Plants on rock or terrestrial, forming long or spreading mats or rarely cushionlike mats. Stems radially symmetric, long to moderately short-creeping to decumbent, not readily fragmenting, irregularly forked, without budlike arrested branches, tips straight; main stem indeterminate, lateral branches conspicuously or inconspicuously determinate, sometimes ascending, 1--3-forked. Rhizophores borne on upperside of stems, throughout stem length, 0.25--0.45 mm diam. Leaves monomorphic, in alternate pseudowhorls of 6 (on main stem) to 4 (on lateral branches), tightly appressed, ascending, green, occasionally reddish, linear or linear-lanceolate, 2.5--4(--4.5) X 0.45--0.6 mm; abaxial ridges well defined; base cuneate and decurrent on underside to rounded and adnate on upperside, pubescent or glabrous; margins long-ciliate, cilia transparent, spreading, (0.05--)0.07--0.17 mm; apex slightly keeled, mostly attenuate; bristle white, whitish, or transparent, puberulent, 0.45--1(--1.5) mm. Strobili solitary, 0.5--3.5 cm; sporophylls deltate-ovate to ovate-lanceolate, strongly tapering or not toward apex, abaxial ridges well defined, base glabrous, margins ciliate to slightly dentate, apex slightly keeled, not truncate in profile, long-bristled. 2 n = 18.
Selaginella rupestris has the widest range of any selaginella in the flora. It is variable in many characteristics, e.g., the hairiness of the margins (which sometimes are not hairy), leaf base pubescence, and shape of sporophylls. The variation in sporangial distribution pattern in the strobili and the number of megaspores per megasporangium are important for understanding reproduction and relationships in this species. Very often the strobili are wholly megasporangiate, with only 1--2 megaspores per megasporangium, suggesting asexual reproduction. In other cases, both types of sporangia are present in a strobilus, suggesting sexual reproduction. R. M. Tryon (1971) correlated sporangial and spore distribution patterns in S . rupestris with distributional ranges and concluded that there are four races. Race A has 4 megaspores per megasporangium, has microsporangia, is sexual, and is distributed from southeastern Pennsylvania south to Georgia and Alabama. Race B has 1--2(--4) megaspores per megasporangium, has microsporangia, has an unknown type of reproduction, and has the same range as Race A, but it extends into New York. Race C has 1--2 megaspores per megasporangium, has microsporangia, is probably asexual, and is distributed throughout the species range. Race D has 1--2 megaspores per megasporangium, lacks microsporangia, is therefore asexual, and is found throughout the species range except where Race A occurs. These patterns suggest the presence of more than one species within S . rupestris in the broad sense. More studies, especially cytologic, are needed, as well as fieldwork, in order to understand these relationships and the variability in S . rupestris . Among the species in the flora, S . rupestris seems to be most closely related to S . sibirica (see discussion) and may also be allied to the S . arenicola complex (see discussion).
Perennial fern ally 2 - 5 cm tall Leaves: stalkless, firm, thick or somewhat fleshy, tightly appressed, ascending, and alternating in pseudo-whorls of six on the main stem and four on the lateral branches. Both sterile, vegetative leaves and fertile, spore-bearing leaves are produced. Stems: moss-like, long- to moderately short-creeping or decumbent, leafy, radially symmetric, not flattened, branched irregularly, forking one to three times (lateral branches sometimes ascending), and forming long or spreading mats, or sometimes cushion-like mats. Roots: very short, very slender, hair-like, borne on modified, leafless, 0.25 - 0.45 mm diameter shoots (rhizophores) on the upper side of stems at branch forks. Sterile, vegetative leaves: all similar in shape and size, green (occasionally with some reddish tinges), 2.5 - 4.5 mm long, 0.45 - 0.6 mm wide, linear to linear-lance-shaped with a strong ridge on the underside, a tapering base on the underside, but a rounded base (attached to the stem) on the upper side, and usually a long-tapering, pointed apex with a white to transparent, hairy, 0.45 - 1.5 mm long bristle at the tip. While the leaf surfaces may be hairy or hairless, the edges are always fringed with long, slender, transparent, spreading, 0.06 - 0.17 mm long bristles.
Similar species: Selaginella rupestris is somewhat similar to S. eclipes, but that species has two different sizes and shapes of vegetative leaves, which are thin, papery, and arranged in distinct ranks; there are axillary leaves at the branching points; and the strobili (clusters of fertile leaves (sporophylls) holding the spore-producing sacs (sporangia)) are flattened rather than four-angled. Species of Selaginella are probably most often mistaken for moss, but some may also confuse them with species in the Lycopodiaceae family. However, members of that family have their spore-bearing sacs (sporangia) borne singly in leaf axils, and those leaves are either the same as the vegetative leaves, or different, in which case they are aggregated into over 3 mm wide, cylindrical clusters located at the branch tips. Also, there is only one size of spore in the Lycopodiaceae family, and these are less than 50 microns in diameter.
Habitat and ecology: Quite rare, restricted to sandy soil in Lake and Porter counties in Indiana.
Occurence in the Chicago region: native
Notes: The classification of the Selaginellaceae family is very confusing and controversial, even at the genus level. Following the Flora of North America (1993), which puts all species in the single genus Selaginella and further breaks groups down into subgenera, this species is part of the largest subgenus in North America, subgenus Tetragonostachys. Selaginella rupestris has the most expansive range of any Selaginella species in North America, and consequently is quite variable in a number of physical characters such as hairiness, shape of the leaves bearing the spore sacs (sporophylls), and the number of and types of spores per strobilus. Some researchers have shown that there are four different races within this single species, thus suggesting that there may be more than one species included under this taxon name.
Stems much branched and with numerous rhizophores, 1 mm thick (excl. lvs), the creeping and the erect (to 6 cm) branches similar except that the latter terminate in tetragonous cones 1-1.5 cm נ1 mm; lvs spirally arranged, appressed, thick and firm, subulate, ciliolate, 1.5-2 נ0.25-4 mm, tipped by a scabrous slender white bristle 1-1.5 mm; sporophylls 4-ranked, evidently broader than the lvs; megaspores yellowish, 0.5 mm wide, faintly reticulate or rugose on all faces, often only 1 or 2 per sporangium, and the plants then usually apogamous and lacking microsporangia; 2n=18. Rocky or sandy, mostly acid soil; s. Greenl. to ne. Alta., s. to n. Ga. and Okla
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Local. Found only on dry, exposed sandstone rocks and in dry sand in the dune area. It has also been reported from Montgomery County. Underwood (Proc. Indiana Acad. Sci. 1893: 257. 1894) says the report from Gibson County in the State Catalogue was an error.