Part 34

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).

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.

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.

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.