Up to now, about 300 species of ascomycetes are known to grow obligately on the gametophytes of mosses or hepatics; these belong to more than 80 ascomycete genera, some of them unknown elsewhere, and at least nine orders. They vary greatly in relation to the mode of nutrition (necrotrophic, biotrophic and gall-inducing parasites), the host organs infected, host specificity, and geographical distribution. Diversity of these fungi is illustrated by spore outlines of 36 species. Hyphae growing superficially, inter- or intracellularly, and various types of appressoria and haustoria are illustrated. Adaptations to the unique substratum, such as the formation of minute reduced ascomata containing gelatinous material, a preference for moist microhabitats such as the interlamellar spaces of leaves in Polytrichales, and phototropic reactions are discussed. Convergent evolution leading to similar morphological features is rather common. The colonization of mosses and hepatics by ascomycetes is a very frequent though generally neglected phenomenon. Some bryophytes seem to grow always with their specific parasites. It is supposed that species diversity among the bryophilous ascomycetes is as high as among the lichenicolous ones. Numerous new taxa remain to be described, but only a small fraction of bryophytes has been proved to be infected so far.
