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.® 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.’ 
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. * British Museum Handbooks. 3 Aitken 
4 Friederick. > Stahlecker, Bachmann. 6 Fink. 7 Stahl. 
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