This is an easy plant to identify, but only if the tell-tale brown raspberry-like gemmae are present on upper leaf tips. Plants growing in sub-optimal situations or at the drier edge of their range often have few gemmae and then may resemble Grimmia trichophylla or G. lisae.
With a bit of practice, the characteristic growth habit of Grimmia hartmanii becomes familiar – loose tufts of erect, mid-green plants with secund leaves at the shoot tips and short hair-points. Careful scrutiny of such plants in the field with a hand-lens may reward the bryologist with gemmae – they can look a bit like tiny grains of soil on the leaves – but sometimes collection of a few shoots (if there is enough of it) and careful examination under a dissecting microscope is the only way to find them. Under the microscope, leaves which have produced and released gemmae are truncate or mucronate (see image below).
Even when gemmae are lacking, G. hartmanii can be identified with microscope characters. A section of the nerve at the bottom of a leaf will show 6 guide cells (G. trichophylla rarely has more than 4). In transverse section the stem has no central strand, unlike G. lisae. For more information about G. hartmanii and its relatives, visit the Resources section (link below).Read the Field Guide account