Around 11am, Frank Dobson gave a fascinating talk on the nature and distribution of lichens. Found in all sorts of environments, including both cold and hot deserts too extreme for any other plants, lichens are important absorbers of carbon dioxide especially in the tundra regions and so make a significant contribution to the state of the world's climate. Regionally, they are key indicators of levels of pollution and locally can indicate the nature of the soil and the minerals beneath.

After lunch, we ventured forth for our lichen foray. Surrounded by London suburbs, Wimbledon Common has seen significant variations in its lichen flora over the past 150 years. Those few species that can cope with high levels of sulphur dioxide are now in retreat and the ones that can cope with modest levels of SO2 and especially with high levels of oxides of nitrogen (the Common is surrounded by roads including the A3) are in the ascendant.

We spent some time studying a number of different varieties and types of lichen on an oak, with Frank pointing out the variations caused by aspect, light levels, enrichment (e.g. those at the level of the legs of passing dogs) as well as the changing nature of local pollution.

Then we headed for the benches around the Windmill Centre - but not to sit! Several of these benches provided rich sources of lichens for study. This was just as well as our enthusiasm at the first bench we visited proved distracting for the golfers preparing to tee off in a competition close by, so we moved round the corner out of their earshot.