13. Selaginella arenicola L. Underwood, Bull. Torrey Bot. Club. 25: 541. 1898.
Plants terrestrial or on rock, forming clumps. Stems radially symmetric, underground (rhizomatous) and aerial, not readily fragmenting, irregularly forked; rhizomatous and aerial stems often with 1 branch arrested, budlike, tips straight; rhizomatous stems mostly ascending; aerial stems erect or ascending. Rhizophores borne on upperside of stems, restricted to rhizomatous stems, 0.2--0.33 mm diam. Leaves dimorphic, in pseudowhorls of 4. Rhizomatous stem leaves persistent, appressed, scalelike. Aerial stem leaves tightly or somewhat loosely appressed, ascending, green, narrowly triangular-lanceolate or narrowly lanceolate, 2--3 X 0.4--0.5 mm; abaxial ridges present; base cuneate, strongly decurrent, pubescent or glabrescent; hairs restricted to base; margins short-ciliate, cilia transparent, scattered, spreading at base, dentiform and ascending toward apex, 0.02--0.07 mm; apex plane, attenuate; bristle white to whitish, straight, coarsely puberulent, 0.25--0.85(--0.9) mm. Strobili solitary, (0.5--)1--3(--3.5) cm; sporophylls ovate-lanceolate to lanceolate, often abruptly tapering toward apex, abaxial ridges not prominent, base glabrous, rarely with few hairs, margins ciliate, apex often recurved, bristled.
Subspecies 2: only in the flora.
Selaginella arenicola and related species have been considered as forming a species complex. This interpretation has been the center of much taxonomic controversy (R. M. Tryon 1955; G. P. Van Eseltine 1918). Tryon recognized one species in the complex, S . arenicola , with three subspecies: subsp. arenicola , subsp. riddellii , and subsp. acanthonata . Other authors (e.g., R. T. Clausen 1946) treated the subspecies as species. I recognize two well-defined species within this complex, S . arenicola and S . acanthonota , which are readily distinguishable by the characteristics given in the key. Some specimens reported by R. M. Tryon (1955) as intermediate between S . arenicola and S . acanthonota appear to be hybrids between S . acanthonota and S . rupestris . In particular, more detailed studies are needed to assess whether populations from Georgia are hybrids or variants of S . acanthonota or of S . rupestris . Future studies are also needed to determine relationships and proper taxonomic rank of Selaginella arenicola subsp. arenicola and subsp. riddellii , which are provisionally recognized here.