Walk into damp, acid woodland and the low banks and rotting stumps are often dressed in a sombre dark green, the shoots packed upright and the leaves a little crisped if the day has been dry. That tuft is very likely swan's-neck thyme-moss, Mnium hornum, among the most familiar mosses of the temperate wood and, for once, one whose parts are large enough to puzzle out by eye.
Leaves you can almost read
A microscope is usually the toll for naming a moss with any confidence. This one is kinder. Its leaves run to four or five millimetres, broad and tongue-shaped, and held up to the light they show their workings plainly: a clear rim of long narrow cells running right round the edge like a hem, and along the upper margin a row of small teeth set two by two. A single midrib reaches almost to the tip. A pocket lens shows all of it, which makes the moss a good one to learn the anatomy of a leaf on before you tackle the fiddlier species in how to identify a moss. Fresh spring shoots come up a clean yellow-green and stand out sharply against the older, almost black-green growth beneath them.
Where it settles
This is a plant of lime-free ground. It wants acid soil, peaty humus, the crumbling wood of old logs and stumps, the damp lower trunks of trees and the shaded sandstone of a stream bank, and it turns its back on chalk and mortar. You will meet it throughout Britain and Europe and right round the cooler Northern Hemisphere, sometimes in such quantity that it sheets a whole woodland bank in green. Because it leans towards old, undisturbed, acid woodland, a deep stand of it is a small clue to the age and character of the wood around you, much as certain wildflowers come to stand for the ground they favour.
The swan's neck
The common name fixes on the moss in fruit. From the tip of a female shoot a slender stalk, the seta, rises a couple of centimetres and then bends sharply over at the top, carrying the spore capsule face down so that it hangs and nods like the crooked neck of a swan. The capsules swell through spring into early summer, green at first and ripening to a warm brown, and once the little lid drops away the spores drift out on dry air. A bank of Mnium hornum in May, pricked all over with these nodding heads on their fine stalks, is one of the quiet pleasures of the spring wood.
Splash cups and the rain
The moss keeps its sexes on separate plants, so a male shoot and a female shoot must somehow be brought together. The males manage it with rain. Each male shoot is tipped by a flat rosette of leaves spread open like a tiny green flower, a shallow cup holding the sperm-making organs at its centre. When a raindrop lands square in the cup it splashes outward, flinging a fine spray of sperm-laden water a hand's breadth across the colony, far enough to reach a neighbouring female and fertilise her. It is a tidy piece of plumbing, and well worth crouching to watch after a shower. The two-part life that follows, the leafy plant giving rise to the stalked capsule, runs through every moss and is set out in what a moss actually is.
Telling it apart
Several relatives share the dark green and the bordered, toothed leaves, and the group as a whole answers to the name thyme-mosses. The likeliest to confuse are the creeping Plagiomnium mosses, which throw out long, arching, almost bare runners across the ground and often carry leaves with a wavy, puckered surface. Swan's-neck thyme-moss holds its shoots more upright and tufted and keeps its leaves flat rather than crimped, and those paired marginal teeth are a dependable mark once your lens is on them. Put the hemmed border, the double teeth and the upright habit together and you can name it without hesitation.