DIVISION II.
MUSCINEÆ (MOSSES).
In this Division a well-marked alternation of generations is to be found. The development of the first or sexual generation (gametophyte),[16] which bears the sexual organs, antheridia and archegonia, commences with the germination of the spore, and consists, in the Liverworts, of a thallus, but in the true Mosses of a filamentous protonema, from which the Moss-plant arises as a lateral bud. The second or asexual generation (sporophyte), developed from the fertilised oosphere, consists of a sporangium and stalk.
The sexual generation, the gametophyte. The protonema in the Liverworts is very insignificant, and not always very sharply demarcated from the more highly developed parts of the nutritive system. In the true Mosses the protonema is well-developed, and consists of a branched, alga-like filament of cells, the dividing cell-walls being always placed obliquely. In the parts exposed to the light it is green, but colourless or brownish in those parts which are underground (Fig. 186). The protonema is considered to be a lower form of the stem, and grows in the same manner by means of an apical cell; at its apex it may directly develope into a leaf-bearing stem, or these arise from it as lateral branches (Fig. 186 k).
The more highly differentiated part of the vegetative system, the “Moss-plant,” which is thus developed from the protonema, is in the “thalloid” Liverworts generally a dichotomously-branched thallus without any trace of leaf-structures (Fig. 194); in Marchantia (Fig. 197) and others, scale-like leaves (amphigastria) are found on the under surface. The higher Liverworts and the Leafy-Mosses are differentiated into a filamentous, ramified stem with distinct leaves arranged in a definite manner, resembling the stem and leaves of the higher plants (Figs. 186, 195, 200).
True roots are wanting, but are biologically replaced by rhizoids. These are developed on the stems or thallus: in the Liverworts they are unicellular, but in the Leafy-Mosses generally multicellular and branched. In the latter group they are considered identical with the protonema, and may become true protonema, and new plants may be developed from them (Fig. 186 b).
Fig. 186.—A Lower portion of a Moss-plant with rhizoids (r), one of which bears a reproductive bud (b). The dotted line indicates the surface of the ground; the portions projecting above this become green protonema (p); k is a young Moss-plant formed on one of these. B Germinating spore of Funaria hygrometrica, with exospore still attached. C, D Older stages of the protonema.
The internal structure of the sexual generation is very simple. The leaves in nearly all cases are formed of a single-layered plate of cells; in the Leafy-Mosses, however, a midrib is very often formed, and sometimes, also, marginal veins; and along these lines the leaves are several layers of cells in thickness. The stem is constructed of cells longitudinally elongated, the external ones of which are narrower and sometimes have thicker walls than the more central ones. Vessels are not found, but in several Mosses there is in the centre of the stem a conducting strand of narrow, longitudinal cells, which represents the vascular bundle in its first stage of development. This strand contains elements for conveying water as well as sieve-tubes. Stomata are entirely wanting in the sexual generation of the Leafy-Mosses; they are found in a few Liverworts (Marchantia), but their structure is not the same as in the higher plants.
Vegetative reproduction takes place by gemmæ or buds which arise on the protenema, the rhizoids, the thallus, or the shoots, and become detached from the mother-plant; or else the protonema and the older parts of the plant simply die off, and their branches thus become independent plants. This well-developed vegetative reproduction explains why so many Mosses grow gregariously. In certain Marchantiaceæ special cupules, in which gemmæ are developed, are found on the surface of the thallus (Fig. 197 A, s-s). Again, protonema may also arise from the leaves, and thus the leaves may act as reproductive bodies. Certain Mosses nearly always reproduce vegetatively, and in these species the oospheres are seldom fertilised.
Fig. 187.—Marchantia polymorpha: a mature antheridium.
Fig. 188.—Spermatozoids.
The first generation bears the SEXUAL ORGANS; both kinds are found either on the same plant (monœcious), or on separate plants (diœcious). In the thalloid Liverworts they are often situated on the apex of small stems (gametophores), springing from the surface of the thallus. In the Leafy-Liverworts and true Mosses the leaves which enclose the sexual organs often assume a peculiar shape, and are arranged more closely than the other leaves to form the so-called “Moss-flower.” The male sexual organs are called antheridia. They are stalked, spheroid, club- or egg-shaped bodies whose walls are formed of one layer of cells (Fig. 187), enclosing a mass of minute cubical cells, each one of which is a mother-cell of a spermatozoid. The spermatozoids are self-motile; they are slightly twisted, with two cilia placed anteriorly (Fig. 188), while posteriorly they are generally a trifle club-shaped, and often bear at that part the remains of the cytoplasm, the spermatozoid itself being formed from the nucleus. In the presence of water the ripe antheridium bursts, and its contents are ejected; the spermatozoids, being liberated from their mother-cells, swarm about in the water in order to effect fertilisation.
