If you have looked closely at a grey, frosted-looking moss on a sunny wall or a mountain boulder and wondered why the tips stay white even in warm weather, the short answer is that you are not seeing frost. You are seeing hair. A good many mosses finish each leaf with a fine, colourless point, and when a whole cushion carries them the plant reads silver or grey from across the garden. Those pale tips repay a second look, because they say a great deal about where a moss lives and will often help you put a name to it.
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The frost that never melts
Bring one of these cushions close, or better still lay a shoot under a hand lens, and the white resolves into a bristle running on past the end of the leaf. Botanists call it a hair-point, or an awn, and in the plainest cases it is simply the leaf's midrib carried on beyond the green blade as a clear, glassy thread with no living cells inside it. Some are short and stubby, scarcely longer than a full stop; others, as on the woolly fringe-moss of the high tops, reach half the length of the leaf again and lend the whole plant its felted, hoary coat. Dry weather shows them best, since a parched shoot pulls its green leaves in tight and leaves the pale hairs standing proud of the surface.
What a glass hair is for
These threads are no mere ornament, and the places they turn up tell you their purpose. White hair-points gather on the mosses of the harshest, sunniest, driest perches, the baking top of a wall or an exposed summit where the wind never rests. A clear hair bounces away some of the fierce light that would otherwise cook the leaf below, and a dense fur of them shades the growing tissue much as a pale hat shades a scalp. They also slow the air moving across the leaf, which cuts the rate at which hard-won water escapes. Several seem to catch dew and mist besides, running the beads down to the foot of the shoot where the plant can take them up. For something with no roots and no way to close a pore against drought, that scrap of harvested moisture can tip a bad morning into a workable one.
The mosses that wear them
Once your eye is tuned to the feature you will meet it over and over on hard, open ground. Grey-cushioned grimmia (Grimmia pulvinata), the little hedgehog buns on almost every old wall, is hoary with hair-points. So is wall screw-moss (Tortula muralis), each of whose leaves tapers to a long clear bristle. The silvery carpets of woolly fringe-moss on acid mountain rock owe their colour to hairs drawn out longer still, minutely toothed down their edges. The screw-mosses of the genus Syntrichia, common on limy walls and coastal sand, do much the same, and the desert moss I write about elsewhere carries the trick to its limit on sun-struck ground. A thread runs through the whole list: nearly all are plants of sun and stone rather than of the damp shaded woodland, where such a hair would be little use.
Silver that owes nothing to a hair
Be careful, though, because not every pale moss is hair-tipped. Silvery thread-moss (Bryum argenteum), the whitish-green moss threading through pavement cracks and gutters the world over, looks almost metallic, yet it bears no awns whatever. Its sheen comes instead from the upper cells of each leaf losing their chlorophyll and going clear, so the leaf tip turns colourless blade and all. A moss bleached by hard drought is a third case: there the green has drained from stressed tissue, and a good soaking brings it back within minutes, whereas a true hair-point stays white wet or dry. Sorting these three apart is the first job whenever a pale moss has you stumped.
Using white tips to name a moss
For a beginner the hair-point is a generous clue, because it cuts the field down quickly. A grey, frosted cushion on a sunlit wall is most unlikely to be one of the lush feather mosses of the woodland floor; it belongs to the small company of drought-hardy wall and rock mosses, and that alone sets aside most of the book. From there you can weigh how long the hairs are, whether they run smooth or toothed, and what the capsule gets up to, the sort of patient looking that identifying moss always comes down to in the end. I like it as a feature to start people on: visible without a lens, honest about the plant's way of life, and sharp enough under magnification to settle a question. Come the next dry spell, run your eye along the top of a sunlit wall and notice how much of the grey turns out, close to, to be glass.