Identification notes
Generally, Plagiomnium species are easy to place in the genus, but the round-leaved species can look very similar so what then?
P. affine is normally a species of moist but well-drained habitats such as grassland and woodland, so there is little scope for confusion with P. elatum or P. ellipticum, both of which prefer distinctly wet ground. P. cuspidatum also grows in drier habitats but has distinctive-looking leaves which are sharply and conspicuously toothed from about halfway up.
The main problem with P. affine is that it can closely resemble P. rostratum, which may share some of its habitats. The books say that P. affine has a narrowly decurrent leaf base whereas P. rostratum is not decurrent at all. However, in practice, the short wing of leaf tissue running down onto the stem of P. affine is so narrow that it may be hardly visible with a hand-lens, so it is a good idea to collect a stem and check it properly under a dissecting microscope.
In the event of any doubt, the marginal leaf teeth can be useful – those of P. affine are usually well-developed, sharp and up to 3 cells long, in contrast to the teeth of P. rostratum which are short, blunt and only 1 cell long. Of course, if you’re lucky enough to find plants with capsules, then the chances are it won’t be P. affine, which very rarely has them. Capsules with a long, beaked (rostrate) lid will belong to P. rostratum (the clue is in the name!) which quite often produces them.
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