GENERAL CHARACTERS OE BRITISH LICHENS. 49 



to the light, the distribution of colour is generally uniform ; 

 but in the horizontal thallus, the inferior surface, which is 

 opposed to, or in union with, the base of support, is much 

 paler than the superior or free surface, which is exposed to 

 the sun, and not unfrequently of a different colour. While 

 it is more highly coloured, the surface exposed, to the light 

 and to atmospheric changes is also, as we have mentioned, 

 from the compactness and thickness of the cortical cells, 

 firmer in texture. Hence species growing in the shade 

 usually present a dull colour in their thallus, while those 

 exposed to the sunlight exhibit bright or deep tints. A 

 marked change in colour may be artificially produced by 

 transferring a species growing in a damp shady locality, to 

 a dry light habitat, or vice versa. But there are a few ex- 

 ceptions : some Lichens, whose normal habitat is the shade, 

 and which are said there to exhibit very bright tints, grow 

 pale on exposure to a greater amount of light ; and some tro- 

 pical Lichens are known to possess very dull colours. It 

 can scarcely be doubted that various mineral matters are, to 

 a slight extent at least, absorbed from the soil or from 

 bases of support, and contribute materially to the colour 

 the thallus ; this has been particularly observed in regard to 

 peroxide of iron, which gives a rusty or reddish colour to the 

 thallus of many species. e 



