Plant 273

Polytrichum commune Hedw. (Polytrichaceae)

Common hair moss

Common hair moss is a cosmopolitan species of temperate and boreal latitudes in the Northern Hemisphere, with a range that extends into Central America, New Zealand and Australia. It is typically a plant of nutrient-poor bogs, wet heaths and forest water courses. Furthermore, its striking appearance means it is unlikely to be confused with any other mosses.

Historically, few mosses have been either visible enough or useful enough to have had common names bestowed upon them. One of the old English common names of the hair moss, great goldilocks, is a reference to the golden-brown, hairy cap that covers the spore-producing capsule. Numerous medicinal properties have been attributed to hair moss but, in the eighteenth century, Johann Dillenius, the first Sherardian Professor of Botany in Oxford and an expert on mosses, was sceptical of the 'many ridiculous and unbelievably generous' claims that have been made. Other uses for hair moss, such as pillow and mattress stuffing and in horticulture, have more foundations, although the moss' use in Iron Age and Bronze Age ropes has been disputed.

Polytrichum commune is exceptionally large for a moss, and can reach up to 70 centimetres tall, although more usually it is less than 20 centimetres tall. Because of its size, Polytrichum commune cannot survive using the water that passes across its leaf surfaces alone. Water must be transported from the base of the plant. This means that, despite being a non-vascular plant, it contains a very simple system of water-conducting cells, unlike other mosses.

The long, awl-shaped leaves, sticking out from the main stem, give the damp plant the appearance of a bottlebrush. The upper surfaces of the leaves are covered with ridges of photosynthetic cells, which are an adaptation to drying conditions the moss may experience in windy conditions. Layers of moist air, trapped between the ridges, prevent leaves from drying out completely.

Polytrichum commune is dioecious, there are separate male and female plants. As with all mosses, the most prominent part of the plant is the green, bottlebrush stage. Sperm from male plants swim through water films to fertilise the eggs of female plants to produce embryos. During the summer months, the embryos develop, and female plants produce box-like capsules, on stalks up to nine centimetres long. When the pepper-pot-like capsule matures, tiny spores are produced which are dispersed by the wind. They will germinate, eventually producing the bottlebrush stage once again.

Further reading

Porley R and Hodgetts N 2005. Mosses & liverworts. Collins.

Sarafis V 1971. A biological account of Polytrichum commune. New Zealand Journal of Botany 9: 711-724.

Stephen Harris