Stems radially symmetric or upperside and underside structurally different, not articulate, prostrate, creeping, or erect, few-to-many branched; vessel elements present. Rhizophores borne on upperside of stems, throughout stem length or confined to base of stem. Leaves monomorphic, tightly appressed and spirally arranged, or upperside and underside leaves slightly differentiated; all leaves linear or linear-lanceolate, thick or fleshy (seldom thin); margins dentate, serrate, or ciliate; abaxial groove with stomates, these arranged along vein; axillary leaves absent. Strobili quadrangular. Sporophylls differentiated from vegetative leaves, most sporophylls fertile; megasporophylls and microsporophylls same size, in 4 alternating ranks, appressed, base usually with 2 diverging flaps or auricles, auricles protecting sporangia below.

Distribution

Widespread, Mexico, West Indies, Central America, South America, Asia, Africa including Madagascar

Discussion

This treatment of subg. Tetragonostachys generally follows that of R. M. Tryon (1955). Some problems, however, remain to be resolved, particularly for Selaginella arenicola and S. densa, in which taxa have been recognized at the infraspecific (R. M. Tryon 1955) and specific (L. H. Snyder Jr. and J. G. Bruce 1986; J. W. Thieret 1980; J. M. Beitel and W. R. Buck, pers. comm.) levels. Based on examination of a wide range of specimens in the S. arenicola complex, I recognize two species, one of which contains two subspecies. In the S. densa complex, I recognize three well-defined species: S. densa, S. scopulorum, and S. standleyi. The phylogenetic relationships among the different series proposed by Tryon need further study.

Within the series Arenicolae R. M. Tryon, a tendency toward structural differentiation occurs between the stem's upperside and its underside (e.g., S. rupincola). This feature may link series Arenicolae to species such as S. hansenii and S. wrightii, which I place in the series Eremophilae R. M. Tryon and which may represent an early intermediate stage toward full stem differentiation. According to Tryon, this "transitional stage" is primitive within the series Eremophilae, where it is found in S. peruviana.

The occurrence of hybrids within Selaginella subg. Tetragonostachys is best shown in S. × neomexicana. Hybridization may be a more common phenomenon, however, than previously acknowledged.

Species ca. 50 (26 species and 1 hybrid in the flora).

Selected References

Clausen, R. T. 1946. Selaginella subgenus Euselaginella, in the southeastern United States. Amer. Fern J. 36: 65--82.

Van Eseltine, G. P. 1918. The allies of Selaginella rupestris in the southeastern United States. Contr. U.S. Natl. Herb. 20(5): 159--172.

Tryon, R. M. 1955. Selaginella rupestris and its allies. Ann. Missouri Bot. Gard. 42: 1--99, plates 1--6.

Tryon, R. M. 1971. The process of evolutionary migration in species of Selaginella. Brittonia 23: 89--100.

Lower Taxa

Selaginella acanthonota, Selaginella arenicola, Selaginella arizonica, Selaginella asprella, Selaginella bigelovii, Selaginella cinerascens, Selaginella densa, Selaginella eremophila, Selaginella hansenii, Selaginella leucobryoides, Selaginella mutica, Selaginella oregana, Selaginella peruviana, Selaginella rupestris, Selaginella rupincola, Selaginella scopulorum, Selaginella sibirica, Selaginella standleyi, Selaginella tortipila, Selaginella underwoodii, Selaginella utahensis, Selaginella viridissima, Selaginella wallacei, Selaginella watsonii, Selaginella weatherbiana, Selaginella wrightii, Selaginella ×neomexicana

