Part 33

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.

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. 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).

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.=