Haplohymenium is a genus of small, delicate plants that have erect-imbricate, often fragile leaves.
Haplohymenium triste (Ces. ex De Not.) Kindb., the most widespread species, has this feature especially
well developed, and as a result fragile leaves are often considered of primary importance in
defining the genus. Fragile leaves, however, are not consistently present within Haplohymenium, and
since some Anomodon species (e.g., A. thraustus Müll. Hal.) have extremely fragile leaves the genus
has been treated as a section of Anomodon. This view is further supported by the remarkable amount
of character state variation found in Anomodon. Haplohymenium, however, is well distinguished from
Anomodon. The genus has a distinctive aspect due to its slender plants that have leaves erect-imbricate
when dry. In addition, all species have a clear areolation even when the cells are pluripapillose because
the cells are also bulging-mammillose. In contrast, the cells of Anomodon are densely papillose
in such a way that the cells are obscured. The primary reasons for recognizing Haplohymenium at the
generic level, however, are associated with its sporophytes. Haplohymenium has short setae (2–5 mm
vs. 9–22 mm in Anomodon), short, oblong-ovate to subglobose capsules, verrucose exostome teeth,
a rudimentary endostome present only as a very low membrane, and sparsely hairy calyptrae. The
genus was revised by Noguchi (1957) and Granzow-de la Cerda (1997).
The name Haplohymenium combines the Greek haplo (single) with hymen (membrane) in reference
to its much reduced endostome.