SECTION VII.MUSCI, OR MOSSES.

Mossesapproach the higher classes of vegetable life in having roots, and a more or less upright stem or axis of growth. Like all other plants, they are chiefly formed of cellular tissue, yet, in the stems, there is an indication of a separation between the bark and pith by the intervention of a circle of elongated cells approaching to woody fibre, which passing into the branches and leaves form a kind of midrib, either extending to the extremity of the leaf or not. The delicate little leaves, which are arranged with great regularity, consist of a single or double layer of cells united by their flattened sides, and rarely exhibiting any epiderm or skin. Green is the prevailing colour in this order of beautiful little plants, but, when exposed to the sun and much moisture, they sometimes become red.

The urn-like vessels (fig. 47B), containing the spores, are either terminal or lateral on the stem of the moss, and rise on a slender stalk from a rosette of narrow leaves possessing a skin perforated by stomata or breathing pores of simple structure. This is the ultimate result of fructification; for at a very early period, while the rosettes of narrow leaves are merely buds, they contain antheridia and archegonia, the latter or female organs being either inclosed in the same bud, or in different buds placed on the same or on different plants; but however that may be, the buds are invariably placed at the base of the leaves, close to the axis of the plant.

When a male bud is developed, the antheridia are found to be globular, ovoid, or elongated hyaline bodies, set in a cluster of hairs and paraphyses, or sterile cellular filaments.Fig. 48represents the antheridia of Polytrichum commune, with the paraphyses and hairs, from the microscopic observations of Dr. Carpenter. The central antheridium is discharging its spermatozoids; the one on the right is empty, that on the left immature.

Fig. 47.A, plant of Funaria hygrometrica:—f, leaves;u, urns or capsules;S, seta or foot-stalk;o, operculum;c, calyptra.B, sporangia of Encalypta vulgaris:—u, urns;o, opercula;c, calyptra;p, peristome;s, seta.

The antheridia are filled with a mass of mucilage containing a multitude of cells, in each of which there is a spiral filament furnished with cilia, as infig. 49. As soon as the filaments are mature, the cells open; the mobile filaments, or spermatozoids, are free, and come out through a pore in the antheridia, in multitudes like pollen out of an anther.

While the antheridia are in process of formation, the female buds expand and exhibit flask-shaped archegonia, similar to that infig. 45, seated in a rosette of leaves. When this archegonium is fertilized by the spermatozoids, its internal germ cell is developed by cell division into a conical body elevated upon a stalk; and this at length tears across the walls of the flask-shaped archegonium by a circular fissure, carrying the higher part upwards as a calyptra or hood (fig. 47c) upon itssummit, and leaving the lower part to form a kind of collar round the base of the stalk.

Fig. 48. Polytrichum commune:—group of antheridia mixed with hairs and sterile filaments (paraphyses), the central one discharging its contents.

Fig. 49. Polytrichum commune:—B, cellular contents of an antheridium previously to the development of the spermatozoids;C, the same, showing the first appearance of the spermatozoids;D, the same, mature and discharging the spermatozoids.

The urn-shaped organ or sporangium has a double wall, and in its centre a fusiform or deeply winged columella. Between these two there is a tissue of a most delicate texture, divided into spherical cells, each of which usuallyproduces four unripe spores, which get an exterior coat when fertilised, and then it is that the stalk lengthens, and the flask-shaped archegonium is torn across.

The urns are closed by a lid or operculum of a flat, convex, or pointed form, which falls off when the spores are ripe to give them egress. The mouth of the urn in most of the mosses is surrounded by a deciduous annulus of two or three rows of elastic cells, which are supposed to aid in scattering the spores, for the mosses have no elaters. Although the separation of the lid may at once expose the spores, they may be covered with a membrane entire or toothed at the circumference, or there may be one or two rows of teeth surrounding the mouth of the urn like a fringe; these teeth form the peristome, and their number is always a multiple of 4, varying from 4 to 64 (seefig. 47p). Sometimes they are divided half way down, sometimes they are prolonged into straight twisted hairs. The teeth arise from the thickening of the walls of two contiguous cells; when there are two rows of teeth, the outer row frequently arises from the layer of cells which line the outer wall of the urn, the inner row from the outermost layer of the spore sac; sometimes, when the peristome is double, three strata of cells are requisite to form the teeth. The urns of the Encalypta vulgaris are represented atfig. 47B; one is covered with the hood or calyptra, while from the other the lid has fallen off, showing the mouth of the urn with its toothed peristome.

When a spore begins to grow, its outer coat is ruptured, its innermost coats protrude and form a projecting extremity, which becomes developed into a confervoid pro-embryo, analogous to the mycelium of fungi, but is distinguished by the chlorophyll contained in its cells. Many spores may concur or not in its formation; butthe pro-embryo of each spore is capable of transforming one or more of the cells seated upon its various ramifications immediately into buds, which grow up into leafy stems, as the Funaria hygrometrica, so that here we have the peculiar condition of one spore giving rise to the development of a number of plants.[68]In process of growth each of these plants would produce antheridia and archegonia, and, by the process described, a full-grown Funaria hygrometrica with its urns and hoods, as represented infig. 47A, would be the result.

