[Illustration: Fig. 121. _Parmelia revoluta_ var. _concentrica_ Cromb. _a_, plant on flint with detached fragment; _b_, upper surface of three specimens; _c_, three specimens as found on chalk downs; _d_, specimens in section showing central cavity (S. H., _Photo._).] The most notable instance in Britain of the “erratic” habit is that of _Parmelia revoluta_ var. _concentrica_ (Fig. 121), first found on Melbury Hill near Shaftesbury, Dorset, and described as “a spherical unattached lichen which rolls on the exposed downs.” It has recently been observed on the downs near Seaford in Sussex, where, however, it seems to be confined to a small area about eight acres in extent which is exposed to south-west winds. The lichen is freely distributed over this locality. To R. Paulson and Somerville Hastings[925] we owe an account of the occurrence and origin of the _revoluta_ wanderers. The specimens vary considerably in shape and size, and measure from 1 to 7 cm. in longest diameter. Very few are truly spherical, some are more or less flattened and many are quite irregular. The revolute edges of the overlapping lobes give a rough exterior to the balls, which thereby become entangled amongst the grass, etc., and movement is impeded or prevented, except in very high winds. Crombie[926] had suggested that the concentric plant originated from a corticolous habitat, but no trees are near the Seaford locality. Eventually specimens were found growing on flints in the immediate neighbourhood. While still on the stone the lichen tends to become panniform, a felt of intermingling imbricate lobes is formed, portions of which, in time, become crowded out and dislodged. When scattered over the ground, these are liable to be trampled on by sheep or other animals and so are broken up; each separate piece then forms the nucleus of new concentric growth. Crombie[926] observed at Braemar, drifting about on the detritus of Morrone, an analogous structure in _Parmelia omphalodes_. He concluded that nodular excrescences of the thallus had become detached from the rocks on which the lichen grew; while still attached to the substratum _Parmelia omphalodes_ and the allied species, _P. saxatilis_, form dense cushion-like masses. E. PARASITISM _a._ GENERAL STATEMENT. The parasitism of _Strigula complanata_, an exotic lichen found on the leaves of evergreen trees, has been already described[927]; Dufrenoy[928] records an instance of hyphae from a _Parmelia_ thallus piercing pine-needles through the stomata and causing considerable injury. Lichen hyphae have attacked and destroyed the protonemata of mosses. Cases have also been recorded of _Usnea_ and _Ramalina_ penetrating to the living tissue of the tree on which they grew, and there may be other similar parasitisms; but these exceptions serve to emphasize the independent symbiotic growth of lichens.