What exactly is a moss?
Mosses are non-vascular plants. This means they do not conduct water internally... for the most part. Some more advanced species like ones found within the Polytrichum and Mnium genera have primitive vascular tissue called hydroids and leptoids; analogous to xylem and phloem respectively. These tissues can be found in the stems and the costa (midrib) of higher mosses and facilitate in the transportation of water and nutrients. But as a whole bryophytes are considered non-vascular.
Mosses are spore-producing. Rather than spreading their progeny via seeds like vascular plants, mosses use spores. Here's how mosses have sex:
Augmented diagram of the lifecycle of a moss
I numbered each step, I'll do my best to explain this as clearly as humanly possible:
1. If you pluck a piece of moss out of a clump this is what it looks like (though they do come in widely different shapes and sizes) The leafy green stuff that you see growing is the gametophyte (holds gametes) and the wirey thing with a capsule on top is a sporophyte (holds spores).
2. As the sporophyte slowly emerges from the top of the gametophyte it carries with is a piece of the old archegonial wall (see step 10). This little covering is the calyptra and may or may not be very conspicuous.
3. On top of the capsule is a tiny cap called an operculum. It covers the capsule opening while the spores ripen.
4. When the spores are ripe and ready the operculum will pop off revealing a peristome. This row of tiny teeth are thought to aid in the dispersal of spores.
5. The spores are released.
6. When a spore find a happy little spot it will begin to germinate. The filamentous green protonema emerges.
7. Small buds will appear on the protonema which will develop into a leafy plant: the gametophyte. Also forming are rhizoids: small root-like structures used to hold the moss to its substrate.
8. Shown here is a dioicous plant (moss version of being dioecious): male and female gametes (sperm and egg) on separate plants.
9. On the male plant antheridia will form, these house the sperm. On the female plant archegonia will form, each one with an egg. Only when water is available can the sperm be released and swim until they find an archegonium and travel down the neck to the egg - should they only be so lucky.
10. When the sperm fertilizes the egg a baby is born: a sporophyte, and life begins anew.
Sexual reproduction in mosses requires many factors to be in place at certain times and sometimes it just isn't worth the moss's trouble. Depending on the environmental conditions many mosses may choose to reproduce asexually through the use of vegetative propagules: gemma, brood bodies, paraphyllia, leaf fragments...
So how is the life cycle of a moss different from other plants? The difference lies in the dominant generation, i.e. the generation that you see walking around outside - sporophyte or gametophyte. The sporophyte always has a 2n condition (diploid: 2 sets of chromosomes). It is 2n because it is the product of a sperm and an egg combining which are 1n each (haploid: 1 set of chromosomes). In vascular plants the dominant generation is the sporophyte, i.e. the tree, the shrub. It is on the sporophyte (tree) that the gametophytes (sperm/egg) are borne. In mosses it is the reverse: the gametophyte is dominant: the sporophyte is borne and is dependent on the gametophyte.
That was about as simple as I could get it...
That concludes part 2...stay tuned for my next installment!