Hunt for moss for any length of time and sooner or later you pick up something that is not moss at all but a liverwort, its near relation and constant companion. The two keep the same damp corners, yet liverworts belong to an older, separate branch of the plant family, and a fair number look nothing like the soft green cushions people expect. The flat, lobed sheet in the photograph above, studded with little green cups, is one of the commonest.
An older branch of the family
Liverworts, mosses and the obscure hornworts together make up the bryophytes, the small land plants that manage without roots, flowers, seeds or any inner plumbing, the group sketched out in what a moss actually is. Of the three, liverworts are reckoned the most ancient; some botanists set their ancestors among the first plants to creep onto land, well over four hundred million years ago. Around seven thousand species are known, most of them small, low and fond of wet. The name is older still: under the old doctrine of signatures the lobed, liver-shaped thallus was read as a sign the plant would soothe ailments of the liver, and 'wort' is simply the old word for a useful herb.
Two ways to build a liverwort
The group falls into two looks. The thalloid liverworts, the kind most people picture, grow as a flat, branching green ribbon pressed to the ground, forking over and over into rounded lobes, with no stem or proper leaves to speak of; Marchantia on the greenhouse staging and Pellia on a wet bank are the everyday examples. The leafy liverworts are quite another matter. These carry minute leaves along a creeping stem and pass at a glance for a small moss, which is just where the confusion begins and a hand lens starts to pay off.
The leafy ones that pass for moss
Set a leafy liverwort beside a moss and the differences hold up well once you know them. A liverwort's leaves lie in two tidy ranks along the stem, flattened into one plane like a pressed frond, often with a third row of smaller leaves tucked beneath; a moss spirals its leaves all the way round the stem in many ranks. The leaves themselves tend to be rounded or, very often, cleft into two or more lobes at the tip, where a moss leaf usually tapers to a single point. And no liverwort leaf carries a midrib, that pale central nerve you can trace up so many a moss leaf. Under the microscope one further thing clinches it: the cells of a liverwort hold glistening droplets called oil bodies, which the mosses lack entirely. The whole cast of lookalikes, liverwort against moss and lichen against both, is sorted out in moss, lichen, liverwort or algae.
Cups, stalks and elaters
How a liverwort breeds is half the pleasure of finding one. The little cups scattered across the Marchantia in the photograph are splash-cups, each holding a clutch of tiny green discs called gemmae; a raindrop landing in a cup throws them clear to root nearby, a way of spreading that skips spores altogether. When liverworts do make spores, the capsule bears no resemblance to a moss's tough, long-stalked, lidded urn. It rides up almost overnight on a soft, ghost-white stalk that wilts within a day, then splits cleanly into four spreading valves; mixed among the spores sit elaters, slender spiral threads that twist as they dry and flick the dust into the air. There is no lid, and no ring of peristome teeth of the sort a moss uses to dole its spores out by degrees. A liverwort empties in a morning and is spent.
Where they turn up
You meet liverworts wherever the ground stays wet and the air sits still: dripping rock faces, the splashed margins of streams and waterfalls, shaded woodland banks, and the bark of old trees, where the leafy kinds thread flat green ribbons through the moss. The thalloid Marchantia is also a well-known pest under glass, spreading as a slick green crust over the compost of pot plants and seed trays; where it shows up it is reporting standing wet and a packed, overwatered surface, so the answer is to ease off the watering and open up the compost rather than to scrape and curse. Fishkeepers run into liverworts too, since several plants sold for the tank, Riccia and the so-called Pellia among them, are liverworts and not mosses, a tangle unpicked in is aquarium moss really moss. Out in the wild they repay being left exactly as they are. Many want clean, damp, unpolluted air, so a bank thick with liverworts is usually saying something good about the place.