If the moss be annual or biennial it dies after bearing fruit; if perennial, two or more successive crops of archegonia are formed. The mode of fructification, therefore, resembles that of flowering plants, with this difference, that the fructification of the latter produces a young plant from each embryonic cell, while in mosses the fructification of one embryonic cell produces a sporangium or urn containing spores, that is, a multitude of reproductive bodies, which have no trace of cotyledons or axis.[69]

Mosses are also propagated by gemmæ or buds. They are produced in many situations, sometimes on distinct organs, sometimes on the tips of the leaves, or on rootlets which grow on various parts of the plants, and which in some of the mosses form a dense woolly or silky mass of a bright yellow or brown colour varying to purple. On the fibres of this mass, green cells appear, which are developed into reproductive buds. Almost every cell on the surface of a moss is capable of forming, by continued division, a cellular nodule, which falls off and gives rise to a germ which grows into a new plant. These nodules are generally situated at the extremity of the leaves, or on the leaves themselves, while pro-embryo fibres spring from the leaf cells of many mosses. M.von Mohl observes that since the cells in the different parts of mosses are capable of being developed into a bud or embryonic confervoid structure producing a bud, it follows that in these plants, notwithstanding their rather complex structure, the subordination of the individual cell to the purposes of the whole plant is still but small; and even here individual life readily acquires the preponderance. This facility of reproduction possessed by the various individual parts of the plants, accounts for the extensive tracts over which mosses spread themselves; moreover, some are diœcious, and as the spores might not always be fertilized, the gemmæ ensure the continued existence of the species.

Mosses are divided into five principal groups, differing exceedingly in importance, and in comparative numbers. They are chiefly distinguished by the position of their fruit. The Pleurocarpi have their fruit lateral, whether on the stem or branches. They comprise thirteen tribes; many of them are found in the southern hemisphere, but a considerable number, especially of the Hypnei, Drepanophyllei, and Hookeriei, are European. The Cladocarpi are characterised by having their urns seated on the tops of very short lateral branches, and by their double axis of growth. The Acrocarpi are distinguished by their main stems ending in fruit. They comprise twenty-seven tribes, and embrace genera and species having a wide geographical range. In the Syncladei the branches of the plant are fasciculate; this group comprises the Sphagnums. The Schistocarpi are remarkable for their fruit splitting into valves, and consist of the tribe Andræaceæ. The various tribes depend on the structure of the urns and leaves, as well as on the natural habits of the plants. In this numerous class of plants only a few remarkable for peculiarity of structure can be mentioned.

The species of the acrocarpous tribe Phascei are exceedinglynumerous, and contain the simplest of all mosses; they grow on newly turned-up soil, and are chiefly annual. Their leaves generally have nerves, and are bordered by large cells. The urns, which are either sessile, or upon a short stalk, have not a trace of peristome, and sometimes have a columella, sometimes not. The spores are large compared with those of other mosses.

The tribe Dicranei contains numerous species, some of which are the commonest of mosses in Europe. They are easily known by their single peristome of sixteen teeth divided half-way down. The leaves are extremely crisp and convolute, and the hood is spoon-shaped. The Leucobryum is remarkable for pallid leaves; it has three layers of cells, a narrow layer of green cells embedded in the centre of the leaf with a broader layer of colourless cells on each side, whose cell walls are perforated with large round openings, as infig. 50b. The mosses of this order live on sandstone rocks, shady banks, and trunks of trees.Fig. 50shows the microscopic structure of the leaves of various mosses.

In the tribe Grimmiei we have frequently a sessile urn, with a single peristome, and a mitre-shaped hood. The leaves, which are dark green, have minute hexagonal perforated cells on their upper surface, and a white nerve projecting from their extremity.

In the tribe Polytrichei the mouth of the urn is mostly closed by a flat membrane and a hood rough with silky hairs. The leaves are sheathing at their base and spreading at their tips; except in a few cases they are rigid; and the nerve often exhibits lamelliform folds. The Polytrichum dendroides contains scalariform ducts, and starch granules.

The Bryei are of variable size, but a number of European species are among the finest of mosses, onaccount of their large leaves and beautiful double-toothed peristome, the great distinction of the group. The leaves are margined, toothed, and composed of a loose reticulation of large rhomboidal cells; in the genus Timmia they clasp the stem at the base and spread widely at the tip. There are thirty-three British species of the genus Bryum, many of which with their abundant urns are extremely ornamental.

Fig. 50. Microscopic structure of leaves of mosses:—a, Octoblepharum albidum;b, Leucobryum glaucum;c, Sphagnum latifolium;d, Hypopterygium Smithianum;e, Eucamptodon perichætialis;f, Andræa subulata;g, Campylopus lamellinervis.

