Moss is a small, soft, green plant that grows in low cushions and spreading carpets on damp ground, stone, bark and a hundred other surfaces. It is a genuine plant, every bit as much as an oak or a daisy, yet it belongs to a very old and very plain branch of the family, one that has made its way for hundreds of millions of years without most of the equipment a garden flower depends on. That plainness is really the whole of the story.
A plant, and one of the earliest sorts
Moss is one of the bryophytes, the plants that never grew the internal plumbing, the fine tubes carrying sap up through a stem, that a larger plant uses to move water and food from one end of itself to the other. Because that plumbing is what botanists mean by a vascular system, moss is described as a non-vascular plant. Lacking the pipes, it cannot lift water far, and so it stays low and drinks the rain straight off its own leaves instead. Plants of this kind were among the very first to haul themselves out of the water and settle on dry land, back before ferns, conifers or anything that has ever borne a flower, which makes the green film on your garden wall a survivor of one of the oldest experiments in land plants going.
No roots, no flowers, no seeds
Much of what defines moss is a list of things it does without. It grows no true roots; the rusty threads matting its underside serve only to grip, a point worth its own page under do mosses have roots. It never flowers, and it sets no seed. In place of seeds it releases spores, single dust-fine cells shaken from a capsule that a moss lifts on a slender stalk once the two sexes have met in a film of rain. A spore that lands somewhere damp grows first into a green thread across the surface, and only then knits itself into the leafy cushion most people would recognise. It is a slower, more roundabout way of getting about than a seed, and it is why moss so favours the wet.
The scientific name, and why there isn't just one
Searches for "the scientific name of moss" usually hope for a single Latin tag, but no one word covers it. Moss is not a species; it is a whole division of the plant kingdom, called Bryophyta, and it gathers somewhere near twelve thousand separate species across the world. Each of those carries its own two-part Latin name, so the springy stuff on a woodland floor might be Hypnum cupressiforme while the grey dome on a wall is Grimmia pulvinata. In other words the word "moss" works rather like the word "tree": a broad, everyday label thrown over thousands of distinct kinds rather than the name of any one of them.
What it asks for, and where you find it
Give a moss moisture, a little light and any surface it can cling to, and it will generally manage the rest. It draws water and the traces of mineral it needs from rain, mist and settling dust rather than from soil, so it turns up in places nothing else will colonise: bare rock, roof tiles, tarmac, tree bark, brickwork and trodden earth alike. Shade and damp suit it best, which is why the north side of a roof or the foot of a shaded wall greens over first. From the Arctic to the tropics there is hardly a corner of the land without its mosses, and a single lawn or paving crack may hold several species growing side by side.
The things it is often mistaken for
Plenty that carries the name "moss" is nothing of the sort. Lichen, the crusty grey and yellow growth on gravestones and twigs, is a partnership of a fungus and an alga rather than a plant at all. Reindeer "moss" is one of those lichens; Spanish moss is a flowering plant related to the pineapple; clubmosses are a separate, vascular lineage with real roots. Sorting the true article from this crowd of impostors is the business of telling moss apart from its lookalikes. And if this plain sketch leaves you wanting the workings in full, the way the two-stage life cycle turns and how the whole plant is put together, that is set out at length in the biology of mosses.