During a recent visit to Gogerddan, home of the ecology department of the University of Aberystwyth, a small patch of one of the hornworts was found; this was shown to be Phaeoceros carolinianus, new for the county of Cardiganshire.  Subsequent visits to that particular field and several adjacent ones revealed that all 4 British species of hornwort grow there, an occurrence only known from 2 other sites (near Monmouth and somewhere in the Scottish Borders), according to Sam.  The fields are otherwise unremarkable:

Field with 3 of the British species of hornwort.  The fourth species is in the next field.

Between the weeds the soil is quite light, sandy in parts, and fairly well drained.  Hornworts grow in patches, the 2 species of Anthoceros tending to produce rather nice rosettes:

Anthoceros agrestis (left) and A. punctatus (right).

The two species of Phaeoceros are more untidy (often much more so than those in the next picture which have been trimmed):

Phaeoceros laevis (female, left) and P. carolinianus (right).  All the bumps on the surface of the P. laevis specimen are emerging horns.  Small, yellowish antheridial bumps on the P. carolinianus specimen are hard to make out here but quite obvious under a hand lens or a dissecting microscope.

There are many differences between the two genera but Anthoceros have incised margins that appear frilly in the field, the effect being enhanced by numerous small vertical lamellae scattered over the top surface.  Phaeoceros thalli have rounded lobes at the margins and are more or less smooth but beware of the frilly edges to some of the antheridial pits (they start out as small covered chambers on the top surface of the thallus).  There are differences in the antheridia, and the spore colour and ornamentation too if you need confirmation.

It’s a good plan to start by looking for antheridia.  In 3 of the species these are on the same thalli as the spore-bearing horns, it’s only P. laevis that is dioicous and then male plants tend to be very obvious because it also has the largest antheridia of them all (1/4 mm long):

Part of a male thallus of P. laevis, almost covered by male ‘pits’, each containing one or two large yellow antheridia

The antheridia are quite different between the two genera.  In Phaeoceros they are many-celled, the cells arranged apparently randomly:

Phaeoceros laevis antheridia

The antheridia in Phaeoceros are nearly spherical whereas those of Anthoceros are much more elongate and more numerous in the pits.

Anthoceros punctatus antheridia

Note that the granular cell contents can obscure the cell walls until maturity but then the 4 tiers of cells in Anthoceros become easy to see.

To distinguish the two species of Anthoceros it is necessary to measure the antheridia, if the body is more than 100 microns long it is bound to be A. punctatus but see Jean Paton’s book for the precise ranges.  A clue can be obtained from the time of year however since A. agrestis is a summer annual and only likely to be found with horns in late summer to early autumn.  A. punctatus can be found fruiting at any time.

The two species of Phaeoceros can be told by their sexuality having first made certain of the genus but note that it can sometimes be difficult to make out whether the horns are on the same thallus as male pits or are poking out from underneath an overlying male thallus.  In the case of P. carolinianus they have to be proven to be on the same thallus:

Phaeoceros carolinianus thallus showing male pit with dull yellow antheridium just visible inside it and the beginnings of a horn at the bottom of the picture, definitely on the same thallus

In addition, there is a useful difference in size of the antheridia, with those of P. laevis sometimes exceeding 250 microns in body length (down to 180 microns) but in P. carolinianus only about 140 microns typically.  It’s certainly worth looking for elaters too if ripe horns are present as they provide yet another means of verifying the species.

Hornworts are fascinating; they have many fundamental differences compared to mosses and liverworts such as the chlorophyllose horns, roughly equivalent to the seta in a moss.  One feature, which is shared with a few liverworts, is the presence of Nostoc colonies inside the thallus.  These appear as dark blobs:

A Nostoc colony on a fragment of Anthoceros thallus (small dark patch right of centre)

Inside the blob there are numerous small algal cells arranged in short chains.  The Nostoc fixes nitrogen from the air in return for a stable environment (diazotrophic symbiosis):

Nostoc punctiforme cells living as a symbiont inside the thallus of a hornwort

In Sussex, two species of hornwort can be found at Wakehurst Place just near the bridge in the rock garden.  A. punctatus is on the bridge itself and adjacent rocks and P. laevis is on the stream bank downstream from the bridge.  A. agrestis is abundant in a field at Wellingham and has been found elsewhere in arable fields, usually in the late summer.  P. carolinianus has one old record by Nicholson but, as a suggestion, could be searched for in gardens such as Wakehurst or Scotney.  As this article shows, they can all grow together so having found one it could well be worthwhile continuing looking.  An old record for Blasia pusilla at Wakehurst also needs refinding and was possibly in that same general area – it can look like a bit like a hornwort until you get to know them.