MUSCINEAe. The Muscineae are a highly interesting class of plants on account of the important part they play in the economy of nature, and also from the remarkable conditions of their development and formation. In many parts of the world it is principally the mossy covering of forests which, by collecting the rainfall like a sponge, prevents the pouring down from mountains of violent and excessive torrents of water, while the Bog-moss (Sphagnum) plays an important part in the formation of peat; and many other Mosses which grow on rocks produce by the decay of their dead parts a thin layer of mould (humus) in which the seeds of higher plants are able to take root. The importance, however, of Mosses in a morphological point of view proceeds from their position in the botanical system. The Muscineae immediately follow the first division of the vegetable kingdom, the Thallophytes, under which are included the Algae and Fungi, because their vegetative body is a “thallus,”—that is to say, is not divided into stem and leaf like that of higher plants, nor are they possessed of roots like those observed in higher plants (Ferns, Conifers, Monocotyledons, Dicotyledons, t fcc.). Their anatomical structure is also very simple, the individual essential tissue-elements being but little differentiated from each other.
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 exceptional cases that they appear. The leaves of Liverworts consist of a single layer of cells without any mid-rib. They are commonly bifid at the apex, but in many forms this condition is only recognizable in early development, and disappears before maturity.
In general the leaves stand so thickly that they overlap each other with their margins, and quite cover the upper surface of the stem. The way in which the leaves overlap has been employed as a systematic character. Overshot leaves {folia succuba) are those in which the anterior margin turned towards the vegetative point of the stem stands higher than the posterior one, and thus the anterior margin of each leaf overlaps the posterior margin of the leaf which stands before it, while its own posterior margin is overlapped by the anterior margin of the leaf which stands behind it. In the opposite case of undershot leaves {folia incuba), the posterior margin of the leaf is higher than its anterior margin, and the anterior margin, inclining obliquely outwards, is overlapped by the posterior margin of the leaf which comes next before it, as in fig. 4. The mode in which the leaves overlap each other depends on a difference in the relative amount of growth of the upper or dorsal and the under or ventral side respectively. If the growth of the upper side preponderates, then we have the overshot, in the opposite case the undershot mode of covering. Originally the leaves do not stand obliquely to the longitudinal axis; it is only gradually that they are shifted out of their place. The position of the leaves is explained by their origin. The apex of the stem is occupied by a cell which has the form of a three-sided pyramid with a vaulted base. The surface of one side of the pyramid is directed downwards, the two others are directed laterally. This apical cell is divided by walls which are parallel alternately to one of the side walls. Thus segments are cut off from the apical cell and lie in three rows. From each of these segments arises a leaf. The leaves are thus necessarily likewise arranged in three rows, one of which is turned towards the substratum.
The anatomical structure of the stem is also a very simple one. Its exterior cells have their walls somewhat more strongly thickened, and thus form a firmer rind-layer. We must also notice the secretion of dark-brown oily corpuscles, which undergo no further conversion, and are met with in certain leaf-cells. In the thallose forms the ramification is of a forked character (dichotomous) (fig.
1). Frequently too, on the older portions, “adventitious” shoots are formed; they may frequently be seen to spring from the margin and the mid-rib of the thallus of Metzgeria furcata. The ramification of the foliose Liverworts is a very complex one, but is never of a really forked character. The branchlets commonly stand below the leaves on the sides of the stem.
The above-mentioned “adventitious” shoots serve the purpose of vegetative increase. The same end is attained by the gemmae, which are of many different forms. In Aneura individual cells detach themselves from the thallus and grow up into new plants. In the Marchantieae the gemmae are formed in peculiar receptacles which stand on the upper side of the ribbon-shaped thallus. In Marchantia polymorpha these receptacles have the form of a basket open at the top, while in I.·unularia the margin of the same receptacle is semicircular. The gemmae, which are developed on the base of the receptacle, are flat upright cellular bodies, with two indentations at the sides, out of which at a later period, at the time of germination, proceed two growing points. The gemmae, however, are of similar structure on both sides. Which side is at the time of germination to become dorsal with stomata and air-chambers, and which ventral with root-hairs, is decided by its position with respect to the light. It is always the side turned towards the light which develops itself into that which bears stomata. This occurs even when gemmae that swim on water receive light only from below. In that case the under side becomes that which possesses stomata and the air-chambers described above, while the upper side develops root-hairs (rhizoids). Blasia pusilia has also very peculiar receptacles for gemmae, provided with a long channel leading outwards, while in many foliose Jungermannieae — for example, Jungermannia υentricosa, Scapania nemorosa, Jungermannia bicuspidata, and others—gemmae consisting of a few cells are formed on the leaves or the stem respectively.
