Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/78

66 MUSCI [LIVERWORTS. (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, b), while the inferior cell becomes round and forms itself into the oosphere. When ready for fertilization the ovum -cell is a spherical por tion of protoplasm with a cell- nucleus (fig. 6, e). Fer tilization follows on the blending of one or perhaps several spermatozoids 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 para site on the plant that nourishes it. The ventral portion of the archegonium grows along with the embryo. In Riccia, the lowest form of one subdivision of the Liver worts, 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 sporo gonium 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), A} ; 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 con siderably 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 (Polytrichum, 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 Riccia. 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 fre quently 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 ana tomical 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 Muscinese 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 Muscineas the course of development consists in a regular alternation of these two stages in an alternation of generations. The one genera tion 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 Ferns. 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 sporo gonium of the Muscinese. The sexual generation of the Ferns is a small and insignificant expansion, the prothallium, which bears antheridia and archegonia, and thus corre sponds to the sexual generation of the Muscinese, which is here represented by the proper moss-plant. The vege tative 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 fila mentous confervaceous Alga, fig. 15); it is from this pro- embryo that the leafy stems arise as lateral buds. From a systematic point of view the Muscineae are divided into two sections Liverworts (Hepaticse) 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 (Hepaticse). 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 Marchantiacese, as, for example, Fegatella conica, which often forms a con tinuous covering on moist stones, walls, &c. (1) The Organs of Vegetation. With a few exceptions, the vege tative 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 dor sal side. The Liver worts thus belong, with the exception of two species, to the class of dorsi-vcntral plants, or those possessing two sides of different struc ture, a dorsal side and a ventral side, as in vertebrate animals. The vegetative body of the Liverworts is either a thallus or a FIG. 1- Metzgeria furcata (magnified about 10 dia- Ipnfv ulnnt ind we meters). The anterior portion of a thallus, seen leafy plant , and we Qn the right hand from above and on the leffc have consequently to hand from below, m, mid-rib ; s, s , s", the grow ing points ; /, /, the wing-like expanded portion of the thallus, composed of a single layer of cells ; / ,/",/" , its development by ramification. (After Sachs.) distinguish between thallose and foliose Li verworts. The latter have already been termed Hepaticae frondosse. There occur, however, transitional