Part 187. The lichen subdivision, lichens in general. Lichens include about 5,000 species, none of which are of much economic importance. They may be defined as algofungal air-plants. Although made up of plants which belong to different classes of Algae and Fungi, which therefore on theoretical grounds might require to be assigned each to its own class, lichens are in practice more conveniently treated as compound organisms forming an artificial group by themselves.

Their dual nature, indeed, doubles the difficulty already encountered in trying to associate different types of fungi with their nearest algal kin. It is of course always desirable to express as well as we can our knowledge of resemblances and our views of kinship; but all this may be done effectively by using names like Ascolichenes and Basidiolichenes (suggestive of relationship between the lichen-fungi and their non-symbiotic kin) and regarding them as forming a series parallel to the series of fungi, much as the fungal series is parallel to the algal.