Horizontal stems deeply (5--12 cm) buried, 1.5--3.2 mm wide; leaves spatulate to somewhat obovate, 1.8--3.5 X 1.1--1.5, apex faintly erose to irregularly lobed. Upright shoots clustered, branching near base, 17--36 cm; leaves monomorphic, appressed, subulate, 1.9--3.4 X 0.6--1 mm, apex acute. Branchlets square with rounded angles in cross section, 1--2.2 mm wide, annual bud constrictions abrupt and conspicuous; upperside convex, bluish to whitish green. Leaves on branchlets 4-ranked, upperside leaves appressed, needlelike, free portion of blade 1--1.7 X 0.5--0.9 mm; lateral leaves appressed, 3.4--7.2 X 1.1--2 mm; underside leaves appressed, somewhat decurrent, 1--2 X 0.4--0.7 mm. Peduncles (1--)3, 4--15 X 0.4--1 mm; leaves remote, scattered, decurrent, free tips ascending, subulate, 2--3 X 0.2--0.25 mm. Stalks mostly formed by successive forking of peduncle, branches uniformly spaced. Strobili (2--)3--4(--7), 10--28 X 2--3 mm, apex round-tipped, sterile tips absent. Sporophylls deltate, 2.2--3.5 X 1.6--3 mm, apex gradually tapering. 2 n = 46. Sterile, acidic soils in open conifer and oak forests; 50--1800 m; Man., N.B., Nfld., N.S., Ont., P.E.I., Que.; Ala., Conn., Del., D.C., Ga., Ind., Ky., Maine, Md., Mass., Mich., Minn., Mo., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., S.C., Tenn., Vt., Va., W.Va., Wis.; Europe; Asia in w China. The distinctive Diphasiastrum tristachyum has narrow, rounded branches and dull, bluish white color. It is a parent in more hybrid combinations than any other North American Diphasiastrum . The reticulogram shows the known pattern of interspecific hybridization in Diphasiastrum . The hybrids are discussed in detail by J. H. Wilce (1965), and their cytology is summarized by F. S. Wagner (1992). The best known North American hybrids are the four involving D . tristachyum . All of the hybrids have apparently normal meiosis and spores.
Diphasiastrum × zeilleri (Rouy) Holub (= D . complanatum X tristachyum ) is a frequent plant in areas of distributional overlap between the parents, especially in north central and western Minnesota jackpine forests.
Diphasiastrum × habereri (House) Holub (= D . digitatum X tristachyum ) has been overlooked and confused with both parents in zones of overlap. It is found occasionally to frequently in habitats like those of the parents, not necessarily growing close to them.
Diphasiastrum × issleri (Rouy) Holub (= D . alpinum X tristachyum ) is a rare hybrid in North America, reported only from Maine, but much more widespread in Europe.
Diphasiastrum × sabinifolium (Willdenow) Holub (= D . sitchense X tristachyum ) is widespread and frequent in eastern Canada. This hybrid is commonly confused with D . sitchense . It is highly variable, and some individuals approach one or the other parent in morphology (W. J. Cody and D. M. Britton 1989). In the flora, the populations are mainly disjunct from the main range of D . sitchense , including those in Ontario, Quebec, Maine, Michigan, New Hampshire, New York, Pennsylvania, and Vermont.
Two other North American nothospecies are Diphasiastrum complanatum X digitatum and D . alpinum X sitchense .
Perennial fern ally 17 - 40 cm tall Leaves: appressed, overlapping like shingles, stalkless on branchlets, and in four ranks: top, bottom, and sides (lateral). The top rank (i.e. along branchlet upper side) and bottom rank (i.e. along branchlet underside) of leaves are needle-like (1 - 2 mm long, 0.4 - 0.9 mm wide), while the lateral ranks of leaves are larger (3.4 - 7.2 mm long, 1.1 - 2 mm wide), and scale-like with pointed tips. Rhizome: deeply buried under 5 - 12 cm of soil, not green, round in cross-section, 1.5 - 3.2 mm in diameter, and covered with small (1.8 - 3.5 mm long), thin, appressed, scale-like, spoon-shaped leaves. Spores: hundreds per sac, all of one kind, 30 - 38 microns in diameter, thick-walled, veiny, and three-sectioned (trilete) with pointed angles. The spores give rise to the gametophyte (the sexual phase of the plant), which is small (no more than 3 cm long), carrot-shaped, underground, not green, but saprophytic, and inhabited by symbiotic fungi (mycorrhizae). Upright stems: arising from rhizomes about every 1 - 8 cm, often branching near base, rounded in cross-section, and covered with small (1.9 - 3.4 mm long, up to 1 mm wide), awl-shaped, appressed leaves. Branchlets: alternate, lateral from upright stems, ascending, upper side bluish green, underside more whitish green with a grayish white coating (glaucous), cordlike, somewhat four-angled, almost square in cross-section, 1 - 2.2 mm in diameter. The branchlets successively branch again so close together that these branched branchlets are nearly parallel to each other, and subsequently become narrowly fan-like in shape.
Similar species: Diphasiastrum tristachyum is similar to D. digitatum, but that species differs by having flattened and blade-like ultimate branchlets; the leaves on the underside of the branchlets are much smaller and reduced in size compared to the leaves on the upper side and sides; the plant is usually more green in color; the rhizome is above ground and only buried by leaf litter; the branching pattern of the strobili stalks is closely spaced so it looks like a single branching (almost whorl-like) rather than two successive branchings; and the strobili often have sterile areas at the tips. Even more similar is the hybrid between these two species, D. x habereri, which differs from D. tristachyum by being a bit taller (usually 30 - 60 cm tall), having much narrower and smaller leaves along the underside than on the sides and upper side of the shoots, and having flattened or compressed ultimate branchlets.
Habitat and ecology: Very rare, in moist to dry woods, only in the eastern half of the Chicago Region.
Occurence in the Chicago region: native
Notes: Members of this genus were formerly included within the genus Lycopodium . Diphasiastrum tristachyum hybridizes with many other species in the genus, more so than any other North American Diphasiastrum.
Etymology: Diphasiastrum comes from the Latin Diphasium, the name of a genus, and astrum, meaning "incomplete resemblance". Tristachyum means "three-spiked", referring to the three fertile stalks per upright stem.