Polytrichum commune s.str.

A tall, dioicous, acrocarpous moss which grows in damp, acidic habitats in heathland, on moors, wet woodlands, ditches, streamsides and other damp peaty places. It is the largest bryophyte in the British Flora and often grows in large colonies that form mounds and hummocks (see photos). Typical plants are usually unmistakable but confusion with large specimens of Polytrichum formosum is possible. However, the capsules of Polytrichum commune are formed on a long seta and they are four-angled (like a box) with a distinct constriction or collar at their base (see photo). It is common throughout much of Britain except for parts of the south and east of England and it is ubiquitous in Wales and a common moss in the uplands of West Glamorgan. Sporophyte capsules are commmon in summer.

Polytrichum commune is a classic endohydric moss, i.e. its stems have central strands of conduction tissue (hydroids), a primitive 'plumbing' system that makes internal conduction of water possible. The evolution of well developed endohydric systems in mosses like Polytrichum and Dawsonia (not respresented in the British Flora) have allowed them to develop into tall robust species. In contrast many other bryophytes, particularly those of intermittantly moist and dry habitats, are ectohydric and water conduction in them is predominantly external. In yet other terrestrial species, water relations are mixohydric where either internal or external conduction can be predominant.

Polytrichum commune s.str. is otherwise known as Polytrichum commune var. commune.