Twister

To appreciate the true beauty of mosses you really need to explore them with a hand lens or low power microscope. This is Tortula muralis, wall screw-moss and to find out how it acquired that colloquial name you need to take a close look at the spore capsules.

Wall screw moss grows in the mortar-filled cracks in walls, where it produces spore capsules that are carried aloft on stalks that are a couple of centimetres long at maturity. These are capsules in the very early stages of development, before their stalks lengthen, but if you take a really close look at a mature spore capsule....

 ...it looks like this. Notice how the capsule's stalk (seta) has twisted helically. If you take a close look at the capsule (double click on the image for an enlarged version) you can see that most of it is sheathed in a membranous covering - the calyptra. Gently pulling this off with a pair of forceps reveals....

 ... a lidded capsule underneath and if you pull the lid (operculum) away....

... it reveals a screw thread-like arrangement (the peristome) underneath, that gives the moss its common name. These threads twist up tightly in moist air but untwist in a dry atmosphere, allowing the minute spores to be shaken out when the seta trembles in the wind. In this image you can see that the operculum that has been removed has become temporarily stuck to the base of the capsule - normally it will just fall away.

Living sponge

Bright green patches of bog moss (Sphagnum) thrive in wet hollows on the fell tops. Step into one of these and you’ll suddenly find that you’ve got a boot full of water, because this moss acts like a living sponge. Each plant constantly grows from its apex and dies from its base and the accumulated weight of living plant crushes layers of dead moss underneath, which ultimately form peat. It’s the plant’s ability to retain water, even in dry summers, that makes Sphagnum bogs such important wildlife habitats for moisture-loving wildlife. You need to look at the minute leaves under the microscope to see how they do this. Magnify the leaves a little and you can see that each leaf is a network of cells. Increase the magnification a little more and two kinds of cells are revealed – green photosynthetic ones (the living part of the leaf) and empty, transparent dead ones. The photosynthetic cells form a living network, enmeshing the dead ones. Increase the magnification further and you can see the structure of each dead cell, it’s shape maintained by beams of thick cell wall material, with a hole in each cell wall. Once these dead cells fill with water capillarity holds it firmly in place. Squeeze the moss and water flows out like water from a sponge. Tread on one of those bright green patches and the water fills your boot..........and then it's wet socks for the rest of the walk.