(2) The Organs of Sex.— The antheridium in Liverworts is a spherical or ellipsoidal cellular body with a shorter or longer stalk (fig. 5, <z), and consists of a wall-layer and an interior tissue, the mother-cells of the spermatozoids. In each of these cells a spermatozoid is formed, and, as it appears, in the following way :— the peripheral layer of the cell-nucleus becomes thickened, and splits up into a ribbon, either annular or rolled up spirally. The cilia by means of which the spermatozoid moves in water appear, on the other hand, to be formed from the protoplasm in which the cell-nucleus lies. The process, however, requires still closer investigation. Each antheridium is originally a simple cell, which grows by repeated subdivision and becomes the antheι ’ idial body. By the swelling up of the walls of the spermatozoid mother-cells, the cells of the antheridium-wall are forced asunder at the apex, and the spermatozoids, each imbedded in mucilage, are emitted in jets either free from one another or united into a pulp. This pulp is often ejected by an explosion of the antheridium, as may be easily observed in Frullania dilatata. In water the spermatozoids isolate themselves, and in the most favourable cases are carried over to the archegonia. Since very many spermatozoids are formed in one antheridium, and since one or very few of these suffice for the fertilization of an archegonium, it is evident that an enormous number of them must perish.
The archegonia also proceed from one cell, through the division of which their construction is accomplished. They always take their origin from the cells of the upper surface, and the same is the case with those of Anthoceros, which, however, are sunk into the tissue of the thallus, so that their neck and ventral portions remain united with the tissue of the thallus and do not project freely over it.
The distribution of the organs of sex is either monoecious or dioecious. In the latter case the antheridia and archegonia arc found on different individual plants, as in Sphaerocarpus terrestris, where the male plant is smaller than the female. In the formel case the antheridia and archegonia are found on the same plant. In the thallose forms they are always inserted on the dorsal side of the thallus, and commonly sunk into a cavity in it (fig. 5, B). In Anthoceros the antheridia even stand in closed cavities, which do not open till the antheridium is mature. The organs of sex stand either on ordinary branches, which at a later period vegetate and grow, or on special sexual shoots. These are of especially remarkable construction in the Marchantieae, of which Marchantia polymorpha may serve as an example. In this species the antheridia are found on the upper side of a peltate radially-lobed disk, supported on a stalk (fig. 5, A^). The archegonia stand on the under side of a similar receptacle. These structures may be termed “inflorescences.” They are not simple branches, but systems oi branches which have arisen from the repeated bifurcation of a thallus-shoot. Each indentation in the disk of the “inflorescence” corresponds to the vegetative point of a branch, and this explains the fact that the oldest antheridia are found in the centre of the disk; and from this point to the periphery, where the vegetative points are found, are successive groups of antheridia of younger growth, for the youngest organs of sex always stand next to the vegetative point of the shoot. The stalk of the inflorescence is nothing else than the inferior portion of the branch-system greatly prolonged. In the leafy Jungermannieae the distribution of the organs of sex is also either monoecious or dioecious. The archegonia in this case stand either singly (Lejeunia, &c. ) or in larger groups. The first archegonium always arises from the apical cell of the shoot connected with it, so that with it the latter closes its longitudinal growth. This circumstance has been employed in the classification of the foliose Jιιngermanniese (with the exception of Haplomitrium) as acrogynous in contradistinction to the anacrogynous, in which latter the archegonia never proceed from the apex of the shoot itself.
Whilst the archegonia of the thallose forms are protected by being lodged in an excavation of the thallus, or surrounded by a luxuriant growth of its tissue, the archegonia of the foliose Jungermannieae are enveloped by the leaves of the stem-bud, which on that account are termed perichaetial leaves (folia perichaetialia'), or, collectively, perichaetium. Between these perichaetial leaves and the archegonia there grows up. in most forms a second involucre, of a goblet or pitcher shape, the “perianthium” (fig. 4, B). It springs up in the form of a circular fence, gradually growing upwards. In many Jungermannieae (Calypogeia) the archegonia are enclosed in a kind of sack-shaped structure, which forces its way into the ground, in which, in fact, it takes root. This “fruit-sac” in the Jungermannieae geocalyceae is the hollowed-out shoot from which the archegonia arise. Its extremity becomes thickened and forms a circular fence about the archegonia; thus there arises a structure about two twenty-fifths of an inch long, which is constantly becoming deeper, and at the bottom of which the archegonia are placed. There is always an endeavour to protect and envelop the archegonia. This is notably conspicuous in the Marchantieae, where round every archegonium there grows after fertilization a special goblet-shaped involucre, whilst every group of archegonia is also enveloped by two scales, which incline towards each other like the valves of a mussel-shell.
