Identification notes
Grimmia ovalis is a rare moss, but may be locally common as a member of the thermophilic community of bryophytes to be found on south facing stone roof tiles quarried from the sandstone of the Black Mountains in Herefordshire and adjacent counties. It forms relatively large flat-topped cushions and patches, rather like the shape of half a bun or a teacake. When dry they appear hoary because of the hair points but are more blackish when damp.
The leaves appear dark because the upper lamina is multistratose and thickened, often merging into the indistinct nerve. They seem quite narrowly lanceolate and tapering because the lamina is curved like the underside of a rowing boat, with the plane margins facing upwards, creating a channel down the upper leaf.
The marginal basal cells have transverse walls more heavily thickened than the longitudinal and the basal cells next to the nerve are rectangular, usually 4-8 times as long as wide.
Grimmia laevigata grows in similar habitats and also has a thickened blackish leaf which is a much wider shape, being triangular than obtuse. Grimmia pulvinata is a very common moss on roofs and walls with a broader, elliptical, unicellular and more translucent leaf, and it fruits readily on arcuate setae that curve downwards. Grimmia ovalis rarely fruits.
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