Walkers on the Cateran trail will notice that many of the trunks and branches of older trees beside the paths are heavily encrusted with lichens. It is thought by some that lichens are parasitic, and that they weaken the tree or, like fungi, are an indication of the tree’s poor health. This is not the case. Rather the majority of lichens are the product of a symbiotic or mutually beneficial relationship between algae and fungi, in which the one provides nutrients for the other, assisted by rainwater and minerals from the surrounding environment. The lichens are simply using the tree for support. Lichens come in a variety of shapes, generally described as ‘foliose’ (leaf-like), fruticose (with branching stems), or ‘crustose’ (flaky incrustation). Most lichens are sensitive to pollution, so are much rarer in urban areas and beside busy roads. The presence of lichens, as seen in the glens, is one result of the unpolluted air of the Highlands.