21. Selaginella wallacei Hieronymus, Hedwigia. 39: 297. 1900.
Plants on rock or terrestrial, forming loose or compact mats. Stems radially symmetric, creeping or decumbent, not readily fragmenting, irregularly forked, without budlike arrested branches, tips straight; main stem long, indeterminate, lateral branches determinate, ascending, 1--2-forked. Rhizophores borne on upperside of stems, throughout stem length, 0.23--0.36(--0.4) mm diam. Leaves monomorphic, in ± alternate pseudowhorls of 4, tightly or loosely appressed, ascending, green, linear-lanceolate, (1.5--)1.8--3.5 X 0.39--0.66 mm; abaxial ridges well defined; base rounded and adnate or cuneate and slightly decurrent on fleshy, loosely appressed stem leaves (from wet places), pubescent, seldom glabrous; margins short-ciliate to denticulate, cilia transparent, spreading at base, dentiform, and ascending toward apex, 0.03--0.06(--0.1) mm; apex keeled and obtuse, sometimes attenuate or plane and attenuate, abruptly short- to long-bristled; bristle transparent to whitish, puberulent, sometimes breaking off, (0.16--)0.2--0.46(--0.9) mm. Strobili often paired, 1--4.5(--9) cm; sporophylls deltate-ovate (mostly on exposed and compact mats) or lanceolate-ovate (on loose, spreading mats from wet places), abaxial ridges well defined, base glabrous, margins short-ciliate to denticulate, apex keeled, abruptly short-bristled, seldom tapering into bristle.
On dry, exposed cliffs, rocky slopes, rocky knolls, or sandy-gravelly soil or on moist, shaded, rocky banks or in meadows; 0--2000 m; Alta., B.C.; Calif., Idaho, Mont., Oreg., Wash.
Selaginella wallacei is extremely variable depending on its habitat (R. M. Tryon 1955). Plants in dry, exposed conditions have short stems, form compact mats with tightly appressed leaves adnate to the stem, and have a rather keeled, abruptly bristled apex. Plants from moist habitats have long stems, form rather moderately long-creeping mats, and have less appressed, decurrent, fleshy leaves, with a more plane-attenuate apex that gradually tapers into a bristle. Plants from exposed, dry conditions sometimes are confused with S . scopulorum , but they have a keeled apex with well-defined ridges on the abaxial groove whereas in S . scopulorum the leaf apex is ± plane and attenuate, and the ridges on the abaxial groove are not prominent. Plants from moist habitats somewhat resemble plants of S . underwoodii .
R. M. Tryon (1955) found strobili 9 cm long in Selaginella wallacei , the longest strobili known within subg. Tetragonostachys and comparable only to those of S . oregana .