Sporobolus sect. Clandestini P.M.Peterson, sect. nov. — Type: Sporobolus clandestinus (Biehler) Hitchc. (≡ Agrostis clandestina Biehler, Pl. Nov. Herb. Spreng.: 8. 1807).

Caespitose annuals or perennials, sometimes with rhizomes. Panicles 0.2—1.6 cm wide, contracted, spiciform, spikelike, usually included in the uppermost sheath, terminal and axillary. Spikelets 1.3—9(—10) mm long, often cleistogamous. Caryopses laterally flattened.

Species included: S. aldabrensis Renvoize, S. compositus (Poir.) Merr., S. clandestinus, S. neglectus Nash, S. vaginiflorus (Torr. ex A.Gray) Alph.Wood.

Pilger (1956) delineated this as group 5 in his subgeneric treatment of Sporobolus and Riggins (1969, 1977) investigated most of the members of clade L in a study of the annual cleistogamous species and a biosystematic study of the Sporobolus asper (Michx.) Kunth (= S. compositus) complex. Our results support the derivation of Sporobolus clandestinus, S. compositus, S. neglectus, and S. vaginiflorus (Torr. ex A.Gray) Alph.Wood from a common ancestor. In the ITS tree one individual of S. clandestinus (Waterfall 5881) does not align with three other samples of S. clandestinus; rather, S. clandestinus (Waterfall 5881) and S. compositus form a clade that is the unsupported sister of the rest of the S. compositus complex whereas in the plastid tree all members of the complex are part of a strongly supported clade. This could indicate hybridization with a member of the highly variable S. compositus complex (Peterson & al., 2003, 2009), multiple origins of this taxon, or simply the occurrence of multiple copies since S. compositus (syn. S. asper (P.Beauv.) Kunth) is a known hexaploid (Riggins, 1977). Likewise, in the combined plastid tree S. vaginiflorus var. ozarkanus falls within the S. airoides clade and is sister to S. splendens whereas in the ITS tree it aligns with three other samples of S. vaginiflorus as expected. Acquisition of the S. airoides plastid haplotype is probably the result of an ancient hybridization event. Sporobolus neglectus also does not align within the combined plastid tree with other members of the S. compositus complex but is found as sister to the C plus the J–O clades.

Important characters in the L lineage include panicles included in the uppermost sheath with cleistogamous spikelets and laterally flattened caryopses. In addition, we tentatively place S. aldabrensis Renvoize in this lineage since it shares a spiciform inflorescence (0.2–1.6 cm wide) with other members of the lineage, although we have data from only a single plastid (rpl32-trnL) marker. Important characters in this lineage are panicles included in the uppermost sheath with cleistogamous spikelets and laterally flattened caryopses. Based on only two plastid markers S. conrathii appears as a member of this clade (see Fig. 2B), although we are not convinced it is closely related since it differs morphologically in having diffuse, subdichotomously branched panicles (Gibbs Russell & al., 1991). Ortiz–Diaz & Culham (2000) sampled only S. compositus from this complex, which was placed in a clade with Calamovilfa, in contrast with our ITS results.