Key

1	Stems prostrate, undersides and uppersides differentiated; leaves conspicuously to slightly dimorphic; rhizophores throughout stem length.	> 2
1	Stems pendent, erect, ascending, or rarely prostrate, radially symmetric or undersides and uppersides slightly differentiated (if so, leaves decurrent as in S. densa complex); leaves not to rarely dimorphic; rhizophores throughout stem length or restricted to stem base.	> 6
2	Underside and upperside leaves abruptly adnate to stem; leaves slightly dimorphic.	> 3
2	Underside leaves decurrent, upperside leaves abruptly adnate to stem; leaves strongly to moderately dimorphic.	> 4
3	Apex of leaves with white or whitish bristle 0.5–1.4 mm; marginal cilia white to whitish, strongly ascending; leaves green, usually with red spots or wholly reddish wine-colored; sporophylls ovate-deltate, short-attenuate toward apex.	Selaginella hansenii
3	Apex of leaves with yellowish bristle 0.2–0.5 mm or absent; marginal cilia transparent, spreading; leaves green, never reddish; sporophylls lanceolate, long-attenuate toward apex.	Selaginella wrightii
4	Leaves with tortuous (twisted) bristle at tip, becoming acute to mucronate; upperside leaves lanceolate; plants forming dense mats.	Selaginella eremophila
4	Leaves acute to bristled, bristle straight; upperside leaves linear-lanceolate; plants forming rather loose mats.	> 5
5	Underside leaves lanceolate, widest at middle; leaf apex acute or with short and flattened bristle 0.1–0.3 mm (mostly at branch tips or buds); sporophylls acute to acuminate.	Selaginella arizonica
5	Underside leaves narrowly linear- lanceolate, widest at base; leaf apex with long round bristle 0.3–0.8 mm; sporophylls bristle-tipped.	Selaginella peruviana
6	Plants epiphytic, seldom terrestrial; aerial stems long-pendent, usually forming festoonlike or overlapping mats; leaves loosely appressed; strobili 1–6 cm.	Selaginella oregana
6	Plants on rock or terrestrial, never epiphytic; aerial stems erect, ascending, long- to short- creeping, decumbent, radially symmetric or slightly differentiated, forming long- or short- spreading mats, cushionlike mats, or cespitose mats; leaves usually tightly appressed; strobili 0.2–4.5(–9) cm.	> 7
7	Aerial stems erect or ascending, sometimes decumbent or creeping; rhizome or rhizomatous stem present; budlike arrested branches usually present on rhizome or lowermost aerial stem.	> 8
7	Aerial stems creeping or decumbent, never erect, radially symmetric or lower stem and upper stem slightly differentiated; rhizome or rhizomatous stem absent; budlike arrested branches absent.	> 15
8	Base of leaf abruptly adnate.	> 9
8	Base of leaf decurrent or long-decurrent.	> 11
9	Leaf margins short-ciliate throughout, cilia 0.02–0.08 mm; leaf base cordate to almost peltate.	Selaginella bigelovii
9	Leaf margins long-ciliate at least at base, cilia 0.06–0.2 mm; leaf base rounded.	> 10
10	Leaf bristle 0.65–1.85 mm; marginal cilia long and spreading throughout, 0.1–0.2 mm; sporophylls strongly tapering toward apex.	Selaginella rupincola
10	Leaf bristle 0.3–0.46 mm; marginal cilia long and spreading at base, short to dentiform and ascending toward apex, 0.06–0.17 mm; sporophylls not strongly tapering.	Selaginella ×neomexicana
11	Scalelike leaves on rhizome loosely appressed; stem leaves without hairs along abaxial groove.	> 12
11	Scalelike leaves on rhizome tightly appressed or rhizome absent; stem leaves with hairs along abaxial groove.	> 13
12	Scalelike leaves on rhizome incurved; leaf apex bristle-tipped; sporophyll bristle-tipped.	Selaginella weatherbiana
12	Scalelike leaves on rhizome straight; leaf apex acute or obtuse; sporophyll apex acute to obtuse.	Selaginella viridissima
13	Leaf bristle tortuous (twisted); leaf base glabrous; abaxial groove and ridges on leaf not prominent, often obscure; strobili 0.4–0.6 cm.	Selaginella tortipila
13	Leaf bristle straight, never twisted; leaf base pubescent; abaxial groove and ridges on leaf prominent; strobili (0.5–)1–3(–3.5) cm.	> 14
14	Underground (rhizomatous) stem leaves scalelike; rhizophores mostly subterranean; sporophyll base glabrous; leaf and sporophyll apices glabrous.	Selaginella arenicola
