Identification notes
This is now such a familiar species of semi-urban situations that it can be hard to believe that D. nicholsonii was once a nationally scarce moss. Like Syntrichia latifolia and Dialytrichia mucronata, which are also species of situations where intermittent inundation by silt-laden river water is commonplace, D. nicholsonii has a secondary anthropogenic habitat where it is now mainly found: on the damp tarmac of pavements, driveways and little-used rural lanes.
With practice, it is possible to identify large populations of D. nicholsonii on tarmac from a distance – they have a characteristic colour and gregarious habit. Once you are familiar with this, and can recognise the way it holds its leaves, it is not a difficult species to recognise. Didymodon luridus tends to prefer more basic situations and rarely forms large, extensive ‘turfs’ like D. nicholsonii but confusion in the field is sometimes possible. In such situations check the leaf under a microscope – with a good quality dissecting scope illuminated from below you may be able to see the bistratose upper leaf margins of D. nicholsonii, which are lacking in D. luridus, which has unistratose leaves. However, both species have recurved leaf margins which can make this a tricky character to see. Doubt can however be dispelled by cutting a transverse leaf section of the upper leaf – but do use a new razor blade. If you’ve got this far you should also be able to count the number of rows of guide cells in the nerve section – D. nicholsonii has two whilst D. luridus has only one.
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