Robert R. Ireland
Plants glossy, medium-sized to large, in loose to somewhat dense tufts. Stems with a creeping primary stem producing an erect, frondose-dendroid secondary stem, pinnately and bipinnately branched; lamellae on branches and branchlets numerous, 1--4 cells high, attached to epidermal cells of stems and abaxial surface of leaf bases and costa; lamellae cells smooth, hyaline, inflated, thin-walled; rhizoids abundant on primary and secondary stems, whitish or reddish brown, much branched, papillose, those on secondary stems branches and branchlets on apical cells of lamellae, usually dichotomously branched, smooth; paraphyllia and pseudoparaphyllia lacking; Secondary stem leaves complanate to slightly concave, erect, oblong-ovate, apiculate. Secondary stem branch and branchlet leaves erect-spreading, long-decurrent; cells of decurrent regions many, hyaline, inflated; median cells long, vermicular; costae single. Sexual condition dioicous. Seta elongate. Capsule lateral on stem, ovoid, arcuate, cernuous; peristome hypnaceous, double, teeth trabeculate, cilia lacking.
Genus 1, species 1; temperate climates.
The family Pleuroziopsaceae (R. R. Ireland 1968) was established for the genus Pleuroziopsis primarily because of the presence of the unique stem lamellae first discovered by A. Noguchi (1952). The lamellae are abundant on the branches and branchlets and in addition to being attached to the stems, they are attached to the abaxial surface of the leaf bases and proximal part of the costa. Also, the pinnate and bipinnate branching pattern, the ovoid, arcuate, cernuous capsule with a hypnaceous peristome and the lack of paraphyllia were additional morphological characters making it distinct it from Climacium. Recently, D. H. Norris and M. S. Ignatov (2000), discovered (1--)2--3-stratose longitudinal streaks (stem lamellae) on the stems of Climacium dendroides plants growing intermixed with Pleuroziopsis plants from Alaska. These stem lamellae are scattered along the stems of C. dendroides, the only species of Climacium in which they were observed. Whether these lamellae are morphologically similar to those of Pleuroziopsis is debatable. The lamellae of Pleuroziopsis, which are numerous, are attached to the leaf bases, the costa and the epidermal cells of the branch and branchlet stems, continuing proximally toward the leaf below. The lamellae of Climacium dendroides are not attached to the leaf bases or costa but instead only to the epidermal stem cells. In addition to the lamellae the authors noted that the structures referred to as paraphyllia in Climacium are rhizoids, just as they are in Pleuroziopsis. However, it is primarily because of the stem lamellae that the authors believe Pleuroziopsis should be placed back in the family Climaciaceae.
Ireland, R. R. 1968. Pleuroziopsidaceae, a new family of mosses. J. Hattori Bot. Lab. 31: 59--64. Noguchi, A. 1952. Mosses of Climaciaceae. Oita Daigaku Gakugeigakubu Kenkyu Kiyo 1: 65--77.