2<5o BIONOMICS 



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 Somer- 

 ville Hastings 1 we owe an account of the occurrence and origin of the revo- 

 luta wanderers. The specimens vary considerably in shape and size, and 

 measure from I 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 2 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 2 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 3 ; Dufrenoy 4 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 para- 

 sitisms; but these exceptions serve to emphasize the independent symbiotic 

 growth of lichens. 



There are however some lichens belonging to widely diverse genera that 

 have retained, or reverted to, the saprophytic or parasitic habit of their fungal 

 ancestors, though the cases that occur are generally of lichens preying on 



1 Paulson and Somerville Hastings 1914. 2 Crombie 1872. 3 See p. 35. 



4 Dufrenoy 1918. 



