Plants terrestrial, on rock, or rarely hemiepiphytic (initially terrestrial, becoming epiphytic) or epiphytic (in S. oregana). Stems prostrate, creeping, decumbent, cespitose, climbing, or fully erect, articulate or not, slightly to greatly branched. Rhizophores usually present, stout or filiform. Roots branching several times dichotomously from rhizophore tips. Leaves on aerial stems dimorphic or monomorphic; if monomorphic, then linear to narrowly lanceolate, highly overlapping, spirally arranged; if leaves on aerial stem dimorphic, then round or oblong to lanceolate, arranged in 4 ranks, 2 ranks of larger spreading lateral leaves and 2 ranks of smaller, appressed, and ascending median leaves, often with axillary leaf at base of each branching dichotomy. Megasporangia lobed to ovoid; microsporangia reniform to ovoid. Megaspores tetrahedral, ovoid, or globose, variously sculptured, (127–)200–1360 µm diam.; microspores tetrahedral, variously sculptured, 20–75 µm diam. x = 7, 8, 9, 10, 11, 12.
Worldwide, mainly tropical and subtropical regions.
The generic and infrageneric classification of Selaginella is controversial, and more than one genus may be recognized (see R. E. G. Pichi-Sermolli 1971 for information on generic synonyms). A. C. Jermy (1986, 1990b) proposed a subgeneric classification similar to that of J. G. Baker (1883, 1887). Despite some reservations, I consider Jermy's system useful for our purpose; therefore it is followed here. Three of the five subgenera proposed by Jermy occur in the flora area: subg. Selaginella, subg. Tetragonostachys, and subg. Stachygynandrum. One of the species in the flora, Selaginella eatonii (see discussion), may eventually prove to be best classified within a fourth, subg. Heterostachys Baker.
Some characteristics used in the keys and descriptions are best observed in fresh specimens or by soaking a sample of a dried specimen in water, using material at branch forks or buds. This is particularly true for members of subg. Tetragonostachys. Use a minimum of 20X (40–60X better) magnification and take measurements of both young and old leaves. Measurements of leaf length include the bristle and the most basal portion.
Selaginella subg. Tetragonostachys has a tendency for stem and leaves close to the substrate surface to be morphologically different from those on the side away from the substrate. In this case, the leaves on the side of the axis away from the surface are called upperside leaves, and those on the side toward the surface are called underside leaves. Otherwise, the leaves are designated only as leaves. In the subg. Stachygynandrum, however, which has complete structural differentiation between stem sides, the upper leaves are called median leaves, and the lower ones are called lateral leaves.
Species probably more than 700 (38 in the flora).
|1||Leaves on aerial stems dimorphic, arranged in 4 ranks (2 median, 2 lateral), axillary leaves present at branching points; rhizophores present.||Subg. Stachygynandrum|
|1||Leaves on aerial stems monomorphic, not in distinct ranks, axillary leaves absent at branching points; rhizophores present or absent.||> 2|
|2||Strobili cylindric; sporophylls spreading; leaves thin, soft, margins short-spiny; stomates throughout abaxial surface of leaf; rhizophores absent.||Subg. Selaginella|
|2||Strobili quadrangular; sporophylls usually appressed; leaves thick or fleshy (seldom thin), firm, margins dentate, serrate, or ciliate (never spiny); stomates in abaxial groove; rhizophores present.||Subg. Tetragonostachys|