SECT. iv. ERRATIC LICHENS. 305 



down, amidst which are scattered round white spots, 

 which penetrate to the medullary strata. 



The erratic lichens are among the peculiarities of 

 the genus Parmelia. The Parmelia saxatilis, common 

 on Atones and boulders of the primary and metamorphic 

 formation, curls up into a ball, only fixed to its matrix 

 by a slender thread, which soon gives way, and the 

 ball being dry and as light as a balloon, is driven 

 bounding by the winds, over the sheep walks and downs 

 of England. The globular Lecanora esculenta some- 

 times suddenly covers large tracts in Armenia, Persia, 

 and Tartary, where the plants are eaten by the cattle and 

 by the nomade tribes. This species, with Lecanora 

 affinis, is largely used by the inhabitants of the countries 

 east of the Levant, and in some parts of Africa is mixed 

 with wheat in grinding. These species are found, scat- 

 tered over the ground without any attachment, in the 

 form of rugged truffle-like objects of the size of walnuts. 

 Sometimes they are piled together in strata a few 

 inches thick, by the whirlwinds, after traversing the 

 air for many miles, which gives rise to the histories of 

 the miraculous descent of food. During a scarcity, a 

 shower of these lichens fell at Erzeroum, and there are 

 other well-authenticated instances recorded. 9 



The Peltigeri, or the third group of Parmeliacei, are 

 so named from the target-like discs on their surface, 

 covered by a veil, which afterwards disappears. The 

 species frequently spread their beautifully foliaceous 

 fronds upon the ground, and as the fruit is marginal, 

 it gives the thallus a digitate appearance. They are 

 often spotted by a little red fungus. The genus Solorina 

 has the fruit scattered over the frond ; in the Solorina 

 saccata it is at first superficial, but a number of fibres are 

 formed on the under-side of the discs, which penetrate 

 the soil, and draw them down below the general surface 



9 Berkeley's ' Introduction to Cryptogamie Botany.' 

 VOL. I. X 



