•374 INTEODUCTION TO GEYPTOGAMIC BOTANY. 



stances. In Parmelia parietina, when growing in the shade 

 it is changed into a greyish tint, but when exposed to the full 

 sun it acquires the same tint as the outer crust itself. The 

 modifications of form and colour in this species, according to 

 outward circumstances, are fully described by Meyer, Entwicke- 

 lung der Flechten, p. 220. 



410. Lichens, like Fungi, spring in the first instance from 

 a branched mycelium, consisting of numerous intricate, often 

 anastomosing, threads. They gradually give rise to a compact 

 mass, assuming -various forms and texture. The outer coat is 

 merely a protective surface, consisting of dense compact cells, 

 or threads so closely interwoven as to simulate cells ; the 

 inner, though differing in various species, retains more of the 

 original structure, and gives birth beneath to the fibres by 

 which the plant is often attached to the surface (hypothallus), 

 or to a spongy mass, as in Placodium, and sometimes to an 

 intermediate stratum of gonidia, with or without a compact 

 mass, similar to the cortical layer, or that which immediately 

 underlies the hymenium ; while above it produces on the one 

 hand the gonidia, or on the other the pycnidia, spermatogonia, 

 and shields or other form of fruit. Where the development 

 is abnormal, in consequence of peculiar conditions of the atmo- 

 sphere, or from defect of light, the filamentous portion may be 

 alone developed, or the whole plant may be broken up into a 

 myriad of dustlike bodies, or the filamentous and pulverulent 

 condition may exist together, and hence arise the dusty Lepra- 

 rice, the semi-filamentous Le]praria latehrarum, and some 

 confervoid conditions. In other cases the thallus may be 

 indefinitely extended, with an abundant development of 

 patches of gonidia, as in the various kinds of Variolarla. On 

 exotic barks many curious forms occur, which it is impossible 

 at present to connect with their original parents. The different 

 exotic Hypochni are, probably, mere degenerations of some 

 Lichens ; and there are other hyraenomycetoid expansions, as 

 Thelepliora pedicellata, Schweinitz, which owe their origin to 

 the same cause. Chrysothrix is probably a state of some 

 Lichen, and the same is clearly the case with Byssocaulon, 

 Mont. The genus Gephaleurus arises from an abnormal 



