A few woodland mosses reward a closer look, and this one rewards it twice. Seen from above it spreads in loose ferny fronds across the leaf litter; brought up close, each translucent strap of a leaf is crimped into small waves you can make out with the naked eye. Hart's-tongue thyme-moss (Plagiomnium undulatum) ranks among the largest and most recognisable mosses of damp British woodland, and those wavy tongue-shaped leaves give it away long before a lens comes out.
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Wavy leaves set in two rows
The leaves are the whole story. Each is large for a moss, strap-shaped or tongue-shaped, a translucent mid-green, and rippled across its width into shallow waves, as though the leaf had been pressed and then crumpled like tissue. They sit in two ranks along the stem, so the shoot flattens out and reads like a small fern frond or a green feather laid over the ground. That combination, a flattened ferny shoot bearing broad leaves you can see are wavy, belongs to nothing else common in our woods. Put a leaf under a lens and you will find a distinct border of narrow cells running round the margin, edged with small teeth, and a firm nerve reaching almost to the tip.
Two kinds of shoot
The plant works by running two different shoots at once. Long arching and creeping stems, the ferny ones, trail over the ground and root down where their tips touch, pushing the patch outward across a bank; shorter upright stems gather their leaves into a looser rosette at the top and carry the sex organs. Most of what you meet is the creeping, barren kind, which is how a single plant can carpet a good stretch of damp ground without ever making a spore. Watching that quiet spread is a reminder of how much moss achieves by simply creeping and layering, a theme picked up in how fast moss grows.
Where it grows
Damp, shaded ground on the richer soils is where to look: woodland floors and stream banks, the sides of ditches and shaded hedge banks, flushed slopes, lush wet grassland and the mossy corner of a lawn that never quite dries. It is common and widespread across Britain and Ireland, at its most luxuriant in the humid western woods where mild wet air keeps everything green, and it is a familiar sight in the mossy oakwoods described under temperate rainforest. The largest fronds I know sit on a bank under hazel that stays wet all summer, where the moss lies in open ferny drifts you could mistake at a glance for seedling plants.
The undulate muddle
Two common woodland mosses carry a name for their wavy leaves, and newcomers routinely swap them. The other is common smoothcap (Atrichum undulatum), and side by side they are quite unalike. Smoothcap grows as upright, unbranched shoots in loose tufts on bare woodland soil, its narrower leaves rough to the touch and toothed, and it fruits generously with tall capsules; hart's-tongue is altogether bigger, its leaves broad and strap-like, its shoots arching and ferny rather than erect. A closer relative, swan's-neck thyme-moss, can also cross your path, though it keeps to acid banks and rotting logs in dark stiff tufts, its leaves plain and unwaved. Both comparisons live in common smoothcap and swan's-neck thyme-moss.
Fruit, and a moss for a first outing
Capsules are not a common sight on this species. It carries the sexes on separate plants, so fruiting depends on the two growing close, and when it does happen the capsules hang down, pendulous and sometimes several together, from the erect shoots. Far more often you meet it barren, which is no loss when the leaves name it so plainly. That plainness makes it a fine moss to put in front of anyone starting out, since its best feature is large enough to see across a path and needs no equipment to enjoy, a gentle entry into identifying moss. Leave it where it grows; it fares poorly in a pot and thrives instead in the damp shade it chooses for itself, sheltering the small lives explored in life in a moss cushion.