_a._ FOLIOSE AND FRUTICOSE LICHENS. These larger lichens are provided with rhizinae or with hold-fasts, which are only absorptive to a very limited extent; the main source of water supply is from the atmosphere and the salts required in the metabolism of the cell must be obtained there also—from atmospheric dust dissolved in rain, or from wind-borne particles deposited on the surface of the thallus which may be gradually dissolved and absorbed by the cortical and growing hyphae. That substances received from the atmospheric environment may be all important is shown by the exclusive habitat of some marine lichens; the _Roccellae_, _Lichinae_, some species of _Ramalina_ and others which grow only on rocky shores are almost as dependent on sea-water as are the submerged algae. Other lichens, such as _Hydrothyria venosa_ and _Lecanora lacustris_, grow in streams, or on boulders that are subject to constant inundation, and they obtain their inorganic food mainly, if not entirely, from an aqueous medium. Though lichens cannot live in an atmosphere polluted by smoke, they thrive on trees and walls by the road-side where they are liable to be almost smothered by soil-dust. West[843] has observed that they flourish in valleys that are swept by moisture laden winds more especially if near to a highway, where animal excreta are mingled with the dust. The favourite habitats of _Xanthoria parietina_ are the walls and roofs of farm-buildings where the dust must contain a large percentage of nitrogenous material; or stones by the sea-shore that are the haunts of sea-birds. Sandstede[844] found on the island of Rügen that while the perpendicular faces of the cliffs were quite bare, the tops bore a plentiful crop of _Lecanora saxicola_, _Xanthoria lychnea_ and _Candellariella vitellina_. He attributed their selection of habitat to the presence of the excreta of sea-birds. As already stated the connection of foliose and fruticose lichens with the substratum is mainly mechanical but occasionally a kind of semiparasitism may arise. Friedrich[845] gives an instance in a species of _Usnea_ of unusually vigorous development. It grew on bark and the strands of hyphae, branching from the root-base of the lichen, had reached down to the living tissue of the tree-trunk and had penetrated between the cells by dissolving the middle lamella. It was possible to find holes pierced in the cell-walls of the host, but it was difficult to decide if the hyphae had attacked living cells or were merely preying on dead material. Lindau[846] held very strongly that lichen hyphae were non-parasitic, and merely split apart the tissues already dead, and the instance recorded by Friedrich is of rare occurrence[847].