Fig. 189.—Marchantia polymorpha. A A young, and B a ripe archegonium with open neck. C An unripe sporangium enclosed by the archegonium a: st the stalk; f the wall of the sporangium. Elaters are seen between the rows of spores.
The female sexual organs are termed archegonia. They are flask-shaped bodies (Fig. 189), the lower, swollen portion (venter) having a wall, in most cases from 1–2 cells thick, enclosing the oosphere (Fig. 189 B, k): the long neck is formed of tiers of 4–6 cells, enclosing a central row of cells—the neck-canal-cells (Fig. 189 A). When the archegonium is fully developed, the walls of the neck-canal-cells become mucilaginous and force open the neck of the archegonium. The mucilage thus escapes, and, remaining at the mouth of the archegonium, acts in a somewhat similar manner to the stigma and conducting tissue of a carpel, by catching and conducting the spermatozoids to the oosphere (Fig. 189 B, m), with whose cell-nucleus they coalesce. With regard to the formation of the oosphere, it may further be remarked that the lower part of the archegonium originally encloses the so-called “central cell”; but shortly before the archegonium is ripe, this cuts off a small portion, the ventral-canal-cell, which lies immediately beneath the neck, and the larger, lower portion becomes the oosphere.
The organs mentioned here, antheridia and archegonia, are present in the Cryptogams (Pteridophyta) and the Gymnosperms. They have always the same fundamental structure, but with slight modifications of detail. These plants are therefore known as the Archegoniata.
The fertilisation of the Mosses cannot be effected without water. Rain and dew therefore play a very important part in this process, and for this end various modifications of structure are found.
Fig. 190.—Andreæa rupestris. Longitudinal section through a sporangium at the time when the mother-cells of the spores are dividing: p pseudopodium; f foot; v vaginula; h neck; c columella; w wall of the sporangium; e external row of cells; s the spore-sac; t the spore-mother-cells; r the calyptra with the neck of archegonium (z).
Fig. 191.—Andreæa rupestris. Transverse section through a ripe sporangium. In the middle is seen the four-sided columella, surrounded by the numerous spores, drawn diagrammatically. Surrounding them is seen the wall of the sporangium, whose outer layer of cells is thickened and coloured. The layer of cells is unthickened in four places (x), indicating the position of the clefts (see Fig. 193).
Among the sexual organs, paraphyses—filamentous or club-shaped bodies—are to be found.
The asexual generation, the sporophyte (Moss-fruit or sporogonium). As the result of fertilisation the oosphere surrounds itself with a cell-wall, and then commences to divide in accordance with definite laws.[17] The embryo (Fig. 189 C) produced by these divisions remains inside the wall a-a of the archegonium (Figs. 190, 199 D, E), and developes into the sporogonium, which remains attached to the mother-plant, often nourished by it, as if the two were one organism. The lower extremity of the sporogonium, the foot (Figs. 190 f; 199 D), very often forces its way deep down into the tissue of the mother-plant, but without an actual union taking place. The central portion of the sporogonium becomes a shorter or longer stalk (seta), while the sporangium itself is developed at the summit. At a later stage, during the formation of the spores, the sporangium very often assumes the form of a capsule, and dehisces in several ways characteristic of the various genera (Figs. 192, 193, 194, 195, 200). The basal portion of the archegonium grows for a longer or shorter period, forming a sheath, the calyptra, in which the capsule is developed, but eventually it ceases to enlarge, and is then ruptured in different ways, but quite characteristically, in each group. Anatomically, the asexual generation is often more highly differentiated than the sexual; thus, for instance, stomata are present on the sporangia of the true Mosses, but are absent in the sexual generation.
