Andy Hall wrote:
There are a whole spectrum of lichens and some will grow in moderately
polluted conditions. Others are more sensitive to conditions, so for
example you will see a richer set of lichens in a forest in Wales than
you will in town.

I heard recently on some environmental thing on R4 about the
effects of pollution on buildings. All this is subject to
the vagiaries of my memory, but I think I've remembered the
gist of it right: In Victorian times, they
burnt loads of sulphorous coal, and got gypsum (or something)
deposits on buildings. Now, with all the petroleum fuel
being used, there's hardly any sulphur, but loads of
nitrogen-containing stuff -- which means that these days it's
easier for plants/algae/etc to grow on buildings.

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#Paul