STRUCTURES PECULIAR TO LICHENS 149 



genetically young, though, as Lindau' has pointed out, soredia are all com- 

 paratively recent. They probably did not appear until lichens had reached 

 a more or less advanced stage of development, and, considering the poly- 

 phyletic origin of lichens, they must have arisen at more than one point, 

 and probably at first in circumstances where the formation of apothecia was 

 hindered by prolonged conditions of shade and moisture. 



That soredia are ontogenetic in character, and not, as Nilson^ has asserted, 

 accidental products of excessively moist conditions is further proved by the 

 non-sorediate character of those species of crustaceous lichens belonging to 

 Lecanora, Verrucaria, etc. that are frequently immersed in water. Bitter'' 

 found that the soredia occurring on Peltigera spuria were not formed on the 

 lobes which were more constantly moist, nor at the edges where the cortex 

 was thinnest: they always emerged on young parts of the thallus a short 

 way back from the edge. 



Bitter" points out that in extremely unfavourable circumstances — in the 

 polluted atmosphere near towns, or in persistent shade — lichens, that would 

 otherwise form a normal thallus, remain in a backward sorediose state. He 

 considers,however,thatmany of these formless crusts are autonomous growths 

 with specific morphological and chemical peculiarities. They hold these 

 outposts of lichen vegetation and are not found growing in any other localities. 

 The proof would be to transport them to more favourable conditions, and 

 watch development. 



4. ISIDIA 



A. Form and Structure of Isidia 



Many lichens are rough and scabrous on the surface, with minute simple 

 or divided coral-like outgrowths of the same texture as the underlying thallus, 

 though sometimes they are darker in colour as in Evernia furfuracea. They 

 always contain gonidia and are covered by a cortex continuous with that 

 of the thallus. 



This very marked condition was considered by Acharius* as of generic 

 importance and the g&nviS,Isidium,vfa.s established by him, with the diagnostic 

 characters: "branchlets produced on the surface, or coralloid, simple and 

 branched." In the genus were included the more densely isidioid states of 

 various crustaceous species such as Isidium coralliniim and /. Westringii, 

 both of which are varieties of Pertusariae. Fries^ with his accustomed insight, 

 recognized them as only growth forms. The genus was however still accepted 

 in English Floras^ as late as 1833, though we find it dropped by Taylor'' in 

 the Flora Hibernica a few years later. 



1 Lindau 1895. ^ Nilson 1903. ^ Bittg^ 1904. * Acharius 1798, pp. 2, 87. 



'' Fries 1825. ^ Hooker 1833. ' Taylor 1836. 



