Most mosses wind their leaves round the stem in a loose spiral, so a shoot looks much the same whichever way you turn it. The pocket-mosses break that habit. They lay their leaves flat in two ranks, like the vanes of a small green feather, and each leaf clasps the one above in a little sheath that gives the group its name. Get your eye in for that flatness and common pocket-moss (Fissidens taxifolius) starts appearing on more damp banks and path edges than you would ever have guessed.
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Leaves laid flat in two rows
A pocket-moss shoot is pressed into a single plane. The leaves leave the stem in two opposite ranks and all lie in that one plane, so the whole shoot reads as a miniature frond rather than a bushy sprig, nearer to a fern pinna or a bird's feather than to the usual mossy tuft. The plants themselves are small, often only a centimetre or two, dark to fresh green, standing singly or crowding into thin turfs on bare ground. That two-dimensional habit is uncommon enough among British mosses that it does most of the identifying for you: spot a flat, feather-shaped little shoot on damp soil and you are almost certainly holding a Fissidens.
What the pocket actually is
The name rewards a hand lens. Each leaf is built in three parts, and the arrangement is the whole point. Along the lower half of the leaf, on the side facing the shoot tip, a flap of leaf tissue folds back and runs beside the nerve, making a sheath that is open at the top; the leaf above settles down into it as though tucked into a slot. Botanists label the pieces the vaginant lamina for that sheathing flap, the dorsal lamina for the strip running down the back of the nerve, and the apical lamina for the free upper blade. You need none of the Latin to use it. Look for the small clasping pouch at the base of each leaf, because no other common British moss is put together this way, and that pouch is your shortcut straight to the genus.
Where to find it
Common pocket-moss prefers bare, damp, base-rich ground over deep shade or sour peat. Clay banks, the trodden sides of woodland paths, ditch and stream edges, the raw soil of an arable field margin, even the crust of soil on a flowerpot left standing outdoors: these are its regular haunts, and on the heavier soils it is genuinely frequent across most of Britain and Ireland. I first learned it on a shaded clay bank behind a garden wall that stayed tacky well into summer, where a scatter of small green feathers had taken the bare patches between larger cushions. A little disturbance suits it, which is why path sides do so well; ground that is periodically scraped open gives the moss the fresh surface it needs to gain a hold.
Two on land, one under water
Britain holds a good many pocket-mosses, and two cross your path often enough to name. Common pocket-moss (F. taxifolius) is the larger and coarser, its nerve running out beyond the leaf tip as a short spine, and it raises its capsule on a stalk from low down the shoot rather than the tip. Lesser pocket-moss (F. bryoides) is smaller and tidier, fruits freely from the shoot tip, and shares much the same soil banks. There is an aquatic member of the family too, one that aquarists know well: Fissidens fontanus, sold as Phoenix moss, which lives fully submerged and is tied down onto wood and rock in planted tanks. It carries the same flattened, pocketed leaf plan, so anyone with a planted aquarium has already met the group, as covered under the aquarium moss types.
Getting to a species
Naming the genus is easy; naming the species usually is not. What separates one pocket-moss from the next comes down to fine detail, the pattern of cells at the leaf margin, whether a paler rim borders it, exactly how far the nerve runs, and those points want a microscope and a proper key rather than a lens in the field. Most of the time I am content to record a small flat-fronded moss on a clay bank simply as Fissidens and save the species for a day with the scope out. Resting at the genus is a respectable place to stop, a point I make in identifying moss. For the broader knack of reading a moss by its growth form first, telling moss apart follows the same line.