Plants mainly trailing on ground. Roots emerging from point of origin on underside of main stems. Horizontal stems on substrate surface or subterranean, long-creeping. Upright shoots scattered along horizontal stem, 5--16 mm diam., round or flat in cross section, unbranched or with 1--4 lateral branchlets. Leaves not imbricate, linear to linear-lanceolate; leaves on horizontal stems scattered, appressed, membranous; leaves on lateral branchlets mostly 6-ranked or more, monomorphic with few exceptions, appressed, ascending to spreading, margins entire to dentate. Gemmiferous branchlets and gemmae absent. Strobili single and sessile or multiple and pedunculate, apex blunt to acute; peduncle, when present, conspicuously leafy; sporophylls extremely reduced, much shorter than peduncle or stem leaves. Sporangia reniform. Spores reticulate, sides at equator convex, angles acute. Gametophytes nonphotosynthetic, mycorrhizal, subterranean, flat and irregularly button-shaped, with ring meristem around circumference. x = 34. In striking contrast to Diphasiastrum , Huperzia , and Lycopodiella , interspecific hybridization is practically unknown in Lycopodium . Many of the species now recognized in Lycopodium have been segregated from Lycopodium clavatum , L . annotinum , and L . jussiaei Desvaux ex Poiret. The three groups given in the key below should probably be treated as subgenera.
Leafy-stemmed perennial herbs, often evergreen. 400, cosmop. (Diphasiastrum, Diphasium, Huperzia, Lepidotis, Lycopodiella, Pseudolycopodiella) species in a given section or subgenus are cytologically similar and commonly form ±sterile but vegetatively vigorous hybrids that often reproduce effectively by rhizomes or gemmae, sometimes forming considerable populations. Some of these hybrids are: L. xbrucei (Cranfill) Lellinger = L. appressum x prostratum; L. xbuttersii Abbe = L. lucidulum x selago; L. xcopelandii Eiger = L. alopecuroides x appressum; L. x habereri House = L. digitatum x tristachyum; L. xissleri (Rouy) Lawalree = L. alpinum x complanatum; L. xzeilleri (Rouy) Beitel = L. complanatum x tristachyum.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.