(3) Development of the Asexual Generation of the Sporogonium.— Just as in the vegetative classification we are led in a continuous series from the simple thallose forms to the foliose forms, so in the perfecting of the fertilized oosphere into an embryo (here the sporogonium) there is a gradual advance from simple to more complex forms. The first result of fertilization always is that the oosphere becomes surrounded by a membrane, grows, and transforms itself by division into a cellular body. In the simplest case, in Riccia (fig. 7, A), the only differentiation which takes place in this cellular body is that it forms a walllayer which is soon reabsorbed, while the whole of the interior cells form spores in such a way that by division four spores proceed from every mother-cell. In Marchantia (fig.
7, B) we find a separation taking place in the embryo, in which its inferior portion (that beneath the line 1, 1) becomes the short stalk of the sporogonium and its superior portion becomes the capsule. In the Jungermanniese (fig. 7, C, E, F) we find in the young embryo a group of cells from which proceed the mother-cells of the spores (shown by hatched lines in the figures); they are covered by a layer of cells which at a later period becomes the wall of the spore-capsule. Lower down (distinguished by a bracket in fig. 7, C ’) comes that part of the embryo out of which the stalk of the sporogonium is formed, while the cell a represents an appendage of the embryo. The inferior portion of the stalk often swells into a thickened foot, which frequently penetrates deeply into the tissue of the fertile shoot (fig. 8). The upper part of the sporogonium, in which the spores and elaters are formed, swells into a sphere.
In the Jungermanniese, when the sporogonium attains maturity, the stalk becomes very considerably lengthened, the ventral part of the archegonium, which had hitherto kept pace with it in growth, flies asunder, and the capsule emerges. This opens in various ways. In the Jungermanniese the wall breaks up into four valves (fig.
4). The sporogonia of Anthoceros behave in a very peculiar manner. The period occupied by their development is much longer than that of the Jungermanniese. While they are discharging mature spores at their apex they are still growing at their base, and forming new spores. The sporogonium dehisces by two valves, and is traversed in the middle line by a longitudinal string of cells not employed in the formation of spores. This persists till the maturation of the sporogonium as a little column, and is termed the ’ ‘ columella” (see fig. 7, D}. The cells from which the mothercells of the spores proceed have here the form of a dome and enclose an inner portion which becomes the columella, while the inferior portion of the embryo becomes its foot. In Dendroceros, a foreign form of Anthoceros, it drives tubules resembling roots into the tissue of the plant on which the sporogonium is seated. There is no want of transitional steps between this method of formation of the sporogonia and that of the Jungermanniese. The species Notothylas is especially noteworthy in this respect.
The spores, which proceed from one mother-cell by division into four parts, are enveloped by a membrane consisting of two layers— an outer one, which has the character of a cuticle in its great resistance to external agencies, and an inner one, which is composed of cellulose. The plants do not proceed directly from the germination of the spores; there is a pro - embryo of simple structure, which, however, in most cases passes over at its end into the plant itself. In the foliose Jungermanniese the first step in germination is the springing up of a row of cells in the cell at the end of which the growth of the shoot next takes place. The young germ-plant next develops leaves of very simple structure, and at first there generally appear on it only two rows of leaves at the sides. In Radula, however, and Frullania, both of them leafy forms, a cakelike cell-surface is first developed in germination; and it is from one of the cells of the margin of this that the growth of the leafy plant proceeds.
Classification of Liverworts
A. Marchantiaceae.
I. Ricciese.— The vegetative body is a dichotomously branched thallus, on the dorsal side of which are air-chambers, either covered by an epidermis like the Marchantiese, or opening outwards in their whole breadth. On the under side of the thallus is a row of scale-shaped lamellae, which become split up at a later period. The sexual organs are scattered on the dorsal side of the ordinary shoots. In the spore-cavity are neither elaters nor sterile cells
II. Corsinieae. — The organs of sex are united in groups, which stand in cavities on the dorsal side of the thallus. In the sporespace are sterile cells, which in Boschia are transformed into elaters. The dorsal side of the thallus possesses stomata.
