Polytrichum

Polytrichum
Male gametophytes of Polytrichum strictum bearing antheridia.
Scientific classification
Kingdom:	Plantae
Division:	Bryophyta
Class:	Polytrichopsida
Subclass:	Polytrichidae
Order:	Polytrichales
Family:	Polytrichaceae
Genus:	Polytrichum Hedw.

Polytrichum is a genus of mosses — commonly called haircap moss or hair moss — which contains approximately 70 species that cover a cosmopolitan distribution. (Less common vernacular names include bird wheat and pigeon wheat.)

The Polytrichum genus has a number of closely related sporophytic characters. The scientific name is derived from the Ancient Greek words polys, meaning "many", and thrix, meaning "hair". This name was used in ancient times to refer to plants with fine, hairlike parts, including mosses, but this application specifically refers to the hairy calyptras found on young sporophytes. There are two major sections of Polytrichum species. The first — section Polytrichum — has narrow, toothed, and relatively erect leaf margins. The other — section Juniperifolia — has broad, entire, and sharply inflexed leaf margins that enclose the lamellae on the upper leaf surface.

With a distinct appearance the Common Hair Cap Moss gets its name from the hairs that cover, or cap, the calyptra where each spore case is held (1). Looking down on it, the Common Hair Cap Moss has a star shaped appearance because of the pointed leaves arranged spirally at right angles around a stiff stem (3). Like other mosses, it is generally a dark green colour and doesn’t grow very tall. The Common Hair Cap Moss has no woody tissue so it only grows from 4–20 cm tall (2). Growing like a lush green carpet, the average life span of this moss is three to five years, although ten has been recorded, and even dead the moss remains intact, and is what makes up the lower portion of this organism

Mosses in the genus Polytrichum are endohydric, meaning water must be conducted from the base of the plant. While mosses are considered non-vascular plants, those of Polytrichum show clear differentiation of water conducting tissue. One of these water conducting tissues is termed the hydrome, which makes up the central cylinder of stem tissue. It consists of cells with a relatively wide diameter called hydroids, which conduct water. This tissue is analogous to xylem in higher plants. The other tissue is called leptome, which surrounds the hydrome, contains smaller cells and is analogous to phloem.

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