As the capsule developes, an external layer of cells—the amphithecium—and an internal mass—the endothecium—are differentiated. As a rule the former becomes the wall of the capsule while the latter gives rise to the spores. In this Division, as in the Pteridophyta, the name archesporium (Fig. 190 t) is given to the group of cells inside the sporangium which gives rise to the mother-cells of the spores. The archesporium is in general a unicellular layer; in Sphagnum and Anthoceros it is derived from the most internal layer of the amphithecium, but with these exceptions it arises from the endothecium, usually from its most external layer. In the true Mosses and in Riccia only spore-mother-cells are produced from the archesporium, but in the majority of the Liverworts some of these cells are sterile and become elaters (cells with spirally thickened walls, Figs. 196, 189), or serve as “nurse-cells” for the spore-mother-cells, which gradually absorb the nutriment which has been accumulated in them. In Anthoceros, and almost all the Leafy-Mosses, a certain mass of cells in the centre of the sporangium (derived from the endothecium) does not take part in the formation of the archesporium, but forms the so called “column” or “columella” (Figs. 190, 191).
The spores arise in tetrads, i.e. four in each mother-cell, and are arranged at the corners of a tetrahedron, each tetrahedron assuming the form of a sphere or a triangular pyramid. The mature spore is a nucleated mass of protoplasm, with starch or oil as reserve material. The wall is divided into two layers: the external coat (exospore) which is cuticularized and in most cases coloured (brown, yellowish), and the internal coat (endospore), which is colourless and not cuticularized. On germination the exospore is thrown off, the endospore protrudes, and cell-division commences and continues with the growth of the protonema (Fig. 186, B-D).
Fig. 192.—Andreæa petrophila. A ripe sporogonium: a an archegonium which has been raised with the pseudopodium; p the foot; b the neck; d-e the dark-coloured portion of the sporangium, whose outer cell-walls are considerably thickened; c-c the thin-walled portions where the dehiscence occurs; o the lower extremity of the spore-sac; f calyptra; g the apex of the sporangium. (Mag. 25 times.)
Fig. 193.—Andreæa petrophila. An empty capsule; the calyptra has fallen off. (Mag. 25 times.)
The morphological explanation which Celakovsky has given of the sporogonium, and which is not at all improbable, is, that it is homologous with an embryo consisting of a very small stem-portion and a terminal spore-producing leaf. This will be further explained in the introduction to the Flowering-plants (p. 236).
In the Liverworts the young sporogonium lives like a parasite, being nourished by the sexual generation (only in Anthoceros has it a slight power of assimilation). In the Leafy-Mosses, on the other hand, with regard to the power of assimilation, all transitions are found from abundant assimilation (Funaria, Physcomitrium) to almost complete “parasitism” (Sphagnum, Andreæa). In the majority of the operculate Mosses the sporogonium has a more or less perfect system of assimilation, and is able itself to form a large portion of the material necessary for the development of the spores, so that it chiefly receives from the sexual generation the inorganic substances which must be obtained from the soil. The more highly developed the assimilative system of the sporogonium, the more stomata are present.
Apospory. In some operculate Mosses it has been possible to obtain a protonema with small Moss-plants from the seta, when severed from its Moss-plant, and grown on damp sand.
The Mosses are the lowest plants which are provided with stem and leaf. They are assigned a lower place when compared with the higher Cryptogams, partly because there are still found within the Division so many forms with a mere thallus, partly because typical roots are wanting and the anatomical structure is so extremely simple, and partly also because of the relation between the two generations. The highest Mosses terminate the Division, the Muscineæ and Pteridophyta having had a common origin in the Algæ-like Thallophyta.
They are divided into two classes:—
Hepaticæ, or Liverworts.
Musci frondosi. True Mosses or Leafy-Mosses.
Class 1. Hepaticæ (Liverworts).
The protonema is only slightly developed. The remaining part of the vegetative body is either a prostrate, often dichotomously-branched thallus, pressed to the substratum (thalloid Liverworts), with or without scales on the under side (Figs. 194, 197); or a thin, prostrate, creeping stem, with distinctly-developed leaves, which are borne in two or three rows (Figs. 195, 198), viz., two on the upper and, in most cases, one on the under side. The leaves situated on the ventral side (amphigastria) are differently shaped from the others (Fig. 198 a), and are sometimes entirely absent. In contradistinction to the Leafy-Mosses, stress must be laid on the well-marked dorsiventrality of the vegetative organs; i.e. the very distinct contrast between the dorsal side exposed to the light and the ventral side turned to the ground. Veins are never found in the leaves.
