I. LICHEN-ACIDS OF THE FAT SERIES
Group 1. Colourless substances soluble in alkali, the solution not coloured by iron chloride. Exs. protolichesterinic acid (C₁₉H₃₄O₄) obtained from species of Cetraria, and roccellic acid (C₁₇H₃₂O₄) from species of Roccella, from Lecanora tartarea, etc.
Group 2. Neutral colourless substances insoluble in alkalies, but soluble in alcohol, the solution not coloured by iron chloride. Exs. zeorin (C₅₂H₈₈O₄), a product of widely diverse lichens, such as Lecanora (Zeora) sulphurea, Haematomma coccineum, Physcia caesia, Cladonia deformis, etc. and barbatin (C₉H₁₄O), a product of Usnea barbata.
Group 3. Brightly coloured acids, yellow, orange or red, all derivatives of pulvinic acid (C₁₈H₁₂O₅), a laboratory compound which has not been found in nature. The group includes among others vulpinic acid (C₁₉H₁₄O₅) from the brilliant yellow Evernia (Letharia) vulpina, stictaurin (C₃₆H₂₂O₉) deposited in orange-red crystals on the hyphae of Sticta aurata, and rhizocarpic acid (C₂₆H₂₀O₆) chiefly obtained from Rhizocarpon geographicum: it crystallizes out in slender citron-yellow prisms.
Group 4. Only one acid, usninic (C₁₈H₁₆O₇), a derivative of acetylacetic acid, is placed in this group. It is of very wide-spread occurrence, having been found in at least 70 species belonging to very different genera and families of crustaceous shrubby and leafy lichens. Zopf himself isolated it from 48 species.
Group 5. The thiophaninic acid (C₁₂H₆O₉) group representing only a small number. They are all sulphur-yellow in colour and soluble in alcohol, the solution becoming blackish-green or dirty blue on the addition of iron chloride, with one exception, that of subauriferin obtained from the yellow-coloured medulla of Parmelia subaurifera which stains faintly wine-red in an iron solution. Thiophaninic acid, which gives its name to this group, occurs in Pertusaria lutescens and P. Wulfenii, both of which are yellowish crustaceous lichens growing mostly on the trunks of trees.
II. LICHEN-ACIDS OF THE BENZOLE SERIES
The larger number of lichen-acids belong to this series, of which 94 at least are already known. They are divided into two subseries: I. Orcine derivatives, and II. Anthracene derivatives.