17. Selaginella mutica D. C. Eaton ex L. Underwood, Bull. Torrey Bot. Club. 25: 128. 1898.
Plants on rock or terrestrial, forming loose mats. Stems radially symmetric, long- to short-creeping, not readily fragmenting, ± regularly forked, without budlike arrested branches, tips straight; main stem indeterminate, lateral branches determinate, 1--2-forked. Rhizophores borne on upperside of stems, throughout stem length, 0.13--0.23 mm diam. Leaves monomorphic, in ± alternate pseudowhorls of 3, tightly appressed, ascending, green, lanceolate to linear-lanceolate or lanceolate-elliptic, 1--2 X 0.45--0.6 mm; abaxial ridges well defined; base rounded and adnate, sometimes slightly decurrent, pubescent or glabrous; margins ciliate to denticulate, cilia transparent, spreading or ascending, 0.03--0.17 mm; apex keeled, obtuse or slightly attenuate, nearly truncate in profile, blunt to short-bristled; bristle transparent to greenish transparent or whitish, smooth, 0.06--0.45 mm. Strobili solitary, (0.6--)1--3 cm; sporophylls ovate-lanceolate, ovate-elliptic, or deltate-ovate, abaxial ridges well defined, base glabrous, margins ciliate to denticulate, apex strongly to slightly keeled, short-bristled to blunt. 2 n = 18.
Varieties 2: only in the flora.
Selaginella mutica , S . underwoodii (R. M. Tryon 1955; C. A. Weatherby 1943), and S . wallacei all have similar patterns of variation. Study is needed to assess to what degree such variability is caused by environmental or genetic factors. Within S . mutica , two rather distinct, morphologic extremes are recognized here as varieties. Many specimens having leaves with spreading, long, marginal cilia and a short, broken, apical bristle have been considered intermediate between the two varieties, but they belong in S . mutica var. mutica .
Selaginella mutica may be one of the parent species of the putative hybrid species S . × neomexicana (see discussion). Selaginella mutica is often found growing in the same habitat with S . underwoodii , S . × neomexicana , and S . weatherbiana . According to R. M. Tryon (1955), where the two grow together, S . mutica mats gradually entirely replace mats of S . underwoodii over time. Selaginella mutica is sometimes confused with S . viridissima .