The ventral part of the archegonium (calyptra) continues to grow for some time, and encloses the growing embryo, but when the spores are ripe it is finally ruptured by the sporangium, and remains situated like a sheath (vaginula) around its base. The sporangium opens, longitudinally, by valves or teeth (Fig. 194, 195, 197 b), very rarely by a lid, or sometimes not at all. A columella is wanting (except in Anthoceros, Fig. 194); but on the other hand, a few of the cells lying between the spores are developed into elaters (Fig. 196), i.e. spindle-shaped cells with spirally-twisted thickenings, which are hygroscopic, and thus serve to distribute the spores. (They are seen in Fig. 189 C, not yet fully developed, as long cells radiating from the base of the sporangium. They are wanting in Riccia).
Fig. 194.—Anthoceros lævis (nat. size): K-K capsules.
Fig. 195.—Plagiochila asplenioides: a unripe, and b an open capsule; p involucre. The ventral edge of each leaf is higher than its dorsal edge, and covered by the dorsal edge of the next one.
Fig. 196.—An elater with two spores.
Round the entire archegonium, (or group of archegonia, when several are developed on the same receptacle) a sheath—the involucre—is often formed, which persists, and encloses the base of the stalk of the sporangium, together with the sheath of the archegonium (Fig. 195 p). In the Marchantiaceæ each archegonium is enclosed in a loose investment, the perigynium, which is developed as an outgrowth from the cells of its stalk.
The majority of the Liverworts are found in damp and shady places, pressed to the substratum; a few are found floating in fresh water.
Family 1. Marchantieæ.
This embraces only forms with a thallus, which is more or less distinctly dichotomously branched, in some, one or two rows of thin leaves are situated on its under surface. On the upper surface of the thallus are found large air-chambers.
Order 1. Ricciaceæ. The sporogonia are, with the exception of a few genera, situated singly on the surface of the thallus, and consist only of a capsule without foot or stalk. They always remain enclosed by the wall of the archegonium (calyptra), and open only by its dissolution. Elaters are not developed. Some genera are found floating like Duckweed.—Riccia glauca grows on damp clay soil. R. fluitans and R. natans float in stagnant waters.
Fig. 197.—Marchantia polymorpha. A Female plant (nat. size): a and b are archegoniophores in various stages of development; s cupules with gemmæ (see page 183). B An archegoniophore seen from below, the short-stalked sporangia are seen placed in 8–10 double rows. C Male plant, with a young and an older antheridiophore. D Antheridiophore halved vertically to show the antheridia (h); m the aperture of the pits in which they are sunk—the older ones to the left, the younger to the right.
Order 2. Corsiniaceæ. (Not native). Intermediate forms between the preceding and the following order. In internal and external structure mainly resembling the Marchantiaceæ. Corsinia; Boschia.
Order 3. Marchantiaceæ, are large, fleshy forms. The surface of the thallus is divided into small rhombic areas, in the centre of each of which is found a large, peculiarly constructed stoma (Fig. 197 A); beneath each of these a large air-cavity is to be found. From the floor of the air-cavity a number of alga-like cells project into it; these contain chlorophyll and are therefore the assimilating cells. The antheridia and archegonia are each found aggregated on specially formed branches (somewhat resembling Mushrooms) projecting from the surface of the thallus. The antheridia are developed on the upper surface (Fig. 197 C, D) and the archegonia on the lower (Fig. 197 A, B), near the centrally-placed stalk.
Marchantia polymorpha is diœcious (Fig. 197), and very common on damp places. Lunularia (South Europe), frequently found on flower-pots in conservatories; Preissia, Fegatella, Reboulia, Targionia.
Family 2. Anthoceroteæ.
These have an entirely leafless, fleshy, flat, and irregularly-shaped thallus. In its intercellular chambers Nostoc-colonies are often found, which have forced their way through the stomata situated on the under side. The antheridia and archegonia arise from the cells lying inside the thallus. The capsule resembles a long, thin pod; it has two valves and a columella. Anthoceros (A. lævis, Fig. 194, and punctatus).
Family 3. Jungermannieæ.
Some forms in this family have a thallus in which leaf-like structures are found (Blasia), while in others (e.g. Metzgeria, Pellia, Aneura) they are entirely absent. The majority, however, have round, thick stems, bearing dorsally two rows of leaves, and one row ventrally. Some of these have the leaves “underlying” (Fig. 195), while in others (Fig. 198) they are “overlying.” (See Figs. 195, 198, with explanation).
The sporangia are spherical, stalked, and situated singly on the apex of the branches, and open by four valves (in Sphærocarpus they are indehiscent).