The tribe Splachnei contain many of the most singular and beautiful of the whole class of mosses. They have large-celled diaphanous leaves and a straight urn, with the spores radiating from the columella. The urn has a swelling at its base, often of greater dimensions than the urn itself. In the Splachnum vasculosum it is purple and very large, but nothing in comparison of that organ in S. luteum and rubrum, which are the pride of hyperborean Europe and America. The enormous size of the swelling, the variety of colouring, the singularity and elegance of form, and in some cases the unusual dimensions, make the species objectsof great interest. The common S. ampullaceum, when growing in abundance on the shallow peaty banks of some mountain stream where cattle come to drink, is scarcely exceeded in beauty by any cryptogam. Species of this order are abundant in the two hemispheres, but the same species rarely occur in both. Their habits, too, are different, for while those in the north only grow on manure, those in the south grow on the trunks of fallen trees. Three genera occur in Great Britain. Gemmæ are found in the axils of the leaves in most species of this group.

The tribe of Schistostegei consists of but a single most elegant species. It inhabits shady caverns, which are sometimes lighted by a golden gleam from the refractions of the confervoid shoots of its mycelium-like pro-embryo, which is perennial, and produces a new crop year after year. The urn is subglobose without a peristome, and when young the spore cells radiate from the columella as in the Splachna. The leaves show various intermediate stages between a vertical and horizontal insertion, and are sometimes perfectly free, while at other times they are united. This moss is scarce, and confined to the northern hemisphere.

There are various genera of aquatic mosses in the different tribes, most of them floating plants. Of these are the pleurocarpous Fontinalei, which inhabit the northern hemisphere; their urn, with its double peristome of sixteen teeth, forms a beautiful microscopic object, on account of the latticed work in the inner row, and the cross bars on the outer teeth, which are united at their tips by two and two. The common species have sharp-angled triangular stems, and keeled leaves which clasp the stem at their base, and are sometimes cleft along the keel.

The syncladeous Sphagnei are aquatic bog mosses of a pale yellowish green colour. They form but onegenus, Sphagnum, which consists of some eight or ten species and several varieties. The species of Sphagnum, or common bog moss, hold an important place in the economy of nature. They are floating mosses, entirely destitute of roots. The stalk of the full grown plant, like that of land mosses, is constructed of three kinds of cells; one forms the exterior or cortical layer, another forms the central pith or axial system, while the third, which is coloured and somewhat ligneous, comes between the other two. The leaves, which have their origin in the cortical layer while it is yet soft, consist of two kinds of cells, one kind being large, elongated, and colourless, and containing spiral fibres loosely coiled in their interior. The membranous walls of these cells have large round apertures by which their cavities have free communication; for certain animalcules, which sometimes live in the cells, have been seen to pass from one to the other. Between these colourless cells are some thick-walled, narrow, elongated green cells, which give the leaf firmness and colour. According to Mr. Wilson, the fascicles, or bundles of branches, are disposed round the stem in imbricated spirals, so that, for every complete spiral formed by five of these fascicles, there are eight spirals formed of twenty leaves, four leaves being inserted between each pair of fascicles. The fructification in all these floating mosses is immersed in the leaves of the stem, the antheridia being globose, and the spermatozoids having spiral motions, both within the cells of the antheridia and when they come out. The urns, which are the product of fructification, and are borne at the top of what appears to be a long footstalk, but is in reality a pedunculate vaginula, are globular, and their lids have been observed to be driven off when the spores are ripe, with such force as to give a distinctly audible report.

The common bog moss grows so rapidly that, rootless as it is, it soon covers a pool with its matted bundles of branches, and as in a few years it has no room to spread, the lower stems and branches decay, sink to the bottom, and begin to form a peat moss, while the upper parts grow on, so that new stems and branches are perpetually produced. Multitudes of spores no doubt germinate, and, in this way, the pool is filled up, and a peat moss is at length formed.

The Sphagnum moss has such a power of absorbing moisture from the atmosphere, that it forms and maintains the peat mosses and quagmires in the mountains which feed the streams at their feet. Mosses in general are almost as much indebted for moisture to the absorbing nature of their leaves as to their roots; but the loose, large celled, and perforated leaves of the Sphagnum suck up water like a sponge; even during the heat of summer, a quantity of water may be squeezed out of a handful of them. In fact the plant is a perfect hydraulic machine, for a small stem of it put into a glass of water with its drooping terminal branch hanging over the edge, acts like a syphon, and soon empties the glass, pouring the water out through its bending top. Though the peat from Sphagnum is often too spongy for fuel at present, yet that little moss now growing on our mountains will yield aniline, magenta, paraffin, and other illuminating gases to remote generations, although not in such quantities as the richer vegetation of the coal measures, the products of a warmer period.

Like all cryptogams the mosses are exceedingly variable and difficult to distinguish. Not only does the same species show great differences in size, but even in other respects the characters vary on account of climate, soil, and exposure.

Beyond that of becoming converted into peat, the uses of the moss family are not of any great importance. Brooms, mats, and other domestic articles are sometimes formed from Polytrichum, and in Lapland Sphagnum not only sometimes enters into the composition of bread, but is used in place of clothing for new-born babes.

Back to Index Next