Rock Lichens 
So that it will be seen that, though lichens are char- 
acteristic of rocks, they also occur scattered or as 
dependents in some of the higher associations. 
The field botanist will find them exceedingly difficult, 
especially the crustaceous ones, for of Lecanoras, Verru- 
carias, and Lecideas there are some 700 species, and 
there are not ten botanists in Britain who are capable 
of naming them correctly.’ 
The brilliant colour of the cups, and sometimes of 
the lichen itself, has been already noticed. Is it not 
curious to find reds, yellows, and pale colours in con- 
nection with their reproductive spores? Similar colours 
appear in flowers which are in no way related to 
lichens. 
It is, at least in the author’s opinion, a fact that 
insects are attracted to those cups and carry the spores. 
The flaring yellow of Physcia parietina is very con- 
spicuous on seaside rocks. No fly could help proceed- 
ing to one of these patches, and it would surely then 
visit the conspicuous little bright sporecups. 
But there is no explanation of how the colours were 
formed. In these sporecups, when forming, there is 
possibly the same relatively intense respiration, higher 
temperature, and greater production of food material 
that are also characteristic of flowers (see p. III). 
One point about lichen-life is the importance to them 
of atmospheric dust. This is of the most varied char- 
acter and very mixed in origin. A small amount of 
nitric acid is said to be brought down with rain-drops 
(due to electric discharges) ; there are also dust particles 
from roads, from volcanic eruptions, from the smoke of 
towns, and indeed gathered up from everywhere. 
Mr. Aitken did on one occasion find a cubic centi- 
metre (.061 cubic inch) of air which contained only 
one dust particle, but this was only once, and from the 
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