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By Angus Hannah  

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As well as being local VCR (100), I have worked on both dry and mortared walls for many years. Unfortunately, my knowledge of bryophytes is limited to the commoner mosses, and I know little about lichens, which are abundant. However, some notes may be of interest.

Walls here are built of schist or whinstone (or occasionally sandstone) in different parts of the island and tend to be moderately acidic. The drystone style is double build with flattish covers where available and substantial vertical copes. Urban walls are built with lime-mortar in the same general style, but thinner at the base and with some of the facing stones laid vertically.

There is virtually no vascular plant flora on the more exposed drystone walls, though the lower courses may carry some similar vegetation to the adjacent wall-foot, eg Hypochaeris radicata, Achillea millefolium, Agrostis capillaris, etc., and on seriously neglected walls brambles and ivy get a hold. Where the wall is retaining a moist earth bank many species will grow over and through it, eg Chrysosplenium oppositifolium, Geranium robertianum, Glechoma hederacea, etc., indeed almost the whole suite of woodland plants will colonise a damp, shady wall. In addition, the stones of such a wall are usually covered with Hypnum spp., Mnium hornum or a range of liverworts.

The mortar walls of built-up areas, mostly 19th century, carry a varied vascular flora including Erinus alpinus, Cymbalaria muralis, Mycelis muralis, Hieracium vulgatum, Arabidopsis thaliana, Cardamine hirsuta, Epilobium montanum, Dactylis glomerata, Erophila glabrescens, Festuca rubra, Fuchsia magellanica, Hedera helix, Hypochaeris radicata, Hypericum androsaemum, Plantago lanceolata, Sagina procumbens, S. apetala, Sonchus asper, Taraxacum sp., Veronica arvensis, and of course abundant ferns including Polypodium agg., Asplenium trichomanes ssp. quadrivalens, A. ruta-muraria, A. adiantum-nigrum, Ceterach officinarum, and especially where shaded, Dryopteris affinis agg., Athyrium filix-femina and Phyllitis scolopendrium.

There is a rich bryophyte flora on walls of both kinds, varying somewhat according to the reaction of the rock and the degree of shade offered. Drystone walls are often carpeted with Hypnum cupressiforme on the shady side, while in the open Bryum capillare, Tortula muralis, Grimmia pulvinata and Homalothecium sericeum are often abundant on the copes and protuberant cover stones. Brachythecium rutabulum, Eurynchium praelongum and other smaller pleurocarps tend to occupy crevices, especially in the lower parts of the wall. More acid stones will carry Rhacomitrium fasciculare, and occasionally Hedwigia ciliata. Most of the above occur also on mortared walls, as well as more strictly calcicole species including Orthotrichum anomalum, Bryum argenteum, Ceratodon purpureus, Trichostomum brachydontium and other acrocarps I am not competent to identify, which prefer to grow on the mortar itself rather than on the stones.