Magdalen Road at night

Magdalen Road at night
December 2010

Thursday, 19 January 2017

Have you ever noticed white patches on the walls of St Leonard's?



No, it isn’t the work of a careless painter, nor the result of someone disposing of their chewing gum, it’s natural.  And not just white patches, but yellow, green, red and brown; it’s lichen, moss and some kinds of algae. 

One of the must-have DVDs of Christmas 2016 was the newly released film of Swallows and Amazons.  Before the film-makers decided to introduce a hint of espionage, there was a sub-plot in Arthur Ransome’s book about Captain Flint’s book of his travels, “Mixed Moss” by “A Rolling Stone”.  Ransome was referring to the proverb, “a rolling stone gathers no moss”, a reminder that moss grows slowly and likes steady conditions.  Moss can be found in damp, shady parts of the neighbourhood, a nuisance to those whose lawns are “grass mixed with moss”, but a way of softening the hard, harsh surfaces of pavements and walls.  Biologists will tell us that there are over 10,000 species of moss, flowerless plants which spread by spores, not seeds.  Gardeners rely on moss as a constituent of peat – suffering from over-use these days – and florists use moss for decoration and to retain water (many mosses can hold over 10 times their weight of water).  In the First World War, moss was collected and used as a dressing for wounds – a memorial at Widecombe is a reminder of the amount collected on Dartmoor.  Moss was superior to cotton as a dressing for wounds, being mildly antiseptic.  There is speculation that some birds choose to use moss in their nests for the health of their chicks.  So, if the moss on your patio or roof is being pecked by birds, it may be for the insects living there, or to help raise the next generation.

Lichens also generally prefer damp conditions; their biology is more complex than that of mosses, and even now there are some aspects of how lichens live that are not fully understood.  The professional society that studies them explains that “A lichen is not a single organism; it is a stable symbiotic association between a fungus and algae and/or cyanobacteria.”  And I am not going to attempt to explain that here!  

Around the neighbourhood, there are countless patches of what is called a crustose lichen (crustose simply means that it has a crust) which can be mistaken for chewing gum on pavements.  (Not many lichens have common English names, and I will leave Latin in the Neighbourhood News to the editor’s fluency.)  The commonest species here are very tolerant to urban pollution; generally, though, lichens prefer an unpolluted environment, and so you find more different lichens where the air is cleaner.  (My brother points out that he finds many more lichens when he visits Exeter than in his workplace of Sussex.)  Next time you are walking in the neighbourhood, stop to look at the crusty lichens on the walls.  Walls with a lot of lichens are providing the right kind of conditions; acidity, moisture, quality of air, chemicals in the stone.  So glazed bricks don’t grow many lichens; older bricks often do.  Lichens are scarce on local granite features, even though the tors of Dartmoor are home to many.  It is all to do with moisture and pollution.  Lichens colonise some pavements more than others; it may be the type of surface, or simply the amount of foot traffic from passers-by.  Have a look under a roadside bench to see how they spread where they are undisturbed.  Different crustose lichens can be found on the bark of trees, usually on the north side of the trunk or branch, because that is moister.  But you may notice that some species of tree have no lichen, while a neighbouring tree has plenty.  Trees differ in the acidity and nutrient content of their bark, and lichens grow where the conditions are just right.

Older garden trees may have fruity or fibrous lichens, and again, the number and type of lichen depends on the species of the tree.  Generally, the tree will not suffer from the presence of lichen, so if you have some, rejoice – your garden has pure air.  

What’s the use of lichens?  Of course, you can feed your pet reindeer on lichen; they enjoy it.  Humans sometimes eat lichen, but it is not for everyday diets – unless you are desperate, and even then, take care.  But from prehistoric times, lichens have been used to make dyes for wool, and some weavers and spinners still do this.  Brown, gold, orange, green, purple, blue and red colours are all possible, depending on the species of lichen used and the type of extraction process.  And if your favourite perfume contains oakmoss, you are scented with an extract from that lichen – despite the name!  

The poet John Clare wrote about the natural world.  It is perhaps appropriate that his gravestone in Helpston churchyard in Northamptonshire is covered with lichen.  The lichen has spread into the inscription so that his epitaph now reads “A POET IS BORN NOT MAD”, hiding the final “E”.

(Published in Neighbourhood News, Jan-Feb 2017)