it possesses two rows of leaves inserted obliquely to the longitudinal axis of the flattened stem. In the foliose Jungermannieae, in the narrower sense of the term, the stalk is thin and thread-like, and bears distinctly defined leaves. In this case also—with the exception of the upright growing Haplomitrium Hookeri— the stem grows in close connexion with the substratum. The leaves stand in three rows: two lateral, and one ventral towards the substratum. The ventral leaves are in this case also, as in Blasia, termed amphigastria or under leaves. They are smaller than the side leaves, and are sometimes reduced to the form of mere hairs, or even are entirely wanting, as in Jungermannia bicuspidata, where it is only in rare...