Identification notes
When A. punctatus has ‘horns’ (slender bivalved sporophytes) it is immediately identifiable as one of our four species of hornworts. Without them, it can look like a number of thallose liverworts (especially Blasia pusilla). Its identity can be pinpointed by a hand-lens search for the small scattered opaque dots in the thallus that are colonies of Nostoc, a cyanobacterium. All of our hornworts have these, as does Blasia pusilla, although the colonies in that species are restricted to a single line on each side of the thallus rather than being scattered about. Hornworts also lack oil-bodies and are differentiated from all liverworts by large solitary plate-like chloroplasts in gametophyte cells.
All but one (Phaeoceros laevis) of our hornworts are monoicous and even if horns are lacking or immature, antheridial cavities can usually be found – these look like small warts or pits scattered on the upper thallus surface. Each contains a number of small orange stalked antheridia which can be forced out by applying gentle pressure to the mouth of the cavity with a pair of forceps. Measuring their width under a high power microscope is important in determining the species, and even the genus – the antheridia of Anthoceros being much smaller than Phaeoceros. Well-grown A. punctatus is also usually obviously frilly-looking and slightly transparent, whereas Phaeoceros species have opaque, leathery-looking thalli that are not deeply incised.
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