Polytrichum commune Hedw.   

Kingdom: Plantae
Phylum: Bryophyta
Class: Polytrichopsida – hair-mosses
Order: Polytrichales

British distribution: Throughout Britain, common in the north and west.
World distribution: Throughout most of the world.

A common moss of wet, base-poor situations, primarily on moorland and in acid, swampy woodland. Usually it grows mixed with Sphagnum species, especially S. palustre and S. recurvum, as here, photographed on the edge of a plantation on Gleniffer Braes (Renfrewshire), or in the lagg area at the edge of a raised bog.

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.

Stem, transverse section, from commercial, prepared microscope slide.

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.
Surrounding the hadrom is a ring (as seen in cross-section) of smaller cells, constituting the leptom. This is analogous to phloem.

(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.)


Leaf, transverse section
, from commercial, prepared, stained, microscope slide.

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.

Chloroplasts can be seen here in the cells of the lamellae. Whereas most moss leaves consist of a single plate of cells, exposed to the environment on both surfaces (enabling water uptake but also water loss), the leaves of Polytrichum are thicker and have differentiated photosynthetic tissue. Humid air is trapped between the lamellae, the terminal, thick-walled cells no doubt acting to protect the tissue beneath. There is sufficient gas exchange for photosynthesis, while water loss is minimised.

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.

P. commune is generally an easily identified species but, in drier situations on moorland and in base-poor woodland, smaller plants are not easy to tell from Polytrichastrum formosum. Capsule characters usually provide ready distinction, being consistantly 4-angled in P. commune and mostly 6-angled (but some individual capsules 4-angled) in P. formosum. When well-formed capsules are unavailable, a cross section of the photosynthetic lamellae can be necessary. The terminal cells of the lamellae are distinctly notched, as shown above. In P. formosum, these cells are rounded in cross-section. The generic difference lies in fine details of the peristome.

© A.J. Silverside
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