GENERAL NUTRITION 233 



from atmospheric dust dissolved in rain, or from wind-borne particles de- 

 posited 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^ has observed that they flourish in valleys 

 that are swept by moisture laden winds more especially if near to a high- 

 way, 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^ found on the 

 island of Riigen 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' 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^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 =. 



That the substratum does have some indirect influence on these larger 

 lichens has been proved once and again. Uloth", 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 has, more 

 recently, compared the acid content of a form of Evernia furfur acea on rock 

 with that of the same species growing on the bark of a tree. In the case of 



1 West [905. 2 Sandstede 1904. ^ Friedrich 1906. * Lindau 1895^. 



5 See p. 109. ® Uloth 1861. " Zopf 1903. 



