There has been much confusion between D. acutus s.str. and D. icmadophilus in Britain, and it was recently realised that many of our records for D. acutus are in fact D. icmadophilus. The links for the Field Guide account and 2014 Atlas distribution are included here as it is likely that the distribution in particular relates more to D. icmadophilus than D. acutus. Further information can be found in the resources given below; the descriptions in Smith (2004) are no longer considered accurate.
The D. icmadophilus/D. acutus species pair is normally quite easy to recognise; both are spiky-looking, erect plants with leaves that point upwards more than D. rigidulus and D. insulanus, both of which also have lanceolate leaves. It’s necessary to examine the ventral (uppermost) cells of the costa in the upper leaf under a microscope to affirm they are short – quadrate to shortly rectangular. Leaves of both species are also completely unistratose (leaf sections are the best way to check this character).
The differences between the two species can be quite subtle but the most reliable characters are the papillosity of the mid-upper leaf cells and dorsal surface of the costa (D. icmadophilus has papillae, D. acutus has smooth cells) and the excurrency of the costa in the upper leaves (usually long and slender in D. icmadophilus, short or long but not needle-like in D. acutus). The shape of the mid-upper leaf cells can also be helpful – in D. icmadophilus they are quadrate or at least angled but in D. acutus they are rounded or lacking angles. There is however some overlap in this character so it cannot be relied on. The colour of the plants can be helpful too – D. icmadophilus tends to look green-brown, whereas shoots of D. acutus are commonly quite deep brown.Read the Field Guide account