The character of the substratum also affects to a great extent the comparative development of the different thalline layers: the hyphal tissues in silicicolous lichens are much thinner than in lichens on limestone, and the gonidial zone is correspondingly wider. In a species of _Staurothele_ on granite, Stahlecker[333] estimated the gonidial zone to be about 600 µ thick, while the lower medullary hyphae, partly burrowing into the rock, measured about 6 mm. Other measurements at different parts of the thallus gave a rhizoidal depth of 3 mm., while on a more finely granular substratum, with a gonidial zone of 350 µ, the rhizoidal hyphae measured only 1-1/2 mm. On calcareous rocks, on the contrary, with a gonidial zone that is certainly no larger, the hyphal elements penetrate the rock to varying depths down to 15 mm. or even more. Lang[334] has recorded equally interesting measurements for _Sarcogyne_ (_Biatorella_) _latericola_: on slaty rock which contained no mixture of lime, the gonidial zone had a thickness of 80 µ, a considerable proportion of the very thin thallus. Fünfstück[335] has indeed suggested that this lichen on acid rocks is only a starved condition of _Sarcogyne_ (_Biatorella_) _simplex_, which on calcareous rocks, though with a broader gonidial zone, has, as noted above, a correspondingly much larger hyphal tissue. Stahlecker’s theory is that the hyphae require more energy to grow in the acid conditions that prevail in siliceous rocks, and therefore they make larger demands on the algal symbionts. It follows that the latter must be stimulated to more abundant growth than in circumstances favourable to the fungus, such as are found in basic (calcareous) rocks; he concludes that on the acid (siliceous) rocks, the epilithic or superficial condition is not only a physical but a biological necessity, to enable the algae to grow and multiply in a zone well exposed to light with full opportunity for active photosynthesis and healthy increase. C. CORTICOLOUS LICHENS The crustaceous lichens occurring on bark or on dead wood, like those on rocks, are either partly or wholly immersed in the substratum (hypophloeodal), or they grow on the surface (epiphloeodal); but even those with a superficial crust are anchored by the lower hyphae which enter any crack or crevice of wood or bark and so securely attach the thallus, that it can only be removed by cutting away the underlying substance. _a._ EPIPHLOEODAL LICHENS. These lichens originate in the same way as the corresponding epilithic series from soredia or from germinating spores, and follow the same stages of growth; first a hypothallus with subsequent colonization of gonidia, the formation of granules, areolae, etc. The small compartments are formed as primary or secondary areolae; the larger spaces are marked out by the encounter of hypothalli starting from different centres.