EXTERNAL INFLUENCES 361 



South Lancashire, a district peculiarly suitable for such an inquiry, as nowhere, 

 according to the observations, are the evil effects of impure air so evident 

 or so wide-spread. The unfavourable conditions have prevailed for a long 

 time and the lichens have consequently become very rare, those that still 

 survive leading but a meagre existence. The chief impurity is coal smoke 

 which is produced not only from factories but from private dwellings, and 

 its harmful effect goes far beyond the limits of the towns or suburbs, lichens 

 being seen to deteriorate as soon as there is the slightest deposition of coal 

 combustion products — especially sulphur compounds — either on the plants 

 or on the surfaces on which they grow. The larger foliose and fruticose 

 forms have evidently been the most severely affected. "While genera of 

 bark-loving lichens such as Calicium, Usnea, Ramalina, Graphis, Opegrapha, 

 Arthonia etc. are either wholly absent or are poorly represented in the 

 district," corticolous species now represent about 15 per cent, of those that 

 are left; those that seem best to resist the pernicious influences of the smoky 

 atmosphere are, principally, L ecanora varia, Parmelia saxatilis,P.physodes and 

 to a less degree P. sulcata, P . fuliginusa var. laetevirens and Pertusaria aniara. 

 ^ Saxicolous lichens have also suffered severely in South Lancashire; not 

 only the number of species, but the number of individuals is enormously 

 reduced and the specimens that have persisted are usually poorly developed. 

 The smoke-producing towns are situated in the valley-bottoms, and the smoke 

 rises and drifts on to the surrounding hills and moorlands. The authors 

 noted that crustaceous rock-lichens were in better condition on horizontal 

 surfaces such as the copings of walls, or half-buried stones, etc. than on the 

 perpendicular or sloping faces of rocks or walls. This was probably due 

 to what they observed as to the effect of water trickling down the inclined 

 substrata and becoming charged with acid from the rock surfaces. They 

 also observed further that a calcareous substratum seemed to counteract the 

 effect of the smoke, the sulphuric acid combining with the lime to form 

 calcium sulphate, and the surface-washings thus being neutralized, the 

 lichens there are more favourably situated. They found in good fruiting 

 condition, on mortar, cement or concrete, the species Lecanora urbana, 

 L. campestris, L. crenulata, Verrucaria muralis, V. rupestris, Thelidium 

 microcarpum and Staurothele hymenogonia. Some of these occurred on the 

 mortar of sandstone walls close to the town, "whilst on the surface of the 

 sandstone itself no lichens were present." 



Soil-lichens were also strongly affected, the Cladoniae of the moorlands 

 being in a very depauperate condition, and there was no trace of Stereocaulon 

 or of Sphaerophorus species, which, according to older records, previously 

 occurred on the high uplands. 



The influence of human agency is well exemplified in one of the London 

 districts In 1883 Crombie published a list of the lichens recorded from 