III. Marchantiese.— The thallus is ribbon - shaped, and has a layer of air-chambers on the dorsal side opening outwards through a stoma. On the ventral side are two rows of scales. In the sporespace are elaters. The formation of the “inflorescences” in the lowest forms agrees with those of Corsinia ; in the highest forms they appear as branch-systems.
B. Jungermanniacese.
I. Jungermanniese.— The vegetative body is a thallus without air-chambers, or is a leafy stem. Between these are transitional forms. The sporogonium is differentiated into capsule, stalk, and foot; the capsule dehisces by four valves.
(a) Anacrogynous. —The apex is not employed in the formation of the archegonium. To this division belong all the thallose forms (including Blasia and Fossombronia), as well as Haplomitrium Hookeri. In B,iella and Sphaerocarpus the capsule is without elaters, but possesses sterile cells morphologically corresponding to them. All the others have elaters.
(δ) Acrogynous.— The apex or the apical cell itself is employed in the formation of the archegonium. All the forms are foliose, except Haplomitrûim, which also, in the development of its leaves, does not correspond with the foliose forms, but with Fossombronia.
II. Anthoceroteae. — Thallose forms. The archegonia, immediately on their first formation, are sunk into the tissue of the thallus. The antheridia stand in closed cavities. The formation of the sporogonium shows no distinction between stalk and capsule, but is pod-shaped and fixed by its swollen foot into the thallus. At maturity the sporogonium opens with two valves, between which the columella becomes visible. At its base it possesses a long and permanent growth.
II. Mosses (Ji≡i or Musci Frondosi).
The Mosses proper play a much larger part in the economy of nature than the Liverworts; they occur in much greater quantity and are more widely distributed, the conditions of their production not being confined within such narrow limits. While the Liverworts, for the most part, thrive only in localities that are not too dry, though some forms that grow upon bark, such as Radula and Frullania, can even endure drought, Mosses, on the other hand, have an almost ubiquitous character. Many grow in water (Conomitrium, Fontinalis, <fcc.), in swamps (various kinds of Hypnum, and others), on dry rocks (Grimmia, Andreaea), on roofs, in fields, and on trees,—in short, under favourable circumstances, a growth of Mosses develops itself almost everywhere. This is connected with the fact that very many have the capacity of enduring drought, and of suspending their growth while it continues and resuming it again in moist weather. Besides, a few, such as Hypnum cupressiforme, Ceratodon, Barbula ruralis, can grow upon any kind of substratum. Others grow by preference on organic substrata: as Buxbaumia indusiata on decayed trunks, Splachnum on old cowdung or on a damp humussoil, Orthotrichum and the species of Neclcera on the firm bark of trees, Phascum in manured fields, a great number like Grimmia and Andreaea on rocks. As regards the chemical quality of the substratum, we can distinguish between Mosses that live on chalk (Seligeria, the species of Gymnostomum, and some Hypna) and those that avoid chalk (Andreaea, Dicranum). The species of Polytrichum and Thuidium abietinum are fond of sandy soils, Ephemerum, Fissidens taxifolius, and others of loamy soils, while Archidium phascoides grows on muddy ground. This wide extension of the Leaf-mosses is rendered possible by their uncommonly great capacity for reproduction. Not only is the formation of spores in most species very extensively carried on, but we have to add to this the manifold production of asexual means of multiplication, such as gemmae, &c. The sexual as well as the asexual generation of Mosses attains to a higher grade of perfection than that of Liverworts.
(1) The Sexual Generation.— The extent of the sexual generation of the Moss-plants proper varies within wide limits. While the stem of Ephemerum and some species of Phascum appears in the form of a small bud, often scarcely visible to the naked eye, there are species of more than 4 inches in length, as Sphagnum, Fontinalis, and the tropical species Spiridens. In every case, however, we have here a really leafy stem; no thallose forms exist.
