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Comparison with Most Similar SpeciesFor about 50 years after Blake described H. virginicum in 1936, its distinctness from H. autumnale was doubted, and it was not treated as a good species by leading systematists such as Arthur Cronquist. Opinion changed when common garden comparisons, reciprocal transplant studies, and DNA sequence comparisons revealed that the two are distinct in morphology, ecology, development, and ITS sequence (Knox, Castanea 1987; Knox et al., Syst. Bot. 1995; Simurda et al., Syst. Bot. 2005). Key character differences between the two species are that the basal stem and leaves of H. virginicum that form above water are conspicuously pilose, where H. autumnale is glabrous, H. virginicum has lance-linear, mostly entire mid-stem leaves that scarcely taper toward the base and scarcely show secondary veins, while H. autumnale has leaves that are more frequently toothed, that taper more toward the leaf bases, and that show conspicuous secondary leaf veins. Basal cauline and rosette leaves of H. virginicum are sometimes deeply lobed, while those of H. autumnale are not. Achenes of H. virginicum are longer (mean 1.6 mm), and wider (mean 0.83 mm) than those of H. autumnale (mean length 1.3 mm, width 0.6 mm), and at maturity most corollas of H. virginicum abscise as they turn brown, while nearly all corollas of H. autumnale are retained on the achenes, though brown. Virginia populations of H. virginicum grow in sinkhole ponds that are lined with acid gray clay overlying 100 m or more deep acid alluvium that is perched on bedrock of limestone or dolomite. Erosion of the bedrock causes subsidence of the basin at the soil surface, permitting it to capture water, which may stand continuously for many months at wet times, but the basins do go completely dry either seasonally or supra-annually. Missouri populations of H. virginicum are much less well studied than Virginia populations, but a recent study (Rimer and Summers, Southeastern Nat. 2006) suggests that the habitat there is similar to that in Virginia in presenting a basin with acid soil overlying dolomite or limestone. Ecological studies of Virginia populations indicate that H. virginicum grows year round, forming linear basal leaves below water, and elliptic to obovate entire or lobed leaves above water; the plants are self-incompatible; seeds do not germinate in the dark or below water but may remain viable in the seedbank for more than a decade; plants do not survive to reproduce in the shade (Knox, J. Torrey Bot. Soc. 1997; Messmore and Knox, J. Torrey Bot. Soc. 1997; Adams et al., Biol. Conserv. 2005). Helenium flexuosum has a similar vegetative morphology to H. virginicum, though the plants of H. flexuosum seem to be shorter and there is no record of them growing in sites that flood for long periods. H. flexuosum has neuter ray flowers, in contrast to the pistillate ray flowers of H. autumnale and H. virginicum. Disc flowers of H. flexuosum are purple distally or throughout, while those of H. autumnale and H. virginicum are yellow proximally and yellow to yellow-brown distally (Flora of N.A., 2006).