Identification notes
Every bryologist’s bedtime reading is, of course, Smith’s flora, arming them in advance with a knowledge of juicy bryological gems they may encounter on their travels. A study of the account of this moss leaves you with the impression that it is pretty much impossible to tell from the very common Rhytidiadelphus squarrosus, and therefore best left to the experts to find. You are, of course, unlikely to encounter it very often in Britain, as it is extremely rare, but if you do, on damp soil by a stream bank in the west, it may well catch your eye. What Smith doesn’t point out, is that this species doesn’t really look much like R. squarrosus in its ‘jizz’. In fact, you might pick it up as possible Loeskeobryum brevirostre, worth a second look through the hand lens. Then you will notice that it doesn’t look quite right for that species, as the leaves on the shoot tips are squarrose. More importantly, in L. brevirostre the stems and branches are covered in tiny paraphyllia, like fine fur, structures which are absent from R. subpinnatus. It doesn’t look right for R. squarrosus either, as the stem is visible between the leaves, the bases of which do not sheathe the stem as they do in that species. Also, the branch leaves in R. subpinnatus are of a different shape from its stem leaves and not squarrose, looking like the branch leaves of Loeskeobryum brevirostre, but without the kink which characterizes the leaves of this species when viewed side-on. That is why everyone says it catches your eye as a weird Loeskeobryum at first sight.
At this stage, it is recommended to take a stem or two (no more) back home and to ask the referee to confirm your identification. This is one of those mosses that has a distinctive look in the field, but little extra information is revealed by the microscope as there isn’t really a ‘clincher’ character to separate it from R. squarrosus. Some say that the outer layer of the stem cross-section, ‘the cortex’ is thicker (4 or 5 layers of cells) in R. squarrosus than in R. subpinnatus ((1-)2 to 3 layers), but not every modern flora mentions this. The leaf base orientation, branch leaf shape and general appearance will point you in the right direction though, and we all enjoy picking out a moss that is easily overlooked.
Read the Field Guide account
