Wall Mosses at Monk Bretton Priory
For me, before hoverflies there were bryophytes, as it was in evolutionary history. Bryophytes are a group of plants including mosses, liverworts and hornwort which have been on planet earth for half a billion years, evolving from aquatic algal ancestors. One of the earliest diverging lineages of extant land plants.
I got into bryophytes whilst studying at university, a project on a field course and I have been fascinated with them ever since. The surprise and wonder that first captivated me was that these tiny plants, of which there are 1000 plus in the UK and over 24,000 in the world, show so much diversity that is not obvious with a glance. These tiny flowerless plants helped shaped the existence of life on earth and could do so in the future with the colonisation of Mars, more about this later. They are as fascinating and wondrous as any other living organism if one takes the time to look.
This article is about wall mosses; any wall will do it doesn’t have to be a wall on an English Heritage site such as a priory, but I couldn’t bring a group of Barnsley Naturalists to a wall outside my house or someone else’s garden, so this became a wonderful setting for a field meet.
Wall mosses are a group of bryophytes that are specialised in dealing with desiccation. They only need some form of anchorage which can be formed by the natural stone surface or with assistance from lichens, which release acids creating little pores within the stone, from which the rhizoids (not true roots) of a moss can get into.
Wall mosses are an easy group to study, especially for beginners because they are easy to spot, humans providing many a substrate for them to attach to, be it roofs, garden walls, tarmac paths or, as in this case, ruins.
Mosses are poikilohydric which means they are very much dependent on their surrounding environment. They are only one cell thick with no system for transporting water from the soil, or waxy cuticle to prevent water loss from their leaves.
Poikilohydric – is term referring to plants that cannot regulate their own internal water content making them entirely dependent on the surrounding environment for hydration.
So, to prevent water loss - wall mosses have some fascinating adaptations:
They grow tightly packed together forming dense cushions or mats, this traps moisture between the shoots and reduces air movement which slows down evaporation. Below is a photograph of Anomalous Bristle Moss (Orthotrichum anomalum) which form these pincushions, especially when they produce sporophytes as in the photograph below.
Many species of wall moss have a hair point (Hyaline point) at the end of the leaf. When the leaf begins to dry out these hair points turn in to prevent water from evaporating and trapping the water between the leaves. The Hedgehog Moss (Grimmia pulvinata) below typically forms these lovely, compact cushions, edged with a silvery grey created by the leaf hair points, which makes it look like a hedgehog. They hide their sporophytes within the cushion.
Water is held in tiny spaces between the leaves and stems by capillary action, helping moss stay wet for longer after rain or dew. The photograph below shows Wall Screw Moss (Tortula muralis) with sporophytes, it is a very common wall moss. Notice how it collects and holds rainwater, each of the sporophytes and hyaline points holding a droplet of water, which can be absorbed through their cell walls.
Mosses can also cling to the wall like this Silky Wall Feather Moss (Homalothecium sericeum) below, where the water will be trapped underneath.
On the flip side, instead of trying to stop water loss completely mosses can survive drying out. When conditions are dry, they shut down metabolism and become dormant. When water returns the moss, being one cell thick, can quickly rehydrate and start photosynthesis. There is a species of moss called the Steppe Screw Moss (Syntrichia caninervis) a desert moss capable of surviving desiccation, freezing and even radiation making it a possibility this could be the first plant to terraform the planet Mars. In the links below is explanation and video of how this moss can extract water from the air to hydrate itself. We don’t get this moss in the UK, but we do have similar species Syntrichia intermedia and ruralis which grow on my roof. Amazing to think that they could have been keeping themselves wet and alive last summer, whilst the garden was bone dry.
See how plants drink water from the air
A lot of bryophytes at this time of year exhibit sporophytes, these they produce to create the next generation. They are often visible more in winter and early spring because the season provides the moist cool conditions that mosses need to fertilise each other, the sperm needing to swim through water to reach the egg. In winter there is also reduced competition and less shading from other plants. For wall mosses there are very few plants that will out shade them, but they still need room to release their spores to the winds, which when they finally settle need moisture to develop.
Below are two beautiful mosses with sporophytes visible at this time of year. I was delighted people got to observe both the Pear Drop Moss (Bryum capillare) with those wonderful green pear-shaped sporophytes and Thickpoint Grimmia (Schistidium crassipilum) with red caps that just poke out from between the leaves.
Eventually these plants will die and compost into soil from which higher plants such as Whitlowgrass (Erophila verna) and Hairy Bittercress (Cardamine hirusta) will get a foothold, a process in ecological terms known as succession. But for now, they provide a wonderful spectacle early spring which should be enjoyed.
If you get a chance in the next few weeks do go and investigate a wall nearby, I highly recommend it. I am positive you will find some, if not all the mosses, I have mentioned in the article above.
If you would like more articles about bryophytes then please let me know in the comments below.
Further reading and sources:
Edwards-2006-Mosses-and-Liverworts-of-Towns-and-Gardens.pdf
FB104_Confusing-urban-mosses-part-1-ground-dwellers.pdf
(PDF) Perspectives of moss species in urban ecosystems and vertical living-architecture: A review
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