The dual nature of the lichen thallus introduces at the outset a classificatory difficulty. Theoretically the lichens may be classified on the basis of their algal constituent, on the basis of their fungal constituent, or they may be classified as if they were homogeneous organisms. The first of these systems is impracticable owing to the absence of algal reproductive organs and the similarity of the algal cells (gonidia) in a large number of different forms. The second system is the most obvious one, since the fungus is the dominant partner and produces reproductive organs. The third system was that of Nylander and his followers, who did not accept the Schwenderian doctrine of duality. In actual practice the difference between the second and third methods is not very great since the fungus is the producer of the reproductive organs and generally the main constituent. Most systems agree in deriving the major divisions from the characters of the reproductive organs (perithecia, apothecia, or basidiospore bearing fructification), while the characters of the algal cells and those of the thallus generally are used for the minor divisions. The difference between the various systems lies in the relative importance given to the reproductive characters on the one hand and the vegetative characters on the other. In the system (1854-1855) of Nylander the greater weight is given to the latter, while in more modern systems the former characters receive the more attention.
A brief outline of a system of classification, mainly that of Zahlbruckner as given in Engler and Prantl’s Pflanzenfamilien, is outlined below.
There are two main divisions of lichens, Ascolichenes and Basidiolichenes, according to the nature of the fungal element, whether an ascomycete or basidiomycete. The Ascolichenes are again divided into Pyrenocarpeae or Pyrenolichenes and Gymnocarpeae or Discolichenes; the first having an ascocarp of the nature of a perithecium, the second bearing their ascospores in an open apothecium.
Pyrenolichenes
Series I. Perithecium simple not divided.
a. With Pleurococcus or Palmella gonidia. Moriolaceae, Verrucariaceae, Pyrenothamnaceae.
b. With Chroolepus gonidia. Pyrenulaceae, Paratheliaceae.
c. With Phyllactidium or Cephaleurus gonidia. Strigulaceae.
d. With Nostoc or Scytonema gonidia. Pyrenidiaceae.
Series II. Perithecia divided or imperfectly divided by cross-walls. Mycoporaceae with Palmella or Chroolepus gonidia.