To begin with, we look at a few mosses commonly found on walls,
especially walls containing basic rocks, rocks and wood in a country
(Wall Screw-moss) (Pottiales)
Above and below: Tortula muralis. This is one of the easiest mosses for beginners to find. This moss is autoicous (autoecious) meaning that male and female organs are in separate clusters on the same plant. When dry the leaves press and twist together, but spread out when moist. Each leaf ends in a whitish translucent hyaline hair. This moss is acrocarpous: forming erect shoots which may end in the reproductive organs and sporophyte (seta + capsule).
The capsule of Tortula muralis is narrow and cylindrical.this moss consists of small cushions (less than 1 cm tall) of upright shoots, each of which may bear a terminal seta and spore capsule (it is an acrocarpous moss). The peristome teeth are long fibers that twist together when moist, closing the capsule ( a hygroscopic lid) but untwist to open the capsule in dry conditions, to allow wind to shake free the spores for dispersal. This spiral closing of the capsule in damp conditions gives the genus its name of Tortula (the Screw Mosses). External link: http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artmar03/dcmoss.html.
The leaves (or phyllids as moss leaves are sometimes called) spread out when moist and turgid, but twist together when dry. Like many mosses they can survive desiccation and revive when moisture returns. Being small plants without the waxy water-proofing cuticles and other adaptations that the leaves of flowering plants have, mosses can not easily retain their water content in dry conditions and so must tolerate periodic desiccation or exist in permanently wet environments. The central nerve of each leaf extends as a silvery hairlike projection from the leaf tip.
Grimmia pulvinata (Grey-cushioned
This is the most common species of the genus Grimmia with an almost global distribution and is particularly common in Britain and on the west coast of North America. It forms round cushions 1 to 2 cm tall. Each leaf bears a silvery hairlike extension at the tip. Grimmia is also autoicous and acrocarpous.
The capsules (below) are cylindrical with distinct longitudinal lines and a very beaked cap (calyptra). The seta recurves (bends back on itself) so that the capsules lie buried within the leaves, but straighten in old and dry mosses to facilitate spore dispersal.
Grimmia pulvinata is a xerophytic moss (adapted to dry conditions) and in experiments it has survived 60 weeks in a desiccator to be revived upon rehydration. This moss grows on basic rocks, including walls and is fairly resistant to pollution so is characteristic of urban environments. It may occasionally be found growing on the bark of woody plants. The capsule has apparent air spaces within it for photosynthesis. The capsules of Grimmia pulvinata have characteristic longitudinal lines and when dry they can be seen to possess 8 longitudinal ribs.
Grimmia pulvinata is a xerophytic moss, meaning that it can tolerate dry conditions, or more specifically periodic drying out. Mosses have differing resistance to desiccation: aquatic mosses like Fontinalis squamosa, which grows in streams and rivers attached to rocks and tree roots, will frequently be unable to recover after a week of air-drying, whilst Grimmia pulvinata has been known to withstand sixty weeks in a desiccator at 20 degrees C (Watson, E. V. 1964 The Structure and Life of Bryophytes, Hutchinson University Library). Thus, the apparently miraculous ability of mosses to revive upon rehydration has its limits and so some mosses have adaptations to reduce the severity of drying out, such as the folding or twisting up of leaves as they dry, although the benefits of thsi are not clear: it possibly helps the moss retain the last drop of water for longer, or protects the brittle dry leaf from mechanical damage or perhaps speeds up water transport by capillary action once water becomes available again. The hair-like (excurrent) extensions of the nerves at the leaf tips is also thought to be a xerophytic adaptation, though the function of such appendages is not clear (I suggest that they may serve to conserve moisture by increasing boundary layer thickness).
The adaptations of xerophytic mosses has been reviewed by Watson, W (New Phytologist, 1914). Tortula muralis is also xerophytic. These include the formation of dense cushions, to retain moisture, as in Grimmia and Tortula Clearly resistance to periodic drying is a useful adaptation for wall-dwelling mosses. Forms of Tortula growing on damp walls may have longer setae when growing on damp walls, presumably to position ripe capsules in the drier air for spore dispersal.
The photo below shows Grimmia pulvinata growing alongside what appears to be Tortula muralis in the hydrated state (with leaves expanded). Note the terminal cups of leaves, these are the perichaetia (sing. perichaetium) consisting of a circle of perichaetial leaves or bracts (which are more or less modified in different mosses) enclosing the female reproductive organs. (A perichaetium encloses either both male or female organs or female organs only, a perigonium of perigonial leaves encloses male organs only).