The leaves are simple and always small. They either consist of one layer in their whole extent, or are traversed by a mid-rib of several layers, which is often of rather complex structure, and consists of thick-walled and thinwalled tissue-elements, of which the former give mechanical firmness to the leaf, while it is principally in the latter that the conveyance to the stem of the matter formed in the leaf takes place. In the species of Polytrichum the middle nerve occupies the greatest part of the leaf, and is covered with lamellae, which consist of cells bearing chlorophyll. Instead of these lamellae, we find in some other Mosses simple rows of cells, as in Pharomitrium subsessile. A peculiar structure of the leaf is also found in Leucobryum glaucum and Sphagnum. The first-named Moss forms thick greenish-white turfs in damp spots. A transverse section through a leaf shows it to be composed of two (or three) layers of cells. Above and below is a layer of large cells devoid of contents, the membrane of which is perforated in various places, so that the cavities of the individual cells communicate with each other; thus arises a system of capillary tubes which suck up water and retain it like a sponge. Between the angles of every two of these colourless cells we find inserted a cell containing chlorophyll. The colourless cells are filled with air instead of water, and thus the whole plant has a whitish appearance (Leucobryum), while the green hue comes out at the approach of moisture. The leaves of Sphagnum possess a similar structure. In these too are perforated cells devoid of contents, and surrounded by the meshes of cells containing chlorophyll. The inner wall of the empty cells devoid of chlorophyll is for the most part set with peculiar thickenings of annular or spiral form (fig. 9,∕), which give it the necessary firmness, as in the vessels of higher plants. In this case too the empty perforated cells are intended for the purpose of absorbing water; and the same end is attained by cells of similar structure, which form the rind of the stem.
The stem of Mosses is distinguished by its slight but uniform thickness, which for the most part does not exceed that of a thick thread. Compared with this delicacy, the compact, firm, and tough quality of the corresponding forms in higher plants is so much the more striking. It depends on the fact that the exterior layers of cells in the stem have thickened cell-walls, generally of a brown colour, while the central parts have thin walls. The differentiation of the tissues reaches its highest point in Polytri∙ chum, Hookeria, Splachnum, and others. Strings of separated cells pass from the leaves into the stem, and apply themselves to its central tissue, which is of different conformation from the rest of the tissue of the stem,—a circumstance which requires mention because this is the first indication of the fact of such extensive occurrence among “vascular” plants that the vascular bundles which run off from the leaf pass into the stem and there apply themselves to others. The stems are thickly set with leaves. It is but seldom that these stand in two rows on the sides, as in Fissidens, Conomitrium, and the sterile shoots of Schistostega. The last-named plant is of special interest as possessing two kinds of shoots of totally different appearance. The fertile ones have their leaves placed on all sides of the stem in a spiral manner; the sterile ones, on the other hand, have leaves placed on two sides, and have the appearance of a fern-leaf. This position, however, only comes to pass through a shifting and twisting of the stem. Originally, in the bud, the leaves of the sterile stems stand in a spiral line. On examination of the end of the stem it is found that the growing point from which the leaves arise contains, as in the foliose Liverworts, an apical cell of the form of a three-sided pyramid with a vaulted base. By means of partition walls which are successively parallel to one of the side walls of the apical cell “segments” are cut off; and from each of these proceeds a leaf, as well as a part of the rind of the stem and of the inner tissue. On the direction of the partition walls in the apical cell depends likewise the arrangement of the leaves. The branches do not spring as in most phanerogamous plants from the axil of a leaf, but from the upper surface of the stem below a leaf, and only out of one of its cells, which becomes the apical cell of the branch.
In the relation of the ramification to the general structure of the moss-plant two leading classes have to be distinguished—the acrocarpous a,nd pleurocarpous Mosses. In the former the growth of the stalk concludes with the formation of a sporogonium; for the archegonium, out of which the former springs, proceeds from the apical cell itself. In the pleurocarpous Mosses the sporogonium stands on the tip of a side-branch, and the growth of the principal stem is thus not interfered λvith by fructification. In many acrocarpous Mosses the stalk dies after fructification, and the Moss is then an annual, as in many Phasca- ceae. In those acrocarpous Mosses which are perennial the further development is taken up by a side-branch, and a so - called “innovation” is formed. By the dying away of the principal stem, these innovation-shoots become at a later period independent plants. The roots consist of simple rows of cells, springing from the surface of the stem, especially at its base (see p. 72).