14	Underground (rhizomatous) stem leaves not scalelike; rhizophores mostly aerial; sporophyll base pubescent; leaf and sporophyll apices often puberulent.	Selaginella acanthonota
15	Leaves on main stem adnate to stem (distinct from stem in color), bases rounded or seldom slightly decurrent and cuneate (on underside leaves or in plants from wet places).	> 16
15	Leaves on main stem decurrent (not distinct from stem in color), bases cuneate or oblique (seldom adnate and rounded on upperside).	> 18
16	Leaf apex abruptly short- to long- bristled, bristle puberulent or sometimes entire, (0.16–)0.2–0.46(– 0.9) mm; leaves in whorls of 4; strobili often paired, 1–4.5(–9) cm.	Selaginella wallacei
16	Leaf apex blunt or acute to acuminate or seldom short-bristled, bristle if present entire, 0.03–0.45 mm; leaves in whorls of 3; strobili usually solitary, 0.2–3 cm.	> 17
17	Leaves tightly appressed, apex keeled, mucro or bristle if present 0.03–0.45 mm; stems radially symmetric; strobili (0.6–)1–3 cm.	Selaginella mutica
17	Leaves loosely appressed, apex plane, not bearing bristle; stems slightly structurally differentiated; strobili 0.2–0.4 cm.	Selaginella cinerascens
18	Main stems with upperside and underside slightly differentiated; upperside and underside leaves unequal in size, bases decurrent and oblique (S. densa complex).	> 19
18	Main stems radially symmetric; leaves equal in size, if stem slightly differentiated then leaf bases decurrent and cuneate.	> 21
19	Leaf apex bearing conspicuously puberulent bristle, (1–)1.25–1.9 mm; leaf margins usually long- ciliate, cilia 0.07–0.17(–0.2) mm; sporophyll margins entirely ciliate.	Selaginella densa
19	Leaf apex bearing slightly puberulent or entire bristle, 0.4–1.25 mm; leaf margins relatively short-ciliate, cilia 0.02–0.07(– 0.15) mm; sporophyll margins short- ciliate or denticulate in parts.	> 20
20	Sporophylls deltate-ovate; apex keeled, strongly truncate in profile; bristle usually yellowish; margins short-ciliate to denticulate on distal 3/4.	Selaginella standleyi
20	Sporophylls ovate-lanceolate to lanceolate or seldom ovate; apex attenuate or slightly keeled, not truncate in profile; bristle usually whitish transparent, seldom yellowish (in old leaves); margins short-ciliate to denticulate on proximal 1/2, lacking cilia toward apex.	Selaginella scopulorum
21	Leaves on main stems in alternate pseudowhorls of 5 or 6.	> 22
21	Leaves on main stem in alternate pseudowhorls of 4.	> 23
22	Leaves on main stem in alternate pseudowhorls of 5; leaf base decurrent and cuneate on upperside; leaf apex truncate in profile; sporophyll apex truncate in profile.	Selaginella sibirica
22	Leaves on main stem in alternate pseudowhorls of 6; leaf base sometimes adnate and rounded on upperside; leaf apex attenuate in profile; sporophyll apex not truncate in profile.	Selaginella rupestris
23	Lateral branches spreading, not ascending; stems forming festoonlike mats or rarely compact mats; dry stems not readily fragmenting; strobili sometimes paired.	Selaginella underwoodii
23	Lateral branches usually strongly ascending; stems forming compact cushionlike, usually rounded mats or less often loose mats; dry stems readily fragmenting or not; strobili solitary.	> 24
24	Leaf apex bearing bristle 0.5- -1.4 mm; leaf base pubescent; sporophyll base often pubescent.	Selaginella asprella
24	Leaf apex mucronate, blunt or acute, bearing bristle or mucro 0–0.6 mm; leaf base glabrous, seldom pubescent; sporophyll base always glabrous.	> 25
25	Dry stems not readily fragmenting; lateral branches 1–3-forked; leaf apex strongly keeled.	Selaginella watsonii
25	Dry stems readily fragmenting; lateral branches 1-forked; leaf apex keeled, slightly attenuate or obtuse.	> 26
26	Leaf apex bearing puberulent bristle 0.2–0.6 mm; leaves not in well-defined alternate pseudowhorls.	Selaginella leucobryoides
26	Leaf apex blunt, acute or only ending in very short entire bristle or mucro 0–0.4 mm; leaves in defined alternate pseudowhorls.	Selaginella utahensis

"thick" is not a number.