Wonderwalls

Back in Normal Times, which are now so far in the past they don’t seem normal any more, I cycled an hour each way to work, combining exercise with my commute. Working from home meant I lost part of my fitness regime, so I started cycling for its own sake, exploring the quiet routes around my neighbourhood. My penchant for spotting a beguiling leafy lane and wondering ‘where does this go?’ has resulted in a polite but firm request to get off private land (surely bikes don’t count?) and frequently getting lost (thank heavens for smartphones). It’s also led to the discovery of several plant colonies in what might seem rather peculiar places: walls.

Plants exploiting this seemingly inhospitable habitat are called ‘lithophytes’ – rock-plants. Their natural home is a cliff-face or rocky scree, but if humans obligingly provide a sheltered alternative, they will happily take advantage. While they may appear to be miraculously growing out of nothing at all, most of them do need some kind of fissure, however small, to get going. They rely on their relatives, lichens and mosses, to start this process. These pioneers of bare rock will start to dissolve the stone, creating tiny cracks, and providing the first dusting of organic matter that plants need to get their roots into. Natural erosion and weathering helps too. An old brick wall beginning to crumble, or a dry-stone wall that comes with lots of handy niches built in, provides the foothold these plants need to establish. My own town is a relatively new estate, built from neat, tightly mortared bricks, and I don’t see any wall plants there – so I was delighted to find them on my expeditions.

Wall-rue (Asplenium ruta-muraria) and Rustyback (Asplenium ceterach)

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  Wall-rue

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  Rustyback

These are both little ferns, spreading through tiny wind-borne spores rather than seeds. They need damp places to establish, and the deep cracks of a stone wall seem to suit them well, while dry conditions in summer are perfect for spore distribution – Rustyback gets its name from the prolific coating of spores on its underside. The name Asplenium denotes a member of the spleenwort family, and indeed both of these ferns were once used as herbal remedies for spleen or liver disorders.

Wall pennywort or Navelwort (Umbelicus rupestris)

Mostly found in Wales and the west of England, this extraordinary-looking plant had me braking to a sudden halt when I first saw it in flower. The round, thickened leaves look to me like miniature lily-pads, but they also resemble coins – hence ‘pennywort’. The little dimple where the stalk joins the leaf leads to the name Navelwort, and indeed the Latin Umbelicus. In summer it has long spikes of greeny-white flowers – or to quote John Gerard’s rather enigmatic description, it is ‘beset with many small floures of an overworne incarnate colour’. (?!) These only add to the alien appearance, with the result being a plant that would not look out of place in an episode of Dr Who.

Wall-pepper or biting stonecrop (Sedum acre)

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  Sedum ‘sitting’ on the wall, hence its name

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  Sedum hanging out

‘Sedum’ means sitting – as this plant does on top of a wall – and ‘acre’ means ‘acrid’. This in addition to its common names suggest that this edible plant is quite a piquant addition to salads. The yellow flowers are very cheerful in summer. It has another, quite extraordinary common name; ‘welcome-home-husband-though-never-so-drunk’. Perhaps this was given because, once it has colonised your walls and roofs, the plant is always there when the husband comes home, although the wife might not be.

Ivy-leaved toadflax (Cymbalaria muralis)

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Ivy-leaved toadflax is native to Italy. One suggestion for its arrival here is that it hitched a lift on marble sculptures. From these civilised beginnings it appears to have scrambled up every wall in the country. The pretty purple and yellow flowers look like miniature snapdragons. They grow towards the light, but once the petals fall, the seedheads bend back towards the wall – where they are more likely to find another crack to grow in.  

Tricks of the trade and a CAMazing adaptation

Growing on rocks is tough, so wall plants tend to be specialists. You can recognise this in some of their scientific names. The Latin for wall, murus, appears in muraria and muralis, while rupestris means ‘of rocks’- presumably derived from petra. They employ a number of survival tactics, most importantly various methods for surviving drought. Ferns can go dormant – the rustyback will curl up, apparently dead, but will revive in the next rainfall. Ivy-leaved toadflax has shiny, waxy leaves to help retain water, while navelwort and stonecrop are succulents, having thick, juicy leaves that act as water storage tanks.

Navelwort and stonecrop have an even cleverer trick. They are members of the Crassulaceae, a family which gives its name to a system called the Crassulacean Acid Metabolism, or CAM for short.

Every day, plants carry out the miracle of photosynthesis, in which they take in carbon dioxide and water, and, through the action of sunlight on a special enzyme, use them to create sugars and oxygen. This is the basis of ALL our food and breathable air. Please pause to consider that for a second.

Considered? Good. So, plants take in carbon dioxide from the air, and to do this, they have to open special holes, or stomata, in their leaves. But while carbon dioxide is getting IN, precious water can evaporate OUT. For plants in dry environments, this is a real problem. CAM is an ingenious answer to this eternal plant dilemma. CAM plants open their stomata and take in carbon dioxide at night, when it’s cooler and they lose far less water. They can’t photosynthesise in darkness, so they store the carbon dioxide in the form of malic acid, which builds up overnight. In the morning, once the sun rises, they close the stomata, turn the acid back into carbon dioxide, and start photosynthesising without risk of water loss. Due to the concentration of carbon dioxide in a few cells, this is also far more efficient than ‘normal’ photosynthesis. THIS IS VERY COOL.

I understand that edible CAM plants taste notably more acidic in the morning – when the acid concentration is at its highest – than in the evening, though I haven’t been brave enough to try tasting them at any time! The sap of both sedum and navelwort was once used in herbal remedies to cure skin complaints, and malic acid is still used in some skin creams and cosmetics today.

Wonderwalls indeed!

References

John Gerard – Gerard’s Herbal

Gabrielle Hatfield – Hatfield’s Herbal

Richard Mabey – Flora Britannica

Roger Phillips – Grasses, Ferns, Mosses and Lichens of Great Britain and Ireland