In many organisms, which reproduce sexually, there are two generations: The one has a double set of chromosomes. It is diploid. Plants of this generation now form cells, that have only one single set of chromosomes. These cells are called spores. In normal flowering plants these would be e.g. the pollen.
The spores may now grow up to a plant of the other generation. This generation consequently has only one set of chromosomes - it is haploid. These haploid plants finally develop the so-called gametes (reproductive cells). Two of them now fuse again into one diploid cell, which is the called zygote. From the zygote a plant of the diploid generation can develop.
Practically all flowering plants and ferns, which are to be seen day by day, are diploid. In mosses this is totally different. The generation change discribed here is valid for all mosses, but the concrete shaping of gametophyte, sporophyte and protonema may be rather different in the several taxanomic divisions. Here the generation change of mosses in the narrow sense (not liverworts and hornworts) is described.
Let's begin with a small green moss-plant, as it is normally seen. This is the haploid generation: All its cells only have one single set of chromosomes. It is also called gametophyte.
The gametophyte now forms organs, in which the gametes -the reproductive cells- develop: The archegonia form the egg cells, the antheridia the spermatozoids. Archegonia and antheridia are in many mosses bundled in leaf rosettes similar to flowers - the so-called "moss flowers" or perichaetia. (This has nothing to do with the flowers of flowering plants.) Archegonia and antheridia may also develop on different moss plants, or on different parts of the same plant.
Now it has to rain and the mossflower has to get wet. Then the spermatozoides can swim around. I think it is not necessary to explain, what happens then. In any case - finally there is a fertilized ovum, the "zygote". The fact, that open water is necessary for the fertilization, shows that mosses are not so well adapted as e.g. flowering plants to the life on the land.
The zygote -still lying in a mossflower- now grows up to the diploid generation, the sporophyte. It normally consists of a capsule on a stem.
The sporophyte simply grows on top of the green gametophyte. Normally it doesn't do any photosynthesis, but is nourished at least partially by the gametophyte. If these both wouldn't belong to the same species, the sporophyte could be called a parasite on the gametophyte.
In the capsule now develop the spores, which only have one single set of chromosomes. When the spores are ripe, the capsule opens and sets them free. The way for the next haploid generation is prepared.
In mosses and liverworts, the gametophyte doesn't develop directly from the spores. The spores begin to build some kind of mesh of cell filaments. This mesh is called protonema. The protonema may be rather different in different moss-species. It doesn't have to be an mesh. The protonema of peat mosses e.g. are small thalli.
The protonema now builds some cusps, from which the foliose mossplants arise. This way rather compact cushions may develop.
The following picture shows the generation change in one view:
A spore grows out to the protonema (A). The protonema develops cusps (B), from wich the gemetophytes (C) arise. The gametophytes develop antheridia (D), the antheridia spermatozoides (E), and the spermatozoides fertilize the egg cells in the archegonia (F). Out of the archegonium the sporophyte (G) grows, and at the end the capsule. In the capsule the spores develop, which are finally set free. (H) and (I).
Because of the different shape of gametophyte and sporophyte, the generation change is called heteromorphous. Ferns have a heteromorphous generation change too. But in contrast to the mosses the dominating generation (the green fern plant) is the sporophyte. In many algae however the generation change is homomorhpous, i.e. gametophyte and sporophyte are similar.
In all mosses the archegonia (singular: archegonium) are more or less bottle-shaped organs, which are wrapped by a cover one cell thick. In the interior of this bottle there is a single big central cell, which finally divides into the ovum and a belly-channel-cell, which lies in the bottom of the bottleneck. The neck is closed by a row of neck-channel-cells. Their number varies: Mosses have 10 or more, liverworts 4-8, and hornworts 6.
When the ovum ripes, the archegonium opens, because the uppermost cover cells swell and become slimy. So on top of the archegonium an opening develops. The neck-channel-cells change to slime too, and so there is a channel, through which the spermatozoids can pass through to the ovum.
In all mosses the antheridia are more or less club shaped or spherical. They sit on top of a short stem. They consist of a cover, whichs wraps many so-called "spermatogene cells", each of which finally divides into two spermatids. And those again develop to spermatozoids.
The spermatozoid (on the left) are slightly tortuos filaments, which are substantially filled by the nucleus. On their front there sit two flagella directed to the back.
Archegonia and antheridia may sit in the mossflowers in mixed groups, but they may also grow on different places on the same plant, or only on different plants. In the latter case the "flowers" on the male and on the female plant often look differently.
Sometimes between the archegonia and the antheridia there are some elongated, club shaped cell filaments, which probably shall store water and protect those organs from drying up. The picture on the right shows a such paraphyse, taken from a mossflower of the species Mnium hornum (only german).
Because the spermatozoid is only able to move under water, the fertilization can only take place in the presence of water. They find their way to the ovum, because the neck-channel-cells in the archgonia release certain attractive substances.