Hybrids between species of Diphasiastrum are apparently fertile. They are intermediate in morphology, but can occasionally occur without one or even both parents. The commonest hybrid is D. ×habereri (House) Holub, between the widespread D. digitatum and D. tristachyum. This is best distinguished by having a relatively shallowly buried horizontal stem but with lower leaves of the branches much less reduced than in D. digitatum. The hybrids D. complanatum × D. tristachyum (D. ×zeilleri (Rouy) Holub) and D. complanatum × D. digitatum (D. ×verecundum A. V. Gilman) are much rarer.
1. Strobili sessile; collected long ago in Keewenaw Co.
1. Strobili (or groups of strobili) short to long peduncled; widespread.
2. Strobili 1 (–2) on short to elongated, rarely forked peduncles; base of strobilus with a few sporophylls scattered along peduncle; stomata on both leaf surfaces; known only from Chippewa Co.
2. Strobili 2–4 on forked peduncles; base of strobilus compact and abruptly distinct from peduncle; stomata on lower leaf surfaces only.
3. Horizontal stems almost always buried ca. 4–10 cm in the soil; ultimate (outermost) branches squarish in cross section, 1–2 mm wide; plants usually blue-green (except in deep shade); lower leaves the same size as the upper.
3. Horizontal stems superficial, or slightly buried (< 4 cm) in the litter; ultimate (outermost) branches flat in cross section; 2–4 mm wide; plants not blue-green; lower leaves reduced to a leaf base and a short free appendage.
4. Ultimate branches symmetrically in one plane forming a layered appearance; annual constrictions mostly absent; peduncles of strobili 0.6–1 mm in diameter and staying green until after spore discharge in autumn (Sept. –early Oct.); widespread.
4. Ultimate branches not evenly layered, giving an irregular appearance; annual constrictions uniformly present; peduncles of strobili 0.4–0.7 mm in diameter and losing green by spore discharge in late summer (late July–early Aug.); Upper Peninsula and northernmost Lower Peninsula south (very rarely) to ca. 44° 30’ N.
All species found in Diphasiastrum
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. April 28, 2017. http://michiganflora.net/genus.aspx?id=Diphasiastrum.