After a windy night, pick up one of the small twigs blown down beneath a tree and look closely at the bark. Very often it is studded with neat dark-green cushions about the size of a shirt button, each crowded with tiny stalked urns. These are the bristle-mosses, mostly the genus Orthotrichum and its close relative Ulota, and once you start noticing them you find them on twigs, trunks and old walls almost everywhere.
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Small cushions on bark and stone
Bristle-mosses grow as rounded, rather stiff cushions or tufts, dark green to nearly black when dry and freshening to a duller green once wet. Most are epiphytes, living on the bark of trees, and they lean towards well-lit twigs and small branches rather than deep shade, so the outer, sunnier parts of elder, willow, ash and sycamore often carry them thickly. A good few species have taken instead to stone, settling on mortar, concrete, roof tiles and the tops of walls much as the true masonry mosses do. The cushions stay small, seldom more than a couple of centimetres across, and they sit tight against the surface, which is why they go unnoticed until a fallen branch brings them down to eye level.
Urns you can nearly always find
What makes this group such a gift to a beginner is that it fruits, and fruits heavily, almost whenever you meet it. Nearly every cushion carries a crop of little capsules tucked down among the leaves or lifted just clear of them, rather than held high on long stalks the way many mosses show theirs. Dry, a capsule is narrow and ribbed with about eight lengthwise furrows, like a small fluted barrel or an urn; wet, it swells and the ribs ease out. A hood called the calyptra sits over the young capsule, and in these mosses it is frequently furrowed and clad in pale hairs, which lends the group its bristly look. Because ripe capsules are so reliably there, bristle-mosses are one of the few families where a hand lens shows you the whole life cycle at a single glance.
Orthotrichum and Ulota
Two names cover most of what you will pick up. Orthotrichum keeps its leaves fairly straight as the cushion dries, so the tuft simply darkens and closes a little. Ulota does something you cannot miss: on drying its leaves curl and twist into tight corkscrews, so the whole cushion looks frizzled, then relaxes flat again the instant it is wetted. Common crisp-moss (Ulota crispa) is the one to learn that on, frequent on the twigs of birch and hazel in cleaner, damper districts. Between the many species of Orthotrichum the differences run fine, but the curl-versus-stay-straight split alone gets you to the right genus in the field.
A moss that reads the air
Bristle-mosses carry a story worth knowing, because they are sensitive to what they breathe. Many were badly hurt by the sulphur dioxide of industrial, coal-burning Britain, and through the middle of the last century they dwindled out of towns and much of the polluted lowland, holding on only in cleaner country. Since the Clean Air Acts and the long fall in sulphur pollution they have returned strongly, spreading back along twigs into cities that had lost them for decades. One species in particular, the white-tipped Orthotrichum diaphanum, every leaf drawn out into a fine colourless hair-point, is now a common sight on wayside twigs, fences and bare concrete even in the middle of towns. These comings and goings make the group useful indicators, a theme picked up under moss and air quality.
Getting to a species
The genus you can settle in the field; the exact species is another matter. Where one Orthotrichum divides from the next comes down to small internal points, whether the breathing pores on the capsule wall sit flush at the surface or sunk below it, how far the capsule teeth run, how big the spores are, and reading those wants a microscope and a proper key rather than a lens on a wet walk. So I will usually jot down a bristly, free-fruiting cushion on a twig as an Orthotrichum and save the rest for an evening indoors with the scope. Anyone wanting to build the wider skill of sizing up a moss by its shape will find it set out in how to identify moss, and the other mosses sharing these same trees are gathered under moss on trees.