On the Silurian series of rocks in the same district they describe a somewhat brighter coloured flora: “First Stereocaulons invite attention, and greenish or yellowish shades are introduced by an abundance of Lecanora sulphurea, L. polytropa, Rhizocarpon geographicum and Parmelia conspersa, often beautifully commingled with grey species such as Lecidea contigua and L. stellulata, and reddish angular patches of Lecanora Dicksonii. Also an abundance of orbicular patches of Haematomma ventosum with its reddish-brown apothecia.” A brightly coloured association on the cretaceous sand-rocks of Saxon Switzerland has been described as “Sulphur lichens.” These have recently[1178] been determined as chiefly Lepraria chlorina, in less abundance Lecidea lucida and Calicium arenarium, with occasional growths of Coniocybe furfuracea and Calicium corynellum.
4. OMNICOLOUS LICHENS
Some account must be taken in any ecological survey of those lichens that are indifferent to substrata. Certain species have become so adapted to some special habitat that they never or rarely wander; others, on the contrary, are true vagabonds in the lichen kingdom and settle on any substance that affords a foothold: on leather, bones, iron, pottery, etc. There can be no sustenance drawn from these supports, or at most extremely little, and it is interesting to note in this connection that while some rock-lichens are changed to a rusty-red colour by the infiltration of iron—often from a water medium containing iron-salts—those that live directly on iron are unaffected.
The “wanderers” are more or less the same in every locality and they pass easily from one support to another. Bouly de Lesdain[1179] made a tabulation of such as he found growing on varied substances on the dunes round Dunkirk and they well represent these omnicolous communities. It is in such a no man’s land that one would expect to find an accumulation of derelict materials, not only favourably exposed to light and moisture, but undisturbed for long periods and bordering on normal lichen associations of soil, tree and stones. Arnold[1180] also noted many of these peculiar habitats.
The following were noted by Lesdain and other workers:
On iron—Xanthoria parietina, Physcia obscura and var. virella, Ph. ascendens, Placodium (flavescens) sympageum, Pl. pyraceum, Pl. citrinum, Candelariella vitellinum, Rinodina exigua, Lecanora campestris, L. umbrina, L. galactina, Lecania erysibe, Bacidia inundata. Xanthoria parietina is one of the commonest wandering species; it was found by Richard[1181] on an old cannon lying near water, that was exfoliated by rust.
On tar—Lecanora umbrina.
On charcoal—Rinodina exigua, Lecanora umbrina.
On bones—Xanthoria parietina, Physcia ascendens, Ph. tenella, Placodium citrinum, Pl. lacteum, Rinodina exigua, Lecanora galactina, L. dispersa, L. umbrina, Lecania erysibe, L. cyrtella, Acarospora pruinosa, A. Heppii, Bacidia inundata, B. muscorum, Verrucaria anceps, V. papillosa.