As the trees drop their leaves for winter many reveal the encrustations of lichens covering their twigs and branches.

These are a fascinating world of their own.

The Biodiversity Group hosted an introduction to lichens as part of the Science Festival two weeks ago.

Twenty people joined Lyndsey Mahon from the Saving Devon’s Treescapes project in the Knapp orchard where the trees have a wonderful array of these fascinating life forms.

Lichens are not plants and they are not fungi; they are a bridge between the two scientific kingdoms.

Most lichens are fungi that have trapped algae inside.

The fungal part supports and protects the alga while the algal cells photosynthesise to produce sugar that feeds the fungus; the alga is called the photobiont.

In a few cases, the photobiont is a cyanobacterium instead of an alga, but the outcome is the same.

Most lichens take one of three basic forms: crustose, foliose or fruticose.

As the name suggests, crustose lichens form a crust on the substrate, the surface on which they are growing.

Other lichens lift away from the substrate in flattened leafy or foliose projections.

Fruticose lichens are more bushy; the word has nothing to do with fruits but comes from the Latin for bush.

You find all three forms on trees, particularly in areas with unpolluted air such as west-facing coasts, but they grow on many other surfaces including stone and soil.

Many of the headstones in the churchyard have crustose lichens spreading across them and there are some foliose species as well.

The crustose ones cling tight to the stone but you can peel the foliose ones back to reveal an underside that is often a different colour to the upper surface.

The fungal part of the lichen reproduces either by producing spores or by shedding tiny parts of itself to be scattered by the wind or on the feet of animals.

Sometimes the algal part is carried with the new ‘plant’; sometimes the fungus has to find a new partner.

The different spore-producing bodies are key characteristics for identifying many lichens.

Lichens can be used to dye fabrics; they are eaten in some parts of the world, and they have proven medicinal properties, but most of them are ignored and left to get on with their slow lives.

This means that few have common names and the Latin names can be quite daunting.

There is a string of sausages with bushy lichen where the threads are fattened like a string of sausages.

Xanthoria parietina is a bright orange lichen common at the seaside; its common name is the maritime sunburst lichen.

The world of lichens is strange and rather mysterious, but it is worth taking a closer look if you are passing a tree or a gravestone.