British distribution: Throughout Britain, common in the north and west.
The gametophytes are perennial, often showing zones corresponding to two or three years growth. The stems are strong and tall, often reaching 20-30 cm, and, although mosses are said to be "non-vascular plants", P. commune shows fairly clear differentiation of conducting tissue. The species is endohydric and relies primarily on conduction of water from the base of the plant.
The hadrom is the central cylinder of stem tissue, consisting of wide-diameter cells (hydroids), which conduct water. The tissue is analogous to the xylem of higher plants.
(Note that the stem is gametophyte tissue, so there is no direct phylogenetic connection between the hadrom and leptom of Polytrichum and the xylem and phloem of vascular plants, the latter being sporophyte tissues.)
The whole of the upper leaf surface is occupied by a series of parallel photosynthetic lamellae - imagine a series of closely parallel brick walls, each approximately six bricks high.
The leaves also curve and twist around the stem during dry conditions, as a further mechanism to avoid water loss. The teeth along the leaf edge may aid in this process, or else help keep out small invertebrates (unless anyone has a better idea?).
It may seem odd that a plant of such damp habitats shows xeromorphic adaptations, but on open moorland, the drying action of the wind may be considerable. (Consider heather and other plants of the same habitat.)
The protected micro-environment between the lamellae can be a home for other organisms such as rotifers and for a number of minute parasitic fungi.