Diphasiastrum Holub (1975: 104).

Type:— Lycopodium complanatum L.

Lycopodium subg. Complanatostachys Herter (1909: 29).— Lycopodium subgen. Thujophyllium Boerner (1912: 110).— Lycopodium sect. Complanata Victorin (1925: 34, 89).— Diphasium Rothm. ser. Complanata Rothm. (1944: 64). Type:— Lycopodium complanatum L.

Sporophytes with subterranean or creeping to subscandent main stems. Branchlets dorsiventral, anisophyllous with decussate, trimorphic leaves, one dorsal and one ventral rank of narrow leaves, and two lateral ranks of broad laterally compressed leaves. Branchlet leaves herbaceous throughout; strobili pedunculate. Sporophylls subpeltate. Sporangial epidermal cells with evenly sinuate side walls. Spores reticulate on all faces; gametophytes dauciform.

This description applies to the Tropical American material only. Some species in North America and East Asia, have radial, isophyllous branchlets.

Diphasiastrum occurs in the northern temperate, arctic and alpine regions, mountains in the tropics, and is absent from Australia. The genus includes approximately 20 species. The genus (as Lycopodium section Complanata) was monographed by Wilce (1965). Holub (1975) extensively discussed the relationship to other groups.