Symphyotrichum subg. Virgulus
Perennials, sometimes stipitate-glandular; rhizomatous or with cormoid caudices. Stems brittle at maturity, usually ± densely hairy, sometimes glabrous, sometimes stipitate-glandular at least distally. Leaves: basal withering by flowering, usually sessile, sometimes subpetiolate, blades usually 3-nerved, spatulate or obovate to oblanceolate, sometimes linear-oblanceolate, margins entire; cauline sessile, blades ovate or oblong to lanceolate, oblanceolate, or linear, bases rounded or auriculate, clasping or subclasping (leaves sometimes reduced distally). Heads radiate. Phyllaries unequal or subequal, sometimes ± foliaceous, bases usually indurate, green zones usually diamond-shaped, sometimes lanceolate or apices foliaceous (faces usually hairy, sometimes glabrous, sometimes stipitate-glandular). Ray florets 7–36(–100) in 1 series; laminae (2–)4.5–24(–25) × 0.5–2 mm. Disc corollas usually weakly ampliate, throats narrowly funnelform, lobes erect. Cypselae cylindric to obovoid or oblong, sometimes fusiform, sometimes ± compressed, (5–)6–10-nerved, usually densely strigillose. x = 5, 4.
North America, Mexico.
Species 24 (19, including 1 hybrid, in the flora).
This group of morphologically and cytologically distinctive asters has been treated at ranks from section to genus. J. C. Semple and L. Brouillet (1980) treated them as the separate genus Lasallea, an illegitimate name replaced by Virgulus (J. L. Reveal and C. S. Keener 1981). A. G. Jones (1980) treated the group as a subgenus within Aster in a broad sense. G. L. Nesom (1994b) proposed subgeneric status for the virguloid asters within Symphyotrichum. On the basis of chloroplast DNA phylogenetic evidence, Xiang C. (1994) showed that the taxon is part of the Symphyotrichum group and recommended treating the virguloid asters as a genus. On the same basis, Xiang and Semple (1996) noted that it could be included within a broadly defined Symphyotrichum or treated separately. Semple et al. (2002), using molecular phylogenetic data, showed the group to belong within Symphyotrichum.
Subgenus Virgulus has been divided into four sections. Section Grandiflori (Torrey & A. Gray) G. L. Nesom includes all the stipitate-glandular taxa: S. grandiflorum, S. campestre, S. pygmaeum, S. yukonense, S. fendleri, S. oblongifolium, and S. novae-angliae, sometimes segregated in sect. Polyliguli (Semple & Brouillet) Semple. Section Patentes (Torrey & A. Gray) G. L. Nesom includes S. patens, S. phlogifolium, S. georgianum, S. adnatum, and S. walteri. Section Virgulus (Rafinesque) G. L. Nesom comprises S. concolor, S. plumosum, S. pratense, and S. sericeum. Section Ericoidi (Torrey & A. Gray) G. L. Nesom includes S. ericoides and S. falcatum. Symphyotrichum ×amethystinum is an intersectional hybrid (sects. Grandiflori × Ericoidi) and remains unassigned. Relationships among sections are unresolved.