Wall Screw Moss and Grimmia pulvinata

I am attempting to identify all the life in my garden. In recent postings I have investigated a few of my garden's common birds and a common weed. It is time to take on a more ambitious identification challenge: moss.

I have not hitherto taken much notice of moss! I have to say however, that having begun to look for it, it does have a certain appeal for the amateur naturalist. Firstly, a lot of it is really rather beautiful once you begin to examine it closely, and secondly you can be pretty much guaranteed to find some to study any time you go outdoors at any time of year (c.f. say, wildflowers, butterflies etc.).

Common on my garden wall (around (1.8,1.0 see The Lie of the Land) see are the small moss 'cushions' seen in the photo 1. Following some head scratching (see below) I think I'm confident in stating that there are two distinct species of moss at work here.

Photo 2 shows things more clearly. The left hand moss clearly has elongated thin spore capsules on upright stalks ('setae'). Clicking on photo 2 to enlarge it and looking carefully you'll see the right hand moss is also 'fruiting', but its setae curl back on themselves to 'bury' the spore capsules amongst the leaves. After many hours work I'm reasonably confident in identifying the left as Wall Screw Moss (Tortula muralis) and the right as Grimmia pulvinata (for which I can't find a common name - comments anyone?).


The obvious challenge in identifying a moss is the small size of any distinguishing features. Recently however I purchased a 'Westbury SP30' student microscope from Brunel Microscopes (for anyone looking to do similar, I'd recommend the company - extremely helpful on the phone and they delivered the 'scope in 48hrs).

It turns out that moss leaves make inherently 'good viewing' under the microscope since in most cases the leaf consists of simply a monolayer of cells. Putting a moss leaf under the microscope, you immediately get a view of living cells without any of the 'messing about' with microtomes and stains I believe is often required with other 'higher' plants. Photo 3 shows the fairly regular cells of G. pulvinata (100x; 1 small graticule division = 10um) and photo 4 of T. muralis (450x; 1 small div. =2.5microns) - which remind me of miniature four leaf clovers.

The most striking thing about both the leaves of G. pulvinata (photo 5) and T. muralis (photo 6) under low magnification (100x, 1small div.=10um) are the inordinately long white hairs at the end of the leaf. These hairs give the 'cushions' their silvery look in photo 1 above. With regard to telling the leaves apart, photo 6 shows that the more rounded ('ovate') leaf ending of T. muralis. A second differentiating feature of G. pulvinata are the tiny spines protruding from the leaf hairs (photo 7, 400x) (the leaf hairs of T. muralis are smooth).

To identify my mosses I used the key in British Mosses and Liverworts, E.V. Watson (Cambridge Univ. Press). Helpfully, Andrew Spink has reproduced this key on his website. The images of T. muralis in the British Bryological Society Field Guide were also useful. (I couldn't find G. pulvinata there - does it have another name?). I took the microscopic photos by simply holding the lens of my camera up to the eye piece of my microscope (hence the reason they're not very good!).

Finally - as an illustration of how small the world is (or how big the internet depending on your point of view) - I discovered in the course of my searching that Patrick Roper has also been busy this month blogging about T. muralis. Great minds think alike..?