Part 17

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_).

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_).

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.

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).

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._

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._