Rock Lichens 



very top of Ben Nevis.^ Amongst the dust there will be 

 pollen grains,'" bacteria, fungus spores, and especially 

 yeast-cells, as well as other organic material (see p. 191). 



Such dust particles falling on the surface of the lichen, 

 or washed down by rain, will be extremely useful, for 

 they will furnish the lichen with useful food material. 



It must not be supposed that lichens are unable to 

 obtain mineral salts from the rocks on which they live. 

 Some of them are specially expert in corroding the 

 rock surface. The curious root-like fibres which one 

 finds on the underside of a lichen are sheafs of fungus 

 threads which are well able to absorb nourishment as 

 well as to fix the lichen to its support. 



One of the chalk lichens (Sarcogyne simplex) can 

 eat its way downwards to a depth of some 1 5 mm.* (a 

 little over half an inch) in the solid chalk ; others can 

 penetrate to 6 mm. depth in siliceous rocks (.234 inch). 

 There are some authors who hold that lichens do actually 

 corrode quartz, the hardest of all rocks. Rhizocarpon, 

 which grows upon granite rock, does as a matter of fact 

 penetrate into them. Its rhizoids dissolve the mica and 

 force their way in between the quartz and orthoclase 

 particles, and in this way it succeeds in disintegrating 

 the rock.^ 



As some bacteria are able to produce " weathering " 

 of certain rocks, it is not surprising that special rock 

 lichens should be able to do so. 



Yet the growth of lichens is very slow indeed, especi- 

 ally when one compares them even with mosses. 



A leaf-lichen, Parmelia caperata, grew about 1.5 cm. 

 (3th of an inch) in seven and a half months. Another 

 authority has calculated that .0610 cubic inch of the 



* It has been possible to prove the existence of trees in the time when 

 certain peat deposits were laid down by examining the peat with a micro- 

 scope for blown pollen grains. — Friih. 



68 



