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).
