Horizontal stems on substrate surface or shallowly buried in litter, 1.1--2.2 mm wide; leaves appressed, linear to narrowly lanceolate, 1.4--4 X 0.5--1.2 mm, apices acute. Upright shoots 8--44 cm, branching irregularly successively to 5 times; leaves on upright main stem appressed with decurrent base, narrowly lanceolate, 1.2--3.2 X 0.5--1.1 mm, apex acute to acuminate. Branchlets flat in cross section, narrowly bladelike, 1.8--4 mm wide, annual bud constrictions abrupt and conspicuous; upperside green, faintly shiny, flat; underside dull, pale, flat. Leaves on branchlets 4-ranked; upperside leaves appressed, linear-lanceolate, free portion of blades 0.7--2 X 0.5--1.2 mm; lateral leaves appressed, 2.6--7.3 X 0.8--2.1 mm; underside leaves weakly developed, appressed, narrowly deltate, 0.7--1.5 X 0.4--0.9 mm. Peduncles 1--2 on each upright shoot, 0.5--8.5 X 0.4--0.9 cm; leaves spirally arranged to nearly whorled, linear-lanceolate, 1.4--4.1 X 0.4--1 mm, apex acute to blunt. Stalks forked at uniform distances. Strobili 1--2(--4), 8.3--32 X 2--3 mm, apex blunt, sterile tip absent. Sporophylls broadly deltate to nearly cordate, 2--3 X 2--2.4 mm, apex abruptly tapering. 2 n = 46.
Dry open coniferous or mixed forest alpine slopes; 0--2000 m; Greenland; St. Pierre and Miquelon; Alta., B.C., Man., N.B., Nfld., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska, Idaho, Maine, Mich., Minn., Mont., N.H., N.Y., Vt., Wash., Wis., Wyo.; circumboreal.
Diphasiastrum complanatum forms a hybrid with D . digitatum that is seemingly uncommon and has never received a binomial designation. It is probably far more common than collections indicate, however. Superficially, the hybrid resembles both parents and is often confused with them. Collections are known from Ontario, Quebec, Connecticut, Maine, Michigan, Minnesota, New Hampshire, Vermont, and Wisconsin.
Horizontal stems elongate, mostly shallowly subterranean, 1-2 mm thick, with distant, reduced lvs; erect stems to 3 dm, branched, the branches indeterminate, with annual constrictions, the branchlets obviously flattened, 2-3 mm wide (incl. lvs) shiny, bright green or somewhat yellowish-green; lvs 4-ranked, entire, adnate over half their length, the upper appressed, the lateral having a short, deltoid free part with slender, spreading or incurved tip, the lower much reduced, seldom 1 mm long; cones 1-2.5 cm, 3-5 mm wide; without a sterile tip, 1-4+ on forked, remotely bracteate peduncles; sporophylls yellowish, broad-based, with scarious erose margins; sporangia reniform, 1.4-1.7 mm wide; spores as in no. 11 [Lycopodium clavatum L.]; 2n=46. Woods and rocky slopes in acid soil; circumboreal, s. in our range to Pa. and Minn.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.