ABSTRACT
Early work such as that of Fry (1924,1927) clearly demonstrated that lichen growth on stone and glass surfaces led to the deterioration of these surfaces largely through physical processes associated with the lichen. These processes included hyphal penetration between the mineral grains of rocks, thereby causing their mechanical disruption. This effect was exacerbated by the swelling nature of gelatinous materials and organic salts associated with the mycobiont hyphae, as well as expansion and contraction of the lichen thallus following episodes of wetting and drying, thus bringing about a plucking action at the mineral surface. However, a chemical effect upon the mineral substrates beneath epilithic lichens was not widely accepted until relatively recently. A review by Syers and Iskandar (1973) for example, concluded that chemical effects beneath lichens on rocks were not significant. Elevated carbon dioxide levels associated with lichen growth were thought to be of some importance for rock weathering, but the effects of lichen acids and other organic acids were generally discounted because of their low solubility. Subsequently, largely through the use of scanning electron microscopy and other physical investigative techniques, it became apparent that the lichenrock interface is often, although not always, characterised by evidence of intense chemical interactions between the organism and the primary rock forming minerals.
