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 15 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. 
(gth of ansinch) in seven and a half months, Another 
authority has calculated that .o610 cubic inch of the 
* Tt 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 
