That the substratum does have some indirect influence on these larger lichens has been proved once and again. Uloth[848], a chemist as well as a botanist, made analyses of plants of _Evernia prunastri_ taken from birch bark and from sandstone. Qualitatively the composition of the lichen substances was the same, but the quantities varied considerably. Zopf[849] has, more recently, compared the acid content of a form of _Evernia furfuracea_ on rock with that of the same species growing on the bark of a tree. In the case of 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 2·75 per cent. of atranorin. In both cases there was a slight formation of furfuracinnic acid. He found also that specimens of _Evernia prunastri_ 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[850] found later that this lichen when very sorediate yielded an increased supply of atranoric acid. Ohlert[851], 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 prunastri_ 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.[852] and small depressions are frequently scooped out of the rock by the action of the lichen, thus giving a lodgement to the foveolate fruit.