Fig. 198.—Frullania dilatata. Portion of a branch seen from the under side: r and b are the anterior and posterior edges of the same dorsal leaf; a ventral leaves (amphigastria). The dorsal leaves are “overlying,” i.e. the anterior edge of the leaf overlaps the posterior edge of the preceding one.
All the species in this family were formerly reckoned as belonging to one genus, Jungermannia, but now they are divided into several, arranged as follows:—
I. Anacrogynæ. The archegonia are situated on the upper side of the thallus or stem, placed laterally, and covered by an “involucre,” formed by the calyptra together with the tissue of the stem or thallus.
a. Anelatereæ. Without any elaters: Sphærocarpus, Riella.
b. Elatereæ. α. Thalloid: Aneura pinguis, in damp situations; Metzgeria furcata, on trees; Pellia epiphylla, in damp situations; Blasia pusilla, on damp clay soil, in the shade (scales are present on the thallus). β. Foliose and not dorsiventral: Haplomitrium hookeri.
II. Acrogynæ. The apex of the stem or of certain branches is adapted for the formation of female shoots. The archegonia are most frequently aggregated on the apex of the shoots, and are encircled by their leaves (perichætium). Between these and the archegonia, enclosing the latter, a peculiar cup-shaped organ (the involucre) is formed. This group only includes leaf-bearing genera: Frullania, Radula, Madotheca, Ptilidium, Calypogeia, Lepidozia, Mastigobryum, Lophocolea, Jungermannia, Scapania, Plagiochila.
Class 2. Musci frondosi or veri (True Mosses).
In this class the protonema is well developed, and resembles a branched filamentous Alga, from which it can be easily distinguished by its oblique septa (in Sphagnum it is a cellular expansion). The Moss-plant, which is developed directly from the protonema, generally has an erect, thick, cylindrical stem similarly constructed on all sides. The leaves are arranged spirally, the most frequent divergence being 2/5 or 3/8 (Fig. 200 A). A midrib is often present and also marginal veins formed by longitudinally elongated cells; at these veins the leaf is more than one layer in thickness. In Leucobryum the leaves are generally constructed of more than one layer.
The stem grows by means of a three-sided, pyramidal, apical cell which gives rise to three rows of segments, each segment forming a leaf. The lateral branches arise from the lower portions of the segments, the upper portion of which does not take any part in the construction of the leaf. From their mode of origin the branches are not axillary, and differ in this respect from the Flowering-plants.
The ventral portion of the archegonium is very early ruptured at its base by the growing sporogonium, upon which it remains, and it is thus raised into the air, forming a “hood,” the calyptra (Figs. 192; 200 B). In the Sphagnaceæ the hood is not present; in this order, as in the Liverworts, the archegonium remains at the base of the sporogonium. The sporangium opens by circumsessile dehiscence, the upper portion (operculum) being separated along a specially constructed ring of cells, and falls off like a “lid” (Fig. 200). Only in a few forms (families 2 and 3) does any variation of this take place. Elaters are never found, but (with the exception of Archidium) there is always present in the sporangium a central mass of cells, the columella, which take no part in the formation of the spores. The columella, in some, does not reach quite to the operculum and in these cases the spore-sac is bell-shaped and covers the columella (Andreæa, Fig. 190; Sphagnum, Fig. 199 D); but in the majority of Mosses the columella extends to the lid, so that the space containing the spores becomes a hollow cylinder.
The sporangium is generally raised on a long stalk; in the great majority this stalk is formed from the lower half of the oospore and belongs to the asexual generation—it is then known as the seta. In Andreæa and Sphagnum the seta is very short, and the sporangia are raised upon a long stalk (pseudopodium) developed from the summit of the sexual generation (Figs. 190, 192). In the latter figure an archegonium (a) is seen attached to the pseudopodium, having been carried up with this during the course of its development. The summit of the pseudopodium is enlarged to embrace the foot of the sporogonium (Figs. 192, 199 D).
A. The sporangium is supported on a pseudopodium; the columella does not extend to the operculum.
Fig. 199.—Sphagnum acutifolium.—A The upper portion of a plant: a branches with antheridia; ch branches with terminal archegonia and perichætia; b the upper stemleaves. B A male branch whose leaves are partly taken off in order to show the antheridia. C Group of three archegonia: the central one (a) is formed from the apical cell. D Sporogonium in longitudinal section: the broad foot (sg’) is sunk in the vaginula, v; c calyptra; ar neck of the archegonium; ps pseudopodium. E ripe sporangium with operculum, and the remains of the archegonium situated on the pseudopodium which is still surrounded by the perichætium; to the left is a barren branch. F Portion of a foliage-leaf seen from above: l perforations; b chlorophyll-containing cells; s spiral thickenings.
