The taxonomic revision of the European and African species of Marchantia L., which completes the worldwide revision of the genus, is based upon the study of structural variation in about 1700 specimens. The study of the European species was limited to recently collected plant material; most synonymy was taken from the literature. Of the 28 described African taxa, 6 are retained, and 12 new synonyms are proposed. The taxonomic status of 8 African names remains unresolved.
The taxa are grouped into two subgenera: subg. Marchantia, with two species, and subg. Chlamidium (including sect. Paleaceae, sect. Chlamidium, and sect. Papillatae), with four species. Keys to plants with and without reproductive structures allow identification.
Ninety four morphological characters were analysed. Multivariate statistical analysis permitted investigation of the range of morphological variation in the highly variable species M. polymorpha, M. pappena and M. debilis.
The European and African taxa of Marchantia are adapted to a wide range of microhabitats. Dispersal of some of the taxa seems to be limited by either high or low temperatures, but the availability of water throughout the year, free space and light seem to be the main factors controlling their distribution.
Few species are present in Europe and Africa, as compared to the other continents; only two exclusively inhabit the African continent. Nearly all show a low level of sexual reproduction and seem to rely mainly on vegetative multiplication for their dispersal and survival. Major climatic fluctuations and the repeated shrinking of the humid areas in Africa in the past, may be responsible for the low diversity and for the reproductive behaviour of the African Species.