Theoretically this is quite an easy species to identify when it is growing in dense cushions on limestone or other base-rich rock in the hills and the leaf tips are blunt, as the books tell you they should be. However, it can be remarkably variable and atypical forms can cause confusion with other narrow-leaved acrocarps of base-rich rocks.
When the leaves are more acute-tipped, there are a couple of useful clues that should point you towards Gymnostomum. By far the best of these in the field is colour. The leaves have a really strong, red or pink nerve – it’s much wider than similar-looking acrocarps – and these colours are conspicuous in older leaves, giving the lower parts of plants a rusty red appearance. Under high magnification you’ll notice small projecting papillae on the leaf margin – these give an irregular outline and are characteristic of Gymnostomum species. Near the base of the leaf obscure teeth are sometimes present and then you have to be careful not to confuse it with Eucladium verticillatum, which in any case has a more slender nerve and lacks red pigments.
Did you know that Gymnostomum is named from its gymnostomous capsules, which lack a peristome?Read the Field Guide account