Mosses in the genus Syntrichia growing on trees, walls and paths, can usually be picked out fairly readily – smallish acrocarps, with dark green twisted leaves when dry, ending in whitish hair points. The challenge is identifying which one you’ve got. This is one of the smaller ones (leaves about 2mm.), with a slight constriction of the leaf near the middle and a toothed hair point visible under the hand lens. The hair point is often slightly reflexed and where it joins the leaf, the rounded leaf tip often has a slight notch.
You might come across this moss when looking at trees or walls, but it can be actively sought on grotty paths in churchyards for example, where a little work with the knee pads may prove fruitful. It can be easily overlooked, so be conscientious.
Tortula muralis can be quickly dismissed as the leaf margins are clearly recurved along the whole leaf length and the hair tip is untoothed. S. ruralis is a much bigger plant and does not have a constriction at mid-leaf. On trees S. laevipila can be found, but has a smooth hair point. The most likely source of confusion is S. montana, which often shares pretty much the same habitat as S. virescens and looks very similar. It is usually a bit bigger and does not have the notched leaf tip and reflexed hair point, but it is safest to take back any montana/virescens material for checking. A transverse leaf section shows a very clear difference in the nerve structure: In S. virescens there are only one or two layers of thick-walled ‘stereid’ cells on the abaxial side of the nerve section, whereas in S. montana there will be 5 or 6 such layers.Read the Field Guide account