A delightful, minute moss that is guaranteed to set the pulse racing whenever it is found. It is so small that it is usually only encountered during hands and knees bryologising, and especially during close scrutiny of slightly acid soils where disturbance prevents other species gaining much ground. Think ant-hill tops, entrances to rabbit burrows and eroded banks in sheltered places.
Until 2006 and the publication of the European checklist of mosses, A. muticum included two varieties, segregated by the ornamentation of their spores. Now, however, A. muticum only includes plants with finely granulose spores. Those with spinulose spores, formerly known as A. muticum var. mediterraneum, are now elevated to species status as A. mediterraneum.
All Acaulon species are tiny, bud-like and cleistocarpous. Because of the way the upper leaves of A. muticum are convolute (they wrap around each other to form a sheath) it is not always possible to see the capsules within, nor the diagnostic coarse teeth near the apex of the leaves. Under the microscope, however, A. muticum reveals itself as a very distinctive-looking little moss.
Beware small pretenders! The male plants of some dioicous Bryum species have brown perigonia at the tips of their shoots which can easily pass as capsules. Examination with a dissecting microscope will always reveal the clusters of antheridia within. Tortula acaulon is normally a bigger moss but small forms do exist and also has convolute-looking perichaetial leaves, it is wise to examine the leaves of your plant with a compound microscope to make sure it isn’t that species.Read the Field Guide account