Horizontal stems on substrate surface, 1.3--2.7 mm wide; leaves appressed to ascending, linear to narrowly lanceolate, 1.8--4.5 X 0.6--1.2 mm, apex acute, scarious, often lost. Upright shoots 15--50 cm, branching regularly successively to 3 times; leaves appressed with decurrent base, subulate, 1.8--3.5 X 0.6--1 mm, apex acute. Branchlets flat in cross section, narrowly bladelike, 2.8--3.9 mm wide, annual bud constrictions very rare; underside dull, pale, flat; upperside green, flat, shiny. Leaves of branchlets 4-ranked; upperside leaves appressed, linear-lanceolate, free portion of blade 0.7--1.5 X 0.5--0.9 mm; lateral leaves appressed to spreading (spreading especially in juvenile stages), 3.1--5.5 X 1--2 mm; underside leaves very weakly developed, spreading, narrowly deltate, 0.3--1 X 0.3--0.7 mm, apex pointed. Peduncles mostly 2, 4.4--12.5 X 0.1--1.3 cm; leaves usually somewhat whorled, linear-lanceolate to nearly filiform, 2--3.3 X 0.5--0.9 mm, apex blunt to acute. Stalks mostly pseudowhorled, 2-forked, forks basal. Strobili 2--4 per upright shoot, 14--40 X 2--3 mm exclusive of elongate sterile tip; sterile tips to 11 mm (occurring on ca. 50% of specimens), apex blunt to acute if sterile tip is absent. Sporophylls deltate, 1.7--2.6 X 1.8--2.8 mm, apex abruptly tapering. 2 n = 46. Coniferous and hardwood forests and second growth, shrubby or open fields; 0--1500 m; St. Pierre and Miquelon; N.B., Nfld., N.S., Ont., P.E.I., Que.; Ala., Ark., Conn., Del., D.C., Ga., Ill., Ind., Iowa, Ky., Maine, Md., Mass., Mich., Minn., Mo., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., S.C., Tenn., Va., W.Va., Wis. An endemic in eastern North America, Diphasiastrum digitatum is the most abundant species of Diphasiastrum on the continent, much used for decoration as wreaths. It was long confused with the circumboreal D . complanatum .
Perennial fern ally 15 - 55 cm tall Leaves: stalkless on branchlets, and in four ranks: top, bottom, and sides (lateral). The top rank of leaves (i.e. along branchlet upper side) are appressed, 0.7 - 1.5 mm long, 0.5 - 0.9 mm wide, and narrowly lance-shaped, whereas the bottom rank (i.e. along branchlet underside) are weakly developed but obviously spreading, 0.3 - 1 mm long, 0.3 - 0.7 mm wide, and very reduced or hair-like. Lateral ranks of leaves appressed to spreading, 3.1 - 5.5 mm long, 1 -2 mm wide, somewhat linear with long slender tips. Rhizome: on or above soil surface, or lightly covered by leaf litter, never buried underground, rounded to almost flattened in cross-section, 1.3 - 2.7 mm in diameter, and covered with small (1.8 - 4.5 mm long), thin, appressed to ascending, scale-like, linear to narrowly lance-shaped leaves. Spores: hundreds per sac, all of one kind, 24 - 33 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 1.5 - 7 cm, mostly rounded in cross-section, and covered with small (1.8 - 3.5 mm long, up to 1 mm wide), awl-shaped, appressed leaves. The stems branch regularly one time the first season, and then two times each successive season. Branchlets: nearly opposite, lateral from upright stems, ascending, upper side green and shiny, underside dull and pale, 2.8 - 3.9 mm wide, and flattened or almost blade-like. The branchlets successively branch again, and these branched branchlets are spreading and subsequently become broadly fan-like in shape.
Similar species: Diphasiastrum digitatum is similar to D. tristachyum, but that species differs by having cordlike, nearly four-angled ultimate branchlets; the leaves on the underside of the branchlets are fairly similar in size and shape to the other ranks of leaves; the plant is usually more bluish or gray in color; the rhizome is very deeply buried underground; the strobili stalks branch successively at two separate points; and the strobili never have sterile areas at the tips. Even more similar is the hybrid between these two species, D. x habereri, which differs from D. digitatum by having a more deeply buried rhizome, being quite vertical in habit with an average height of 30 - 60 cm, and having narrower (only up to 2 mm wide) ultimate branchlets. Diphasiastrum digitatum has often been confused with a more northern species, D. complanatum, but that species does not have such regularly fan-shaped branchlets, the strobili stalks are usually non-forked, and the strobili are usually smaller (1.5 - 2.5 cm long) and never have sterile areas at the tips.
Habitat and ecology: Occasional, preferring dry conditions such as sandy fields, thickets, or dry woods.
Occurence in the Chicago region: native
Notes: At one time placed within the genus Lycopodium, this species has also been known as L. flabelliforme. It is the most abundant species of Diphasiastrum in North America, even though it is isolated to the east, where it is endemic. This species is commonly confused with D. complanatum, a more northern species that occurs throughout boreal areas of the world (circumboreal), but the plants are not that similar in branching morphology.
Etymology: Diphasiastrum comes from the Latin Diphasium, the name of a genus, and astrum, meaning "incomplete resemblance". Digitatum is Latin for finger, refering to the branching pattern that resembles fingers on a hand.
Author: The Field Museum
From Flora of Indiana (1940) by Charles C. Deam
Extremely local. Found on moist, rocky slopes. This species is regarded by many authors as a variety of Lycopodium complanatum. Blanchard (Rhodora 13: 168-171. 1911) made a special study of this species and L. complanatum in the field, and after nearly ten years' observation, concluded that the two were distinct species. Victorin (Contrib. Lab. Bot. Univ. Montreal. no. 3: 62-63. 1925) confirms Blanchard's observation of characters which seem to me also to be sufficient to regard this form as a species rather than a variety. These two species have definite geographical ranges which add to this opinion. The range of L. complanatun in North America extends from Newfoundland through the greater part of Canada to Alaska and southward to northern Michigan, northern Wisconsin (not reaching New England), and Washington. L. flabelliforme is much more southern, occurring from Newfoundland, Nova Scotia, and the lower valley of the St. Lawrence River westward to Minnesota, southward to North Carolina and Kentucky.
Horizontal stems surficial or nearly so; erect stems more regularly branched, the branches determinate, without annual constrictions, the branch- systems flattened and fan-shaped; cones 2-4+ cm, commonly (not always) with a slender sterile tip several mm, the typically 4 peduncles tending to be subumbellately clustered; otherwise much like no. 16 [Lycopodium complanatum L.]; 2n=46. Dry open woods or meadows; Nf. and Que. to Man. and Minn., s. to Ga., Tenn., and La. (L. dillenianum; L. flabelliforme; L. complanatum var. f.)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.