Family 1. Sphagneæ (Bog-Mosses).
The protonema has been already described. The stem is regularly branched owing to the fact that a branch, or collection of branches, arises at every fourth leaf. These branches are closely covered with leaves, some are erect, while others hang down and surround the stem. No rhizoids are developed. These Mosses are of a whitish-green colour, and when water is present are always saturated with it like a sponge, the reason for this being found in the construction of the stem and leaves. The stems are covered by an external layer of large clear cells, without chlorophyll, but with annular or spiral thickenings on the walls, which are also perforated by large holes. By means of capillary attraction, water is thus raised to the summit of the stem. Similarly constructed cells are also found in the leaves, but they are surrounded by a net of very narrow, chlorophyll-containing cells (Fig. 199 F), whose colour is thus to a great extent lost amongst those which are colourless. This anatomical structure is an essential condition for the formation of peat. The Bog-Mosses grow by preference on moors, which they cover with a thick carpet saturated with water. The lower extremities of the plants perish very rapidly, and gradually become converted into peat, and the branches thus separated from each other become independent plants. The sporangia (Fig. 199 D, E) are spherical, but with a very short stalk. They open by a lid, but have no annulus. The archegonium (Fig. 199 C) persists at the base of the sporogonium as in the Liverworts. Only one genus, Sphagnum.
Family 2. Schizocarpeæ.
The Mosses which constitute this family are of a brownish-black colour and are found living on rocks. The sporangium resembles that of the Liverworts inasmuch as it opens by four valves, but these continue attached to each other at the apex as well as at the base (Fig. 193).—There is only one genus: Andreæa.
B. The stalk is formed from the lower portion of the sporogonium. The columella is continued to the summit of the sporangium and united with it (Archidium has no columella.)
Family 3. Cleistocarpeæ.
The fruit does not dehisce in the regular way, but the spores are liberated by decay. They are small Mosses which remain in connection with their protonema until the sporangium is mature. The archegonium remains sessile at the base of the short capsule-stalk, and is not raised into the air (compare Hepaticæ).—Phascum, Ephemerum, Archidium, Pleuridium.
Family 4. Stegocarpeæ.
To this belong the majority of the Mosses, about 3,000 species.
The capsule opens as in Sphagnum by means of a lid (operculum), which is often prolonged into a beak. Round the mouth of the opened capsule, a number of peculiar yellow or red teeth are to be found. These constitute the peristome; their number is four, or a multiple of four (8, 16, 32 or 64). The form and thickenings of these teeth are widely different, and on this account are used by Systematists for the purposes of classification. In some Mosses (Fig. 200 C, D) there is a double row of teeth. Except in Tetraphis they are not formed from entire cells, but from the strongly thickened portions of the wall of certain layers of cells belonging to the lid, and persist when this falls off. They are strongly hygroscopic, and assist greatly in the ejection of the lid, in which operation they are considerably aided by a ring of elastic cells with thickened walls, situated in the wall of the lid near the base of the teeth. This ring is known as the annulus. The archegonium is raised into the air like a hood, the calyptra, which either covers the sporangium on all sides (having the shape of a bell), or is split on one side (Fig. 200 B, h).
Among peculiar forms may be mentioned: Splachnum, which is especially remarkable for the collar-like expansion at the base of the capsule. Fissidens deviates in having a flat stem and leaves arranged in two rows. The leaves are boat-shaped and half embrace the stem.—Schistostega has two kinds of stems. The barren ones resemble Fern-leaves; they have two rows of leaves, which are attached together vertically, are decurrent and coalesce at their bases. The fertile ones have an ordinary appearance.—Tetraphis: the peristome is composed of four teeth, which are formed from entire cells. T. pellucida has peculiar gemmæ.
The family is divided into two groups: the Musci acrocarpi, the growth of whose main axis is limited and terminated by the formation of the sexual organs; and the Musci pleurocarpi, whose sporogonia are situated on special lateral shoots, while the growth of the main axis is unlimited.
