Fissidentaceae

One of the most distinctive families of mosses, easily distinguished by their distinctive gametophytes. All species have leaves arranged in two rows, on opposite sides of the stem, with all the leaves flattened in a single plane (complanate) that is perpendicular to the substrate. The leaf morphology is complex and is divided into three sections for descriptive purposes: the vaginant, dorsal and apical laminae. The dorsal lamina section of the leaf is a single layer that extends from the base to the apex on the side of the costa facing the substrate (i.e. the basiscopic side). In some species (e.g. F. dealbatus Hook.f. & Wilson) a costa appears to be lacking, and in such species the dorsal lamina is demarcated by where a costa would be expected which is a line several cells thick. On the other side of the costa, which faces the stem apex (i.e. the acroscopic side), the leaf is divided into the apical lamina and the vaginant laminae. From the apex to where the leaf becomes double layered is referred to as the apical lamina. Basal to this section the leaf is referred to as the vaginant laminae. In this section the leaf has two layers of largely overlapping leaf laminae, each referred to as a vaginant lamina. The vaginant laminae are fused along the costa or along the edge away from the leaf margin where a costa could be expected for species without a costa. In some species, the vaginant laminae are fused only along the costa, a condition referred to as open, while in others the vaginant laminae are fused from the costa to the leaf margin as well and are referred to as closed. The vaginant laminae are always free along the leaf margin and sheathe the stem and often the adjacent leaf closer to the stem apex. In species with open vaginant laminae, one of the vaginant laminae continues as the apical lamina and is referred to as the major lamina in the vaginant laminae section of the leaf, while the other vaginant lamina is referred to as the minor lamina.

In two non-Victorian species, that have been recognised in a separate genus Nanobryum within its own family (e.g. Stone 1982), the dorsal and apical laminae are much reduced or even absent. These species have a distinctive costal anatomy that is shared with some Fissidens species, with typical leaf morphology, that are placed by phylogenetic analyses of chloroplast sequence data well within the Fissidens lineage (Suzuki et al. 2018). This supports inclusion of Nanobryum within Fissidens.

Two main theories have been proposed explaining the derivation of the peculiar leaf morphology in Fissidens (Robinson 1970). In one theory the two vaginant lamellae represent the original leaf and the apical and dorsal laminae were extensions to this original leaf. In the second theory, the original leaf was trilobed, two of the lobes now represented by the vaginant laminae and the third and middle lobe was turned on its axis to become the apical and dorsal laminae.

An important feature for the identification of Fissidens species is the presence and extent of a border of elongated, thick-walled cells, typically with pointed ends, which is often referred to as a limbidium. The term border is used here instead of limbidium. The marginal cells of some species (e.g. F. adianthoides) are differentiated from other cells and hence form a border but are not elongated and are not equivalent to a limbidium. Limbidium presence and extent combined with characters associated with the peristome, costa anatomy and cell size, define genetic lineages that have been classified as separate sections (Suzuki et al. 2018). The subgenera are defined by peristome characters. Subgenus Pachyfissidens has a peristome structure that is not responsive to water content in the air, while the other subgenera Fissidens and Neoamblyothallia have peristomes that respond to higher humidity by bending inwards over the mouth (Suzuki et al. 2018). The upper portions of the peristome filaments are spirally thickened in subgenus Fissidens, while in subgenus Neoamblyothallia they are squamose. Unfortunately, sporophytes are often absent on many of the species of Fissidens and are impractical much of the time for identification.  

The peristome of 16 teeth that are usually split into filaments, suggests a close relationship with the Dicranaceae that often shares this peristome morphology. A close relationship to Dicranaceae is also supported by some phylogenetic analyses of chloroplast data that place Fissidentaceae within the Dicranales (La Farge 2000).