Peristome Variations in the Genus Fissidens: An SEM Study

Abstract

Peristomes of 19 species representing two subgenera of the genus Fissidens were examined using scanning electron microscopy. Seven types of peri- stomes were distinguished on the basis of variations in the trabeculae and the lamellae of the dorsal and ventral surfaces. These seven types are designated the scariosus-, bryoides-, excurrentinervis-, obtusifolius-, taxifolius-, subbasilaris- and manateensis-types of peristomes. In some instances, species having similar peristome types are gametophytically similar; in other instances species having the same peristome type have different gametophytes. In still other instances, species with dissimilar peristome types have similar gametophytes. The genus Fissidens is a homogeneous group of mosses distinguished by an unusual leaf structure. The leaves are equitant and each is composed of a ventral, boat-shaped proximal portion consisting of two vaginant laminae and an undivided distal ventral lamina; dorsally there is an undivided dorsal lamina. In addition, the leaves are distichous and complanate. In the Musci, the peristome is considered an important indicator of natural relationships (Philibert 1884-in Taylor 1962, Dixon 1932, Buck & Crum 1978). The peristome of Fis- sidens, usually described as dicranoid, consists of a single row of 16 peristome teeth that are haplolepideous (having a single row of dorsal scales). Thus, the genus Fissidens is a taxon apparently closely related to the Dicranaceae on the basis of the sporophyte but which possesses a distinctive gametophyte easily separating it from all other haplolepi- deous mosses. Conspectus of the genus Fissidens. The genus Fissidens was established by Hedwig (1801) to accommodate 14 species. Since that time the number of species ascribed to Fissidens has increased to approximately 876 (Wijk et al. 1962, 1969). Along with this increase in the number of species, a large number of subgeneric taxa has been described. Brotherus (1924) recognized 4 subgenera, viz., Polypodiopsis (C. M0ll.) Broth., Eufissi- dens Hedw. (=Fissidens), Pachyfissidens (C. M011.) Kindb. and Octodiceras (Brid.) Broth. Subgenus Fissidens was further divided into twelve sections, viz., Weberiopsis C. Mill., Reticularia Broth., Bryoidium C. Miill., Pachylomidium C. Mill., Pycnothallia C. Miill., Heterocaulon C. Miill., Semilimbidium C. MWll., Aloma C. Miill., Crenularia C. Mill., Crispidium C. Miill., Amblythallia C. Miill. and Serridium (C. Mill.). In the past, subgeneric taxa within Fissidens have been based principally on gameto- phytic characters. The peristome was never utilized in the interpretation of relationships within Fissidens. This is because differences seen in structure of the peristome with the light microscope appear minor. However, Florschiitz (1964) believed the current division of the genus was highly artificial. Species he considered closely related were positioned into disparate sections. Furthermore, there has been disagreement among authors as to the number and rank of the subgeneric taxa within Fissidens, even though all treatments