The message
from "RichardS" noaccess@invalid contains these words:

Apparently lichens only grow in places of low atmospheric pollution,
so it's a good indicator of air quality, as well as any aesthetic
benefit.

but is the same true about moss? In fact, are moss and lichen synonymous?
I know that lichens are rarer than they once were.

No. Moss is a bryophyte, a true plant. Lichen is a symbiotic union of an
alga and a fungus - an unlikely combination as algae are plants and
fungi are in a completely separate order.

I must say that I haven't noticed any evidence of lichens being any less
common than they were, and I've been interested in them since the early
'fifties.

I'm sitting working looking at the roof of the house opposite, and this is
liberally covered in mosses. This is sunny Twickenham, not that far out of
London and close to the Heathrow flightpaths and one of London's arterial
road, so whereas this might not be an area particularly high in atmospheric
pollution it certainly isn't the cleanest environment.

Mosses require some sort of soil to start with, and moisture with it.
Some mosses can dry out for long periods and be revitalised by a shower
of rain, and it is these which you'll find on roofs.

very often their shrinking in dry periods dislodges them, and they can
then colonise damper areas, either in the gutter or on the ground.

--
Rusty
Open the creaking gate to make a horrid.squeak, then lower the foobar.
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