Click images for full size
Above: Grimmia pulvinata and Tortula muralis.
recurvirostrum ( = Barbula recurvirostrum, Red
This moss grows on basic rocks, including walls, and thin calcareous soil and sometimes in tree boles on flood plains near streams and rivers. It belongs to the order Pottiales. It forms loose turfs / sheets and is up to 5 cm tall. The lower leaves often have a reddish color in which case this is a distinguishing feature. The leaf margins also fold inwards.
The cylindrical capsules are held upright on a seta that is often reddish in color, especially in its lower parts.
This moss has both male and female organs borne on the same plant, typically interspersed on the same cluster or inflorescence (synoicous) sometimes with the male antheridia naked in the axils of the bracts (modified leaves surrounding reproductive organs) - a condition called paroicous.
crassipilum (Thickpoint Grimmia) (Grimmiales)
Above: Schistidium crassipilum (bottom left of center) growing
with Grimmia pulvinata and Tortula muralis. This plant
is common on walls and tarmac (here on a concrete rock) but can also be
found on limestone and basic sandstone rocks.
In Schistidium the capsules remain enclosed by modified leaves with just their tops showing (they are immersed in the perichaetial leaves that surround the female reproductive organs from which the capsules spring). This moss is auoicous and has no or few stomata on its capsules. The immersion of the capsules and the lack of stomatal pores upon it are again adaptations for dry conditions: protecting developing spore-generating tissues from excessive dehydration (once mature the capsules may dry to release the spores that are generally more resistant to dry conditions and indeed more easily dispersed in dry air). In mosses in general, the capsule usually contains between 3 and 200 stomata, presumably to facilitate photosynthesis which occurs in the developing capsule and presumably provides the sporophyte with some of its resources for spore production. These stomata are largely insensitive but will close when the capsule is very dehydrated.
This moss is almost certainly Bryum dichotomum (Bicolored Bryum). Bryum species can, however, be very hard to tell apart, especially since they can be very variable and to be sure rather painstaking microscopical examination may be required. In this species the leaves are short and concave and widest below their midpoints and have nerves running the whole length. This is a short moss, usually less than 1 cm tall and may form compact tufts.
The capsules of Bryum are typically pendulous with mammillate opercula (i.e. the lid of the moss capsule bears a nipple like projection). In Bryum dichotomum the capsules are short and roundish are borne on short, smooth yellow-orange setae.
Below: Some Bryum species produce gemmae (asexual propagules) in the leaf axils. Bryum dichotomum typically has 1-5 gemmae per leaf axis, if they are present at all, whilst the similar Bryum gemmiferum may have 20-30 per leaf axil. The specimen below has an intermediate number (about 10-12 per leaf axil) but this is typical of some Bryum dichotomum populations. The gemmae may each have a pair of leaf primordia and hence resemble miniature immature plants and so are sometimes called bulbils. The length of the awn where the nerve extends from the tip of the leaf is also variable, though generally short.
Bryum dichotomum produces its capsules in autumn and winter and is found on disturbed soil, rocks, walls and cliffs in damp places, such as by water courses. The leaf cells are characteristic short with a number of small chloroplasts.
More on Bryum dichotomum: https://www.britishbryologicalsociety.org.uk/learning/species-finder/bryum-dichotomum-2/
Below: Bryum argenteum (Silver-moss or Silver Thread-moss)
Bryum argenteum is another short moss often forming compact tufts that are usually less than 1 cm tall. To the naked eye it is distinctly whitish or silverish and shiny when damp (grey when dry) due to the whitish leaf tips that make the compact cylindrical shoots appear almost white to the naked eye. The leaves are short, rounded and concave and may have variable extensions at their tips and the nerve does not reach the tip.
Bryum argenteum is found in a variety of disturbed habitats with deposits of shallow soil and rich in nitrate and which are prone to drying, including waste ground, arable fields and a variety of artificial surfaces. For more information see: https://www.britishbryologicalsociety.org.uk/learning/species-finder/bryum-argenteum/
Above: moss growing in a groove on a wooden bench. The slender silvery-white cylindrical shoots with tapered tips belong to Bryum argenteum (the gap on the lower-right is where the Bryum argenteum was sampled). The two other mosses also visible were not sampled or determined. Some of the other mosses present could well be Bryum capillare.
Above: Grimmia pulvinata growing with a Bryum which is probably Bryum capillare - not the spirally twisted leaves of the dry moss and the whitish awns (or hair points, the extensions of the leaf vein) of variable length - both features of Bryum capillare which usually grows on rotting wood and trees. The feather-moss at the back (on the left) was not determined.
Article created: Jan 2020
Article updated: 21 Jan 2021, 29 Jan 2022, 3 April 2022