Identification notes
In the UK, most of the D. tophaceus we find is subsp. tophaceus. However in spring 2021, Didymodon tophaceus subsp. sicculus was found by Peter Martin growing on Lundy Island off the North Devon coast. It was new to Britain and amongst other characters is distinguished from subsp. tophaceus by its nerve being narrower and having shorter cells over the ventral surface.
D. tophaceus subsp. tophaceus is quite a common moss in many areas (more so in the west) but it is very variable and often foxes people. Despite what some floras say, it doesn’t always have blunt leaf tips so that’s not a reliable identification character. Better, in the field at least, is its colour – often tinged pink but approaching deep brown in open situations, especially near the sea. Even green plants in shade usually have a strong pink-tinged nerve, a character visible with a hand-lens.
Another good indicator is its preference for damp, strongly base-rich places, although it seems rather indifferent to substrate otherwise, being found on soil (especially heavy calcareous clay), masonry and, of course petrifying springs (tufa) from which its species name is derived. When it produces its capsules, the seta looks very long for such a small moss and can be quite wavy.
Small plants on damp limestone sometimes resemble Gymnostomum calcareum or G. aeruginosum which also have strong nerves but those species lack the recurved leaf margins of D. tophaceus and have strongly papillose leaf cells.
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