Everyone who learns a little about moss meets its two relations soon enough. Liverworts turn up first and turn up often; hornworts, the third and smallest branch of the bryophytes, are the ones most people never knowingly see at all. They repay the squint, because once you have found the little green horn that gives them their name you will never mistake them for anything else.
The third bryophyte
Mosses, liverworts and hornworts are the three lineages of bryophyte, the small plants that get by without roots, veins, flowers or seeds; what that means in practice is set out in what a moss actually is. Hornworts are the least of the three by a wide margin, with only a couple of hundred species worldwide, and in Britain you are really looking at a mere handful. They like damp, open, disturbed ground: the scraped edge of an arable field, a moist ditch bank, the bare soil of a plant pot or greenhouse staging. After rain in late summer and autumn is when they show themselves, and they are gone again quickly, so a hunt has its season.
A rosette and a horn
The plant body is a flat, dark green rosette or lobed sheet pressed against the soil, smaller and thicker than the pale ribbon of a thalloid liverwort, its margins often crimped or a touch ruffled. Out of that low mat rise the horns, slender upright green spikes an inch or two tall that carry the spores. Here is the oddity worth remembering: the horn keeps growing from its base while it splits and sheds ripe spores from the tip, so a single one can go on releasing spores for weeks together, something no moss capsule manages. And unlike almost every other bryophyte the horn is green and does its own photosynthesis; it even carries breathing pores of the kind you find on a proper leaf.
One chloroplast, and a lodger
Put a hornwort under a microscope and it gives up a feature that startled the early botanists. In many kinds each cell holds a single large chloroplast, rather than the scatter of small ones a moss or a garden plant carries, and set within it sits a pyrenoid, a protein body of the sort you expect in an alga and rarely in a land plant. That algal echo is part of why the group is prized for what it hints about how plants first crept ashore. The second lodger is easier to spot with the naked eye: colonies of Nostoc, a nitrogen-fixing cyanobacterium, live in mucilage-filled pockets inside the thallus and show as tiny blue-green flecks. The bacteria hand the plant usable nitrogen, which is how something so slight can settle on raw, hungry soil that little else will touch.
Telling one from a liverwort
In the field the muddle is with the flat thalloid liverworts, since both hug the same damp bare ground. A hornwort thallus is usually smaller, a duller and darker green, and more rounded, without the neat repeated forking of a liverwort and without the little gemma cups a Marchantia wears; those cups get their due over on liverworts, moss's quieter cousins. What clinches it is the sporophyte. A liverwort throws up a short-lived stalk topped by a rounded capsule that soon shrivels, whereas a hornwort raises that persistent green horn, and nothing else in a damp British corner does quite the same thing.
Not the aquarium plant
A word about the name before it trips you up. The 'hornwort' sold for ponds and fish tanks, Ceratophyllum demersum, is a flowering plant with whorls of stiff needle-like leaves and no connection whatever to these bryophytes; the shared name is pure coincidence, and anyone who keeps a tank has almost certainly met that impostor rather than the real article (the mosses that genuinely belong under water are gathered in aquarium moss). About the true hornworts I will be plain: they are the one bryophyte group I still find hardest to catch in the act. I have turned them up on greenhouse pot soil and along a damp field margin, but for the cell-level detail I lean on the references rather than my own hand lens. If you fancy a small autumn hunt with a genuine prize at the end of it, bare damp soil a few days after rain is where to point your eyes.