250 PHYSIOLOGY 



in the cortical cells and rhizoids of many thalli. In some plants the thallus 

 is brown both above and below, in others, as in Parmelia revoluta, etc. only 

 the under surface is dark-coloured. 



e. COLOUR DUE TO INFILTRATION. There are several crustaceous lichens 

 that are rusty-red, the colour being due to the presence of iron. These 

 lichens occur 'on siliceous rocks of gneiss, granite, etc., and more especially 

 on rocks rich in iron. Iron as a constituent of lichens was first demonstrated 

 by John 1 in Ramalina fraxinea and R. calicaris. Grimbel 2 proved that the 

 colour of rust lichens was due to an iron salt, and Molisch 8 by microscopic 

 examination located minute granules of ferrous oxide as incrustations on 

 the hyphae of the upper surface of the thallus. Molisch held that the rhizoids 

 or penetrating hyphae dissolved the iron from the rocks by acid secretions. 

 Rust lichens however grow on rocks that are frequently under water in which 

 the iron is already present. 



Among " rusty " lichens are the British forms, Lecanora lacustris, the 

 thallus of which is normally white, though generally more or less tinged 

 with iron; it inhabits rocks liable to inundation. L. Dicksonii owes its fer- 

 ruginous colour to the same influences. Lecidea contigua v&r.Jlavicunda. and 

 L. confluens f. oxydata are rusty conditions of whitish-grey lichens. 



Nilson 4 found rusty lichens occurring frequently in the Sarak-Gebirge, 

 more especially on glacier moraines where they were liable, even when un- 

 covered by snow, to be flooded by water from the higher reaches. It is the 

 thallus that is affected by the iron, rarely if ever are apothecia altered in 

 colour. 



1 John 1819. 2 Grimbel 1856. s Molisch [892. 4 Nilson 1907. 