(2) The sexual organs of the Leaf-mosses, the antheridia and the archegonia, in their mature condition correspond in the main with those of the Liverworts, from which, however, they differ somewhat in their development. They occur generally in groups at the extremity of a shoot. Such groups containing, as the case may be, either solely antheridia, or solely archegonia, or a mixture of the two, are termed “flowers.” An exception to this condition of the antheridia is found in Sphagnum (fig. 11, A). Here the antheridia do not stand in groups on the summit of the male branch, but are arranged along it, so as to stand beneath the leaves on the prolonged axis of the shoot. In the rest of the Mosses, with the exception of Polytrichum, the first antheridium (or archegonium) of a group proceeds from the apical cell itself; and thus the growth of the shoot connected with it is closed. The general character of the male “flowers” is very various, in the form of buds, heads, or disks. They are enveloped by a number of leaves, the “perigonium” (fig. 11). The male plants of dioecious Mosses are often considerably smaller than the female ones. This is the case with Funaria hygrometrica, and to a remarkable extent with I)icranum undulatum and Leucohryum glaucum. Dioecious Mosses are frequently sterile on account of the absence of one of the sexes. The female “flowers” (fig. 12) are in the shape of buds, enveloped by a number of leaves, which become smaller towards the inside. The archegonia have in general the same structure as those of Liverworts, but are distinguished by a very much developed stalk, diminishing towards the base in the form of a wedge (fig. 12). They originate, as do the antheridia, from a single cell.
(3) The development of the fertilized oosphere into the sporogonium differs considerably from that of the Liverworts, both as regards the external processes of growth and as regards the differentiation in the interior of the embryo. In the first of these points the greatest resemblance to the Liverworts is exhibited by Sphagnum. Here the sporogonium continues almost till maturity to be enclosed by the ventral portion of the archegonium—the calyptra,—which keeps pace with it in growth, and which is irregularly ruptured by it at the period of maturity. Archidiuτn, one of the Phascaceae, behaves similarly. In all the other Mosses, however, the sporogonium at an early period bursts through the ventral portion of the archegonium, tearing off the calyptra at its base and raising it up as a cap. The sporogonium possesses a stalk which grows up gradually, but which in Sphagnum, Andreaea, and Archidium is very short. In the two first-named genera the shortness of the stalk is made up for by a stalklike elongation of that portion of the stem on which the archegonium is placed. Thus arises a false stalk, a “pseudopodium,” which is to be distinguished from the real stalk, which belongs to the sporogonium. The capsule is often of complex structure. In the Bryineae it possesses a distinct epidermis, often provided with stomata, which is altogether wanting in the vegetative parts of Mosses. A few layers of cells are united to the epidermis, and together with it form the wall. The interior tissue is never entirely employed in the formation of spores, but a part of it remains as a sterile cell-body—the columella (fig. 13, co) —with the exception of Archidium, 1 which in this respect approaches the Liverworts. The embryo, the young sporogonium, is originally fusiform (fig. 14, B, C), and is differentiated into capsule and stalk (seta). The latter commonly penetrates into the tissue of the fruit-shoot, which forms, outside the base of the sporogonium, an exuberant growth like a fence—the “vaginula”—on which we not unfrequently find archegonia that have miscarried (fig. 13, v).
In Mosses, as in Liverworts, the spores proceed from the division of one mother-cell into four parts. The phenomena of their germination are very peculiar. In the great majority of the Mosses there arise from the germinating spore cellular filaments resembling Confervae. They have quite the appearance of Algae, and formerly were even confounded with them. They bear the name of protonema, and often form a thick green covering on the ground in forests. Such a protonema consists of two parts: one above and the other below the ground. Both are rows of cells, but those above the ground contain chlorophyll and have transverse walls placed rectangularly, while those below the ground contain no chlorophyll and their transverse walls are placed obliquely. They serve as roots for the protonema, and of similar structure are the roots (rhizoids) which spring from the surface of the leafy stem, especially from its base. On this protonema, and especially from the basal cells of the threads which are above ground, the moss-stem is formed. This originates from an outward pouching of one of these cells, and this becomes the apical cell of a mossstem. Several mossplants may be formed on one and the same protonema, which has itself proceeded from one spore. The spores of the Bogmoss (Sphagnum) form quite another kind of pro-embryo when they germinate on damp earth. They then transform themselves into an intricately ramified expansion or cell-surface; and any cell of this surface may then become the apical cell of the stem.
When they germinate in water the spores of Sphagnum form a thread-like j pro-embryo. In some other Mosses too, especially Λn- dresea, Tetraphis, and Diphyscium, remarkable forms of pro-embryo occur, which, however, can only be alluded to here. In no case does a moss-spore grow directly into a moss-stem.
