What is a lichen? It is a composite organism consisting of a fungus and one or more algae. The fungus provides the body, and the algae produce nutrients through photosynthesis. There are many different species and forms, and they are difficult to identify, impossible without a hand lens, but once you start looking they are fascinating.
Lichens are extremely sensitive to pollution and are natural indicators of the health of the environment. They have become more widespread in recent years because of a reduction of sulphur in the air. There is, however, also an increase in nitrogen which affects some lichens adversely and others positively, so there may well be more changes in future. Some were identified for me by an ecologist who walked around the Gardens with me last week. We stopped at the Black Mulberry tree near the Five Arrows Gallery and found more than a dozen different species on that one tree alone.
Some areas of the gardens (e.g. Lover's Lane) and some plants (e.g. Azaleas) seem particularly prone to attracting a growth of lichens. Three of the most obvious ones hang in bunches on the twigs, looking from a distance a bit like pale greenish or greyish flowers (left). They are superficially quite similar to each other, but if you look closely you can see quite obvious differences. They do not have universally recognised common names, but Evernia prunastri , also known as "oak moss", (right) is quite a sturdy branching type, with short, flattened very divided branches. It is darker above and paler on the underside (quite obvious when you investigate it), which separates it quite easily from the next species, which is all one colour. It is extensively used in perfume making.
The very similar-looking one, often on the same bush, is Ramalina farinacea, "tree moss", (left), but this species is all one colour, a silvery grey, also very branched and with branches narrowing towards the tip. It is common and widespread. The third one belongs to a genus called Usnea, or "beard lichen". You can see two lichens in the photo (right). The lower one is Usnea subfloridana. It looks a bit like a greyish or greenish fuzzy tassle, its branches much finer and more hair-like than the other two species mentioned. It has medicinal properties.
Just being able to distinguish between these three species provides a lot of interest. But beware! If you get really interested you could be in for a lifetime's study.