Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Heads discoid, the fls all tubular and perfect, purple (white); invol of numerous ± appressed, imbricate bracts, in our spp. constricted at the summit; receptacle flat or convex, naked; style-branches slender, gradually tapering distally, minutely hispidulous outside, smooth inside, inconspicuously stigmatic near the base within; achenes (in ours) ribbed, commonly hairy; pappus double, the inner of numerous long, slender bristles, the outer of short bristles or short narