Diphasiastrum is a genus of clubmosses — ancient, vascular, non-flowering plants — belonging to the family Lycopodiaceae and placed within the subfamily Lycopodioideae under the Pteridophyte Phylogeny Group classification (PPG I, 2016). The genus contains approximately 16 recognized species along with numerous fertile natural hybrids, which arise so readily that some hybrids occur more frequently in the wild than either parent species.
Plants in Diphasiastrum are immediately recognizable among lycophytes by their flattened, branching stems. Leaves are arranged in three parallel planes — dorsal, lateral, and ventral — and come in two or three distinct forms on the same plant, an arrangement opposite and decussate on the stem. The overall habit of many species superficially mimics diminutive gymnosperms, earning them popular names such as ground-cedar and ground-pine. A consistently distinctive feature of the genus is its basal chromosome count of n=23, which sets it apart from all other lycopod genera.
The genus was formally proposed by the Czech botanist Josef Holub in 1975. It is closely related to Lycopodium, and some authorities treat Diphasiastrum as a section of that genus (Lycopodium sect. Complanata); Plants of the World Online (POWO) considers Diphasiastrum a synonym of Lycopodium. Other classification systems, including PPG I, maintain it as a separate genus. The fertile spores of several Diphasiastrum species are harvested as Lycopodium powder, used historically as a flash powder, in the pharmaceutical industry as a pill coating, and in fingerprint dusting.
Etymology
The genus name Diphasiastrum was coined by Josef Holub in 1975, derived from the related genus Diphasium with the Latin suffix -astrum, denoting an incomplete or false resemblance. Diphasium itself combines the Greek di- (two) and phasis (appearance or phase), a reference to the two-plane (dorsal and ventral) arrangement of leaves that characterizes these plants.
Distribution
Diphasiastrum has a subcosmopolitan distribution centred on the Northern Hemisphere, ranging from boreal and temperate forests of Europe, Asia, and North America southward along mountain chains to Jujuy Province in northwest Argentina. The genus also occurs in New Guinea and the Marquesas Islands in the Pacific. All species require high atmospheric humidity throughout most of the year, or, in colder climates, reliable winter snow cover to protect overwintering stems.
Ecology
Diphasiastrum species grow in a variety of open to semi-shaded habitats including heathlands, pine and spruce forests, alpine meadows, and rocky slopes. They reproduce both by spores and vegetatively via creeping rhizomes. The genus is notable for its high frequency of interspecific hybridization: many hybrids are fertile, and several (such as D. × issleri and D. × zeilleri) are more common in parts of their range than the parent species themselves.
Taxonomy Notes
Diphasiastrum was segregated from Lycopodium by Josef Holub in 1975 on the basis of dorsiventral branchlet structure, a distinctive prothallium type, and a basal chromosome number of n=23. The genus is recognized by PPG I (2016) within Lycopodiaceae subfamily Lycopodioideae. Some authors continue to treat it as Lycopodium sect. Complanata, and Plants of the World Online (POWO) considers Diphasiastrum a synonym of Lycopodium. GBIF records 33 accepted descendants. Many named taxa in the genus are of hybrid origin and several are treated as synonyms depending on the classification followed.
