234 PHYSIOLOGY 



the latter, the thallus produced 4 per cent, of physodic acid and 2'2 per cent, 

 of atranorin. In the rock specimen, which, he adds, was a more graceful plant 

 than the other, the quantities were 6 per cent, of physodic acid, and 275 per 

 cent, of atranorin. In both cases there was a slight formation of furfuracinnic 

 acid. He found also that specimens of Evernia pnmastri on dead wood 

 contained 8*4 per cent, of lichen-acids, while in those from living trees there 

 was only 4-4 per cent, or even less. Other conditions, however, might have 

 contributed to this result, as Zopf 1 found later that this lichen when very 

 sorediate yielded an increased supply of atranoric acid. 



Ohlert 2 , who made a study of lichens in relation to their habitat, found 

 that though a certain number grew more or less freely on either tree, rock 

 or soil, none of them was entirely unaffected. Usnea barbata, Evernia pru- 

 nastri and Parmelia physodes were the most indifferent to habitat; normally 

 they are corticolous species, but Usnea on soil formed more slender filaments, 

 and Evernia on the same substratum showed a tendency to horizontal growth, 

 and became attached at various points instead of by the usual single base. 



b. CRUSTACEOUS LICHENS. The crustaceous forms on rocks are in a 

 more favourable position for obtaining inorganic salts, the lower medullary 

 hyphae being in direct contact with mineral substances and able to act 

 directly on them. Many species are largely or even exclusively calcicolous, 

 and there must be something in the lime that is especially conducive to 

 their growth. The hyphae have been traced into the limestone to a depth 

 of 15 mm. 8 and small depressions are frequently scooped out of the rock by 

 the action of the lichen, thus giving a lodgement to the foveolate fruit. 



On rocks mainly composed of silica, the lichen has a much harder sub- 

 stance to deal with, and one less easily affected by acids, though even silica 

 may be dissolved in time. Uloth 4 concluded from his observations that the 

 relation of plants to the substratum was chemical even more than physical, 

 so far as crustaceous species were concerned. He found that the surface of 

 the area of rock inhabited was distinctly marked: even such a hard substance 

 as chalcedony was corroded by a very luxuriant lichen flora, the border of 

 growth being quite clearly outlined. The corrosive action is due he con- 

 sidered to the carbon dioxide liberated by the plant, though oxalic acid, so 

 frequent a constituent of lichens, may also share in the corrosion. Egeling 6 

 made similar observations in regard to the effect of lichen growth on granite 

 rocks ; and he further noticed that pieces of glass, over which lichens had 

 spread, had become clouded, the dulness of the surface being due to a multi- 

 tude of small cracks eaten out by the hyphae. Buchet 6 also gives an instance 

 of glass which had been corroded by the action of lichen hyphae. It formed 



1 Zopf 1907. * Ohlert 1871. 3 See p. 75. 4 Uloth 1861. 



5 Egeling 1881. 6 Buchet 1890. 



