On felt—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 Cl. fimbriata var. tubaeformis growing on the dry dung of rabbits in Epping Forest. On the same type of pellets Lesdain records Physcia ascendens f. leptalea, Cladonia pyxidata, Bacidia inundata and B. muscorum; and on sheep pellets: Physcia ascendens f. leptalea and Placodium citrinum; while on droppings of musk-ox in Ellesmere Land Darbishire found Biatorina globulosa, Placodium pyraceum, Gyalolechia subsimilis, Lecanora 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.
5. 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 surroundings. They are mainly saxicolous, but the presence of sea-water is the factor of greatest influence on their growth and distribution, and they occur indifferently on any kind of shore rock either siliceous or calcareous. Wheldon and Wilson[1185] noted this indifference to substratum on the Arran shores, where a few calcicolous species such as Verrucaria nigrescens, V. maculiformis, Placodium tegularis and Pl. lobulatum, grow by the sea on siliceous rocks. They suggest that the spray-washed habitat affords the conditions, which, in other places, are furnished by limestone.
The greater or less proximity of the salt water induces in lichens, as in other maritime plants, a distribution into belts or zones which recede gradually or abruptly according to the slope of the shore and the reach of the tide. Weddell[1186] on the Isle d’Yeu delimited three such zones: (1) marine, those nearest the sea and immersed for a longer or shorter period at each tide; (2) semi-marine, not immersed but subject to the direct action of the waves, and (3) maritime or littoral, the area beyond the reach of the waves but within the influence of sea-spray. In the course of his work he indicates the lichens of each zone.