378 ECOLOGY 



pyraceum, Lecanora umbrina, Bacidia inundata and Polyblastia Vouauxi var. 

 charticola. 



On linoleum — Xanthoria parietina, Physcia ascendens f. tenella, Rinodina 

 exigua, Lecanora umbrina. 



On indiarubber — Physcia ascendens f. tenella. 



On tarred cloth — Xanthoria parietina, Placodium citrinum, Pl.pyraceum, 

 Rinodina exigua, Lecanora umbrina, Lecania erysibe, Bacidia inundata. 



On io^t— Bacidia inundata, B. muscorum. 



On cloth (cotton, etc.) — Bacidia inundata. 



On silk — Physcia ascendens. Ph. obscura, Placodium citrinum (thallus), 

 Lecanora umbrina, Bacidia inundata. 



On cord — Physcia ascendens f. tenella, Placodium citrinum (thallus). 



On excreta — One would scarcely expect to find lichens on animal 

 droppings, but as some of these harden and lie exposed for a considerable 

 time, some quick-growing species attain to more or less development on 

 what is, in any case, an extremely favourable habitat for fungi and for many 

 minute organisms. Paulson and Thompson found tiny fruiting individuals 

 of Cladonia macilenta and CI. fimbriata var. tubaeformis growing on the dry 

 dung of rabbits in Epping Forest. On the same type of pellets Lesdain re- 

 cords Physcia ascenders f leptalea, Cladonia pyxidata, Bacidia inundata and 

 B. muscorum ; and on sheep pellets : Physcia ascendens f leptalea and Placo- 

 dium citrinum; while on droppings of musk-ox in EUesmere Land Darbishire 

 found Biatorina globulosa, Placodium pyraceum, Gyalolechia subsimilis, Leca- 

 nora epibryon, L. verrucosa, Rinodina turfacea and even, firmly attached, 

 Thamnolia vermicularis. 



It would be difficult to estimate the age of these lichens, but it seems 

 evident that the " wanderers " are all more or less quick growers, and the 

 lists also prove conclusively their complete indifference to the substratum, 

 as the same species occur again and again on the very varied substances. 



S. LOCALIZED COMMUNITIES 



Lichens may be grouped ecologically under other conditions than those 

 of substratum. They respond very readily to special environments, and 

 associations arise either of species also met with elsewhere, or of species 

 restricted to one type of surroundings. Such associations or communities 

 might be multiplied indefinitely, but only a few of the outstanding ones 

 will be touched on. 



a. Maritime Lichens. This community is the most specialized of any, 

 many of the lichens having become exclusively adapted to salt-water sur- 

 roundings. They are mainly saxicolous, but the presence of sea-water is the 

 factor of greatest influence on their growth and distribution, and they occur 