Mosses propagate themselves very extensively, not only by means of spores, but in a vegetative way. Every one of the hair-shaped roots (rhizoids) which spring out of the stem has the power, when exposed to the light, of forming protonema, and moss-buds upon this. On this depends the fact that the Mosses which grow on tiled roofs (Grimmia and others) are hardly to be extirpated. Even if the turf is removed, the roots that have made their way into the pores of the tiles still survive, and develop protonema, and new moss-plants upon this. In Phascum and Ephtmerum, which are apparently annual Mosses, the protonema is perennial, and forms new plants in the following year. Gemmae also arise in abundance on the protonema (e.<j., of the species of Barbula) — cellular bodies which are surrounded by a dark-coloured membrane, and whose cells are densely filled with a store of material. They are able to endure drought, and on germinating they either form moss-buds directly or protonema in the first place. Protonema may also proceed directly from the cells of the leaves and stems of moss-plants, and thus subserve their multiplication. Portions even of the sporogonium may transform their cells back into protonema. If we cut off young sporogonia and place them in damp sand, there will arise from their interior cells (or the wall of the capsule) threads of protonema, on which new plants spring up. Many Mosses also possess special gemmae. In Tetraphis pellucida they are stalked cellular bodies, enveloped by a leafy calyx, from which at a later period they fall away. In Λulacomnium androgynum, they spring from the extremity of a leafless prolongation of the stalk, in Grimmia Hartmanni and Barbula papillosa from the leaves, &c.
Classification of Mosses.
1. Sphagnaceae. — The Turf-mosses are characterized as well by their anatomical structure as by the development of their sporogonia. The stem, which at first is unbranched, possesses roots only in its earliest stage. The stems then stand in a thickly-set turf, and receive water through the whole of their surface. The ramification is very abundant. Two kinds of branches are formed in the terminal buds—long whip-shaped “flagellae” which hang down, and whose duration is annual; and besides these there is formed every year, after the fruit has reached maturity, a side shoot called an “innovation,” which acts in the same way as the main stem, and at a later period becomes an independent plant. The sexual organs, antheridia and archegonia, are developed on the side branches. The peculiar structure of the leaves has been already mentioned. That of the stem is analogous to it. The latter possesses a rind, formed out of cells with thin walls, whose protoplasmic contents have completely disappeared. Like the empty cells of the leaf, they are penetrated by openings, and thus form a system of narrow capillary tubes which stand in connexion with each other and open outwards, and through which the water mounts upwards.
The branches that bear the male “flowers” make an approach to those of the Liverworts in the fact that the antheridia do not stand on the summit of the shoot, but singly close to its leaves. On the other hand, the archegonia agree in their position and development with those of the other Mosses. The embryo—the young sporogonium—is at first a pear-shaped body of cellular tissue, the basal portion of which penetrates deeply into the soft tissue of the fruit-branches on which the archegonium is seated. The sporogonium differs considerably from that of the other Mosses. Those cells which give origin to the mother-cells of the spores—termed collectively “archesporium ”—form a dome-shaped layer in the upper portion of the embryo; thus they are not traversed by the sterile cellular tissue, the columella. Each mother-cell is divided as usual into four spores. The short stalk of the capsule extends itself at maturity only so far as to cause the sporogonium to break through the calyptra, the place of a stalk being practically supplied by the “pseudopodium.” The pro-embryo is a cell-surface.
2. The Andreaeaceae are small blackish Moises growing on rocks. Their antheridia resemble those of the other Mosses. The development of the embryo is anomalous. The archesporium is, as in Sphagnum, a dome-shaped layer of cells. On the extremity of the mature capsule is placed the calyptra, which has been torn away, as a delicate cap. The mode of dehiscence of the capsule here reminds us of the Liverworts. This takes place by four longitudinal clefts, which allow the exit of the spores, and are open in dry and closed in damp weather. In this group also the stalk of the sporogonium remains short, its place being supplied by a “pseudopodium.”
3. The Phascaccae, which are small and generally annual Mosses, are distinguished from the following division by the fact that the capsule does not open by separation of a lid, but remains closed, and the spores are not set free till the wall of the capsule decays. A remarkably anomalous form is Archidium ; the development of its embryo reminds us of that of the Liverworts; the mature sporogonium also possesses no calyptra, but breaks through the ventral portion of the archegonium like that of Sphagnum and that of the Liverworts. The Phascaceae are termed ‘ ‘ cleistocarpous” in contradistinction to the “stegocarpous” Mosses.
