3 6o ECOLOGY 



to successive wetting and drying. Tests were made as to the force of wind 

 required to tear the lichens and it was found that velocities of 77 miles per 

 hour were not sufficient to cause any pieces of the lichen to fly off when it 

 was dry; but after soaking in water, the first pieces were torn off at 50 miles 

 an hour. These figures are, however, considered by Schrenk to be too high 

 as it was found impossible in artificially created wind to keep up the condi- 

 tion of saturation. It is the combination of wind and rain that is so effective 

 in ensuring the dispersal of both these lichens. 



d. HUMAN AGENCY. Though lichens are generally associated with un- 

 disturbed areas and undisturbed conditions, yet accidents or convulsions of 

 nature, as well as changes effected by man, may at times prove favourable 

 to their development. The opening up of forests by thinning or clearing 

 will be followed in time by a growth of tree and ground forms; newly 

 planted trees may furnish a new lichen flora, and the building of houses 

 and walls with their intermixture of calcareous mortar will attract a par- 

 ticular series of siliceous or of lime-loving lichens. A few lichens are partial 

 to the trees of cultivated areas, such as park-lands, avenues or road-sides. 

 Among these are several species of Physcia : Ph. pulverulenta^ Ph. ciliaris 

 and Ph. stellaris, some species of Placoditim, and those lichens such as 

 Lecanora varia that frequently grow on old palings. 



On the other hand lichens are driven away from areas of dense popula- 

 tion, or from regions affected by the contaminated air of industrial centres. 

 In our older British Floras there are records of lichens collected in London 

 during the eighteenth century in Hyde Park and on Hampstead Heath but 

 these have long disappeared. A variety of Lecanora galactina seems to be 

 the only lichen left within the London district: it has been found at Camden 

 Town, Netting Hill and South Kensington. 



So recently as 1866, Nylander 1 made a list of the lichens growing in the 

 Luxembourg gardens in Paris; the chestnuts in the alley of the Observatory 

 were the most thickly covered, and the list includes about 35 different 

 species or varieties, some of them poorly developed and occurring but rarely, 

 others always sterile, but quite a number in healthy fruiting condition. All 

 of them were crustaceous or squamulose forms except Parmelia acetabulum> 

 which was very rare and sterile; Physcia obscura var. and Ph. pulverulenta 

 van, also sterile; Physcia stellaris with occasional abortive apothecia and 

 Xanthoria parietina, abundant and fertile. In 1898, Hue 2 tells us, there 

 were no lichens to be found on the trees and only traces of lichen growth 

 on the stone balustrades. 



The question of atmospheric pollution in manufacturing districts and its 

 effect on vegetation, more especially on lichen vegetation, has received 

 special attention from Wheldon and Wilson* in their account of the lichens of 

 1 Nylander 1866. Hue 1898. 3 Wheldon and Wilson 1915. 



