Polytrichum commune

Polytrichum commune is an acrocarpous moss in the family Polytrichaceae. Its stems usually grow upright, generally without branching, and can form dense clumps or extensive turfs. The foliage is dark green, although it may take on brownish tones with age. The narrow leaves are arranged around the stem and have a sheathing base that wraps around it, as well as a prominent central nerve. Longitudinal lamellae of photosynthetic cells are prominent on the upper surface of the leaf. The leaves change their arrangement with moisture: when hydrated, they spread away from the stem and curve outward; when dry, they move closer to the axis and may fold around it. This variation accompanies its growth in moist environments and its response to drier conditions.

The species grows on soil in moist, often acidic places. Habitats cited for it include peatlands, marshes, pond margins, wet heaths, and forested banks along watercourses. It is also found in swampy forest sites. Its lamellae increase the photosynthetic surface area of the leaves; in addition, water has been described as being conducted from the base through differentiated tissues in the stem. P. commune is dioecious: male and female reproductive organs form on separate plants. Sperm cells are produced on male plants; rain may carry some of them to female plants, where the organs containing the egg cells are found.

After fertilization, the zygote gives rise to a sporophyte that remains attached to the plant supporting it. The sporophyte develops a seta that raises a capsule; the capsule produces spores, which may germinate when released and form a protonema. Shoots appear on the protonema and give rise to new plants with stems and leaves, the visible gametophyte phase of the moss. The capsule is angular and may be covered by a calyptra when young; at maturity, it is elevated on its seta. Thus, the life cycle alternates between the gametophyte, which produces the sex organs, and the sporophyte, which forms the spores. Occurrences of the species have been documented in a boreal swamp forest in Russia’s Arkhangelsk region, on Fengyang Mountain in Zhejiang, China, and at sampling locations in Poland.