4. The Bryineae comprehend by far the greatest number of all the species of Mosses. They are characterized above all by the structure of their sporogonia. Here the parts of the latter are always the longer or shorter bristle - shaped stalk (seta), the capsule, and the calyptra carried up on its summit. That part of the stalk where it passes into the capsule is termed apophysis ; on its epidermis, as well as on that of the capsule, we almost always find stomata. As regards the structure of the capsule, at a very early period a space filled with air makes its appearance, which divides the interior mass from the wall, which is composed of several layers. This interior complex mass of tissue consists of those cells which give rise to the mother-cells of the spores (the archesporium), of a few layers of cells which smτound these only externally, and of an inner mass of tissue, the columella. The latter, however, passes in this case through the cells of the archesporium; these latter form, as in the Phascaceae, a stratum of cells which has the form of a barrel, open above and below (fig. 13), and encloses the columella. The Bryineae are also characterized by the way in which the capsule opens. The upper part of the wall of the capsule is in this case always thrown off as a lid (operculum). This happens in one of two ways: either a layer (or several layers of cells lying upon one another), placed between the inferior portion of the capsule and the operculum, forms a ring (annulus), the cell-walls of which become thickened and partially swollen, so that the annulus becomes loosened and thus causes separation of the operculum and the capsule from each other, or there remains simply a thin-walled annular zone of cells belonging to the wall of the capsule, which splits asunder in dry weather. When the operculum falls away the margin of the open capsule appears in most cases set round with a single or double ring of tooth-like formations, which are termed peristome. This is wanting in Gymnostomum and Hymenostomum. It occurs in its simplest form in Tetraphis. Here the epidermis of the upper conical portion falls off as an operculum, while the whole tissue that lies beneath splits crosswise into four valves, which form the peristome. In the other Mosses, with the exception of Polytrichum, the teeth of the peristome have an essentially different origin. They are in fact nothing but the thickened portions of cell - membranes, whose unthickened portions have perished and been torn asunder. Fig. 16 shows a portion of a longitudinal section through the upper part of the capsule of Funaria hygrometrica.
The capsule possesses a brown epidermis e, whose walls are greatly thickened; se is the tissue that lies between the epidermis and the airspaces h, I of the capsule; sthemother-cells of the spores, and farther inwards follows the columella. Immediately above the air-space rises the stratum of cells which forms the peristome, of one tooth of which a longitudinal section is given in fig. 16. It arises from the great thickening of the outer side of the walls of a row of superposed cells. At maturity the upper portion of the wall of the capsule falls off as an operculum, the cells marked p disappear, and only the thickened parts of the cellmembranes remain as the teeth of the peristome. In Funaria and in other cases the peristome is double, for those cell-walls (fig. 16, i) which look inwards of the stratum of cells which forms the peristome have also in these cases become somewhat thickened, and continue as “cilia,” while the thin portions of membrane placed at right angles to them perish. In other cases (Fontinalis anti-pyretica) the peristome forms a lattice-work. The number of teeth in the peristome is always four, or a multiple of four. They are very hygroscopic; their function is to close up the opening of the capsule in wet weather, and so to hinder the exit of the spores, which are scattered abroad by the wind. In the same way also any germination of the spores in the interior of the capsule is prevented. The columella shrivels up after the formation of the spores, so that at that time we find nothing in the capsule but the spore-dust. The Polytrichaceae have special arrangements for the protection of the spores. Here the opening of the capsule is covered over by a kind of plate ( “epiphragma ”), which is supported by thirty-two or sixty-four teeth, consisting of fibre-like cells.
Retrospective View. — Comparing generally the Mosses with the Liverworts, we see that, however great the variety of the forms they comprehend, they still form two connected series. In several of the subdivisions of the Leaf-mosses conditions occur which remind us of the Liverworts. Such are, in Sphagnum, the form and position of the antheridia, the tearing open of the ventral portion of the archegonium, and the want of a “calyptra” on the sporogonium which proceeds from it. This last is also found in Andresea, in which the method of opening of the sporogonium likewise reminds us of the Liverworts. Among the Phascacese, the Archidium agrees with Liverworts in the absence of a columella. Some few cells in the interior of the sporogonium are formed into mothercells of spores, and push aside the rest of the tissue. Here, too, the calyptra is absent.
The simplest Liverworts make a near approach to the Algae. The sporogonium of Riccia is of scarcely higher organization than the mass of tissue which proceeds from the fertilized ovum-cell of Coleochaete, a green freshwater Alga. Among plants of a higher order we find no group closely allied to the Muscineae. It is true that the Ferns have a perfectly analogous alternation of generations, but it is not a sporogonium which is produced from the fertilized oosphere, but the leafy fern-plant. No transitional forms occur between these two sections; the chasm which divides them is the widest with which we are acquainted in the whole vegetable kingdom. (k. e. g.)
