Wall flowers in Weardale

Dry stone walls, and old stone walls held together with mortar, acquire a flora of resilient species that thrive in their damp crevices. In Weardale these vertical gardens are at their best from mid-May to early June, before conditions become too dry.

This beautiful display, with yellow Alyssum saxatile in the foreground, cascades down a wall in Stanhope, Weardale

Fairy foxglove Erinus alpinus covers large areas of old walls in Stanhope.

Ferns colonise walls too, but usually on the side that's shaded from the sun. This is brittle bladder fern Cystopteris fragilis, growing  in the wall around St. John's Chapel churchyard in Weardale.

Wall rue Asplenium ruta-muraria and maidenhair spleenwort Asplenium  trichomanes sharing the same crevice in a wall at Eggleston in Teesdale

Aubretia and dandelions in a retaining wall near Daddry Shield in Weardale.

Yellow Alyssum saxatile clinging to the narrowest of crevices in Stanhope.

Wallflower Cheiranthus cheiri, living up to its name in Stanhope.

Fairy foxglove Erinus alpinus, on a wall in Stanhope.

Snow-in-summer Cerastium tomentosum on a wall in Stanhope.

A Golden Wall

Thursday's Guardian Country Diary is about this remarkable 'golden' wall at Hexham, in Northumberland's Tyne valley. It's a retaining wall beside the main Carlisle to Newcastle railway line, that crosses the Tyne just beyond that bridge in the distance and passes this point at head height.

The golden covering is caused by an alga called Trentepohlia aurea.

It spends the drier months of the year as a  powdery deposit on rocks and tree trunks but when it's wet the alga grows into a forest of minute filaments, forming a dense mat on the surface.

You can see the filaments in this close-up, grouped together to form golden cushions that are a few millimetres in diameter. This wall is constantly wet, due to water percolating through from the railway track bed, so conditions are near-perfect for the growth of the alga.

When the tufts coalesce into a golden carpet they provides a very striking back-drop for other plants that are colonising the wall, like this ivy and .....

The liverwort Conocephalum conicum, also known as snakewort. Its surface is divided into small polygons, each with an air pore at its centre, giving it a resemblance to green snakes' skin.

There are other liverworts on the wetter sections of the wall, including this one which I think is Pellia endiviifolia.

The crevices are home to mosses and this little fern with leathery fronds - wall rue Asplenium ruta-muraria.

The alga seems to thrive particularly well on the cement but there are patches of lichen on some of the stones. A close look at this one revealed ....

.... these fungal fruiting bodies, which look like tiny pink toadstools. I think this is a species in the genus Baeomyces.

Another lichen, this time .....

... with fruiting bodies (apothecia)  that look like minute disks of liquorice. A species of Lecidea?

This lichen is a Cladonia species, probably C. fimbriata

Lichens are formed by the symbiotic association between a fungus and an alga and it's very likely that the algal symbiont of some of the lichens on this wall is Trentepohlia. 

Back now for a closer look at the alga Trentepohlia, this time under the microscope.

Under low magnification (c. x40) with a stereo-microscope you can see the forest of algal filaments that make up those orange cushions, while ....

....... here, under a compound microscope (x100) you can see the cells that make up the filaments, and .....

.... at a higher magnification still (c. x400) you can see the granular, pigmented contents of the cells.

There are animals living in the crevices in this wall, including the fearsome snake-back spider - but that's another story.

Wildlife on Walls: 5. Wall Rue

There aren't many plants that are so intimately associated with man-made walls that this tough little fern, wall rue Asplenium ruta-muraria.

It's likely that human constructions in brick and stone have extended its range because it's a species that prefers an alkaline substrate - and the mortar between bricks and rocks provides and excellent substitute for limestone in areas with a more acidic natural geology. Stretching the point a little, you could argue that the Romans helped widen the distribution of this fern because they introduced mortar as a means of constructing walls. Without this inadvertent human assistance, this would be a much rarer fern.

It produces spores under those leathery, drought-resistant fronds which have a distinct coincidental similarity to rue Ruta graveolens. 

Wall ferns

Three tough fern species that remain green throughout the winter and survive all-year-round in a very inhospitable habitat. First up, wall rue (Asplenium ruta-muraria) thrives in the mortar of old walls and walls made of limestone. There are often fine displays on the shady side of churchyard walls and it often turns up on old masonry in the heart of cities.

 

Maidenhair spleenwort (Asplenium trichomanes) is an exceptionally graceful species and is also a lime-lover, so thrives in crumbling mortar wherever there is some shade and humidity.

Polypody fern (Polypodium vulgare) is an epiphyte – a plant that grows on other plants – and its natural habitat is on moss-covered branches of trees but in Weardale it often makes itself at home in between the capstones of drystone walls. It’s an evergreen fern and if you turn the fronds over even this late in the year you’ll often get a surprise – bright yellow clusters of sporangia on the underside of the fronds.

For a closer look at these hop over to http://beyondthehumaneye.blogspot.com/2009/11/polypody-fern-with-golden-sporangia.html