Diplazium is a genus of ferns comprising approximately 400 known species, commonly called twinsorus ferns. It belongs to the family Athyriaceae within the order Polypodiales, as established by the Pteridophyte Phylogeny Group classification of 2016 (PPG I). The genus had a complex taxonomic history, having been variously assigned to Athyriaceae, Dryopteridaceae, Aspleniaceae, or Polypodiaceae before its current placement was settled.
The genus is defined by a distinctive double-sorus arrangement: the linear basal sori are paired back-to-back on the same vein, and the name Diplazium itself derives from the Greek diplazein, meaning "double," in reference to the indusia lying on both sides of the vein. The indusium is linear and persistent, and the sporangia are brownish.
Morphologically, the rhizome ranges from creeping to erect and is covered in scales. Fronds are deciduous or evergreen, trophopodic, and either monomorphic or weakly dimorphic. The stipe is green, deeply grooved on the upper surface, and consistently possesses two lunate vascular bundles — a diagnostic feature of the genus. Blades are pinnate, ranging from oblong-lanceolate to deltate in outline, and in texture from herbaceous to papery.
Diplazium has a pantropical distribution, with a handful of species extending into temperate regions. Notable members include Diplazium esculentum, the vegetable fern, whose young fronds are widely eaten across tropical Asia; Diplazium hymenodes, the peacock fern; and Diplazium molokaiense, endemic to Molokai, Hawaii. The taxonomy of the genus is complex and by 2009 had never been the subject of a complete monographic study. Several species once placed in Diplazium have since been transferred: the North American glade fern Diplazium pycnocarpon now belongs to Homalosorus (family Diplaziopsidaceae), and two additional species were moved to the newly erected genus Desmophlebium.
Etymology
The name Diplazium derives from the Greek diplazein, meaning "double," referring to the genus's characteristic arrangement of indusia lying on both sides of the frond vein — giving each fertile vein a paired, back-to-back sorus.
Distribution
Diplazium has a pantropical distribution, occurring across tropical regions of Asia, Africa, the Americas, and the Pacific, with a few species extending into temperate zones. The genus is particularly diverse in humid tropical forests.
Taxonomy Notes
Diplazium was historically assigned to several different families — Athyriaceae, Dryopteridaceae, Aspleniaceae, and Polypodiaceae — before the Pteridophyte Phylogeny Group (PPG I, 2016) definitively placed it in Athyriaceae within the eupolypods II clade of Polypodiales. Taxonomy within the genus remains difficult and poorly understood, with no complete monographic revision completed as of 2009. Several species have been removed: Diplazium pycnocarpon (the common glade fern of North America) was transferred to Homalosorus in family Diplaziopsidaceae, and two further species were placed in the new genus Desmophlebium.
