Lin Bang-juan and Si He
Plants whitish, grayish or bluish green, in dense cushions. Stems erect, simple or branched. Leaves arranged in several rows, thick, fleshy, linear, ligulate or lanceolate from a rather narrow to somewhat broad base; costa broad, filling most of leaf base and apex, in cross section with 2 to 10 layers of large, empty, hyaline and porose cells (leucocysts), enclosing a single, more or less median layer of smaller green cells (chlorocysts) (except in Exostratum where chlorocysts are arranged in three layers); laminae very narrow, consisting of delicate, hyaline, oblong and linear cells, often restricted to leaf base or extending somewhat above the shoulders as a very narrow, inconspicuous border. Dioicous or pseudoautoicous. Setae terminal, usually elongate, straight; capsules erect, symmetric or inclined, asymmetric, often strumose at base; peristome single, consisting of 8 or 16 lanceolate teeth, undivided or bifid, smooth or papillose, or vertically pitted-striolate; opercula conic-rostrate with a long beak. Calyptrae mostly cucullate. Spores small, spherical.
In a broad sense eleven genera and some 200 species are currently known as leucobryoid mosses that belong to the Leucobryaceae. The leucobryoid mosses share a similar leaf structure, where the large, empty, thin-walled leucocysts (or hyalocysts) predominate over the chlorocysts (green cells). The classification of the Leucobryaceae is variable according to different opinions of various taxonomists. Cardot (1899) and Brotherus (1924) recognized four tribes or four subfamilies within the Leucobryaceae in which the family is an unnatural and artificial assemblage of mosses that share only superficial similarities in their gametophytes. Based on the peristome structures, the leucobryoid genera can be classified into two basic families, the Leucobryaceae and Leucophanaceae (= Calymperaceae) (Andrews 1947; Fleischer 1904; Vitt 1984) or placed into several families (Eddy 1990).
The leucobryoid mosses are mainly distributed in the tropical regions, except that some species of Leucobryum extend far northward and southward into temperate latitudes. Four genera and 17 species of leucobryoid mosses are known from China, namely Exostratum L. T. Ellis, Leucophanes Brid. Leucobryum Brid., and Octoblepharum Hedw. These genera are grouped in the traditional sense of the Leucobryaceae (sensu Brotherus 1924) for ease of use in a purely descriptive sense. There is no taxonomic significance implied in this treatment. We are in agreement with the new classification based on the peristome structures.