376 ECOLOGY 



with coral-like Sphaerophorei on the rocks, and on the walls smoky-looking 

 patches of Parmelia fuliginosa and ragged fringes of Platysma glaucum and 

 Evernia fiirfuracea. On the higher scars, flat topped tabular blocks exhibit 

 black scaly Gyrophoreae, dingy green -Lecidea (Rhizocarpon) viridiatra and 

 mouse-coloured L. rivulosa. Suborbicular (whitish) patches of Pertusaria 

 lactea and P. dealbata enliven the general sadness of tone, and everywhere 

 loose rocks and stones are covered with the greyish-black spotted thallus 

 of Lecidea contigua." 



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 o( Lecanora 

 sulphurea, L. polytropa, Rhizocarpon geographicuin 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 Haematonmia 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^ been determined as chiefly Lepraria chlorina, in less 

 abundance Lecidea lucida and Calicium arenariuni, with occasional growths 

 of Coniocybe fiirfuracea and Calicijun 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 con- 

 trary, are true vagabonds in the lichen kingdom and settle on any substance 

 that aff'ords 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 easilj'^ from one support to another. Bouly de Lesdain^ made a tabula- 

 tion 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^ also noted, many of these peculiar habitats. 



1 Schade 1916. ' Lesdain 1910. ' Arnold 1858. 



