Bryodesma arenicola ssp. riddellii

Riddell's Spikemoss is a native perennial in the "Spikemoss" family (SELAGINELLACEAE). It occurs on rock outcrops (sandstone or granite) in north Alabama, and on deep xeric sands in south Alabama. Riddell's Spikemoss is a clump forming perennial from underground rhizomatous stems. Aerial stems are erect and irregularly branched. Leaves (microphylls) are produced in pseudowhorls of 4. Those on the underground rhizomatous stems are white, appressed, and scale like. Those on the aerial stems are green, triangular to lanceolate in outline, pubescent or glabrous, and tightly or loosely appressed. Spores are produced in sporangia located in the axils of the upper leaves (sporophylls). Riddell's Spikemoss is listed as an S2 species in Alabama (typically 6 to 20 occurrences, few remaining individuals, acres, or miles of stream, or factors demonstrably making it very vulnerable in the state), and globally as a G4 species (imperiled globally because of rarity or very vulnerable to extinction throughout its range because of other factors).--A. Diamond

Riddell's Spikemoss is one member of a complex of closely related taxa also including Spiny Spikemoss [Bryodesma acanthonota (Underwood) Škoda], and Sand Spikemoss [B. arenicola (Underwood) Soják ssp. arenicola]. The complex ranges from North Carolina to Texas and members have been treated differently by various authors. Specimens in south Alabama, especially in Geneva County, appear to be intermediate between B. arenicola ssp. riddellii and B. a. ssp. arenicola--A. Diamond & B.R. Keener