Rock Lichens 



common Xanthoria parietina formed about 7,200,000 

 cells in one month ! On a wall which had only been 

 built some thirty years, no less than thirty kinds of 

 lichens were noticed, which of course means a con- 

 siderable rapidity of development. 6 It would be quite 

 impossible for those which grow amongst mosses in 

 forest soil (Peltigera canina) to keep their heads clear if 

 they were not fairly quick in growth. 



Lichens are not often eaten by the higher animals. 

 The tripe-de-roche of the Canadian North is not an appe- 

 tising food, and it seems to be only the reindeer that 

 enjoys reindeer-moss, which is a lichen (Cladina rangi- 

 ferina). Even specialists in omnivoracity like butterfly 

 caterpillars are unable to eat certain lichens, which 

 protect themselves by developing vulpin acid. 7 



The lichens or crottles of the Highlands are still 

 used for dyeing Harris tweeds, but for various reasons 

 it is advisable not to describe the usual processes too 

 minutely. 



So this interesting group is not without direct useful- 

 ness for mankind, whilst for the conquest of dry rocks, 

 or lava-flows, they are the special pioneer form for 

 higher associations. 



1 Fritsch. 3 British Museum Handbooks. 3 Aitken 



* Friederick. 5 Stahlecker, Bachmann. 6 Fink. ' Stahl. 



69 