Fig. 200.—A Hypnum populeum. B and C Sporangia, with hood (h), and operculum (l’), and without these (C), showing the peristome (p). D The mouth of the capsule of Fontinalis antipyretica.
A. Acrocarpi.
Order 1. Weisiaceæ. Peristome, with 16 teeth arranged in one series, rarely wanting. Leaf with midrib. Campylopus, Dicranum (D. scoparium, common in forests), Dicranella, Cynodontium.—Weisia, Gymnostomum (no peristome), Systegium.
Order 2. Leucobryaceæ. Peristome with 16 teeth. Leaves with three or more layers of cells, of which the external ones are air-conducting and perforated (as in the Sphagneæ), the middle one containing chlorophyll. Leucobryum.
Order 3. Fissidentaceæ. Peristome as in the preceding ones. The leaves are arranged in two rows on the plagiotropic shoots; in Fissidens the midrib of the leaf bears wing-shaped outgrowths. Conomitrium, Fissidens.
Order 4. Seligeriaceæ. Peristome with 16 undivided teeth. Very small Rock-mosses. Seligeria.—Blindia.
Order 5. Pottiaceæ. Peristome with 16 teeth, which are divided almost to the base, or with 32 teeth. Calyptra hood-like.—Barbula (B. muralis, B. ruralis), Trichostomum, Leptotrichum.—Ceratodon purpureus.—Distichium.—Pottia.
Order 6. Grimmiaceæ. The leaf-cells are often papillose; in the upper portion of the leaf, small, and of roundish shape. The calyptra is most frequently hood-like or conical. Eucalypta.—Orthotrichum, often with short-stalked capsule, is found on trees.—Coscinodon.—Hedwigia.—Grimmia, Racomitrium.—Cinclidotus.
Order 7. Schistostegaceæ. The stems are of two kinds (see above); Schistostega osmundacea, in caves, has a bright emerald protonema.
Order 8. Splachnaceæ. The capsule has a large, collar-like neck (see above). Splachnum (especially on manure).
Order 9. Funariaceæ. Capsule pear-shaped. Funaria (F. hygrometrica has a very hygroscopic seta, becoming twisted when dry, and straightening with moisture); Physcomitrium; Discelium.
Order 10. Bryaceæ. The capsule is thicker towards the apex; most frequently pendulous. Philonotis, Bartramia.—Aulacomnium.—Paludella Meesea.—Mnium.—Bryum, Webera, Leptobryum.
Order 11. Polytrichaceæ. Single peristome, formed by 16, 32, or 64 teeth. Leaves with longitudinal lamellæ on upper surface.—Polytrichum has long, hairy calyptra. Catharinea (C. undulata, in forests).
Order 12. Georgiaceæ. Peristome with 4 teeth (see above). Tetraphis (T. pellucida has gemmæ).
Order 13. Buxbaumiaceæ. Capsule asymmetrical; double peristome: the interior one conical, with 16 or 32 longitudinal folds.—Buxbaumia (B. aphylla); Diphyscium.
B. Pleurocarpi.
Order 14. Fontinalaceæ. Long, floating Water-Mosses. Fontinalis (F. antipyretica is found in streams). Dichelyma.
Order 15. Hookeriaceæ. Pterygophyllum.
Order 16. Leskeaceæ. Dull-looking Mosses, with papillose or warted leaves.—Thuidium, Thuja-like with regularly arranged 1–3 doubly pinnate stems; Anomodon, Leskea.
Order 17. Pterogoniaceæ. Pterigynandrum filiforme, etc.
Order 18. Fabroniaceæ. Anacamptodon.
Order 19. Neckeraceæ. Stems most frequently with flat, leafy branches. The leaves are smooth, never with longitudinal folds.—Neckera.
Order 20. Hypnaceæ. The leaves are smooth with square, often bladder-like, cells at the edge. Hylocomium (H. splendens, H. triquetrum); Hypnum; Brachythecium; Plagiothecium.—Eurhynchium.—Homalothecium, Isothecium, Orthothiecium, Homalia.—Climacium, Lescuræa, Leucodon.
The Mosses occur all over the globe. Many are found in great numbers, and growing thickly massed together, they form an important feature in landscapes (for example Sphagnum and Polytrichum in the Arctic Tundra). In the Northern and Arctic regions the Mosses are very plentiful, and often form a considerable part of the vegetation, while in the Tropics they are insignificant.
Species of Hypnum and Polytrichum, like Sphagnum, play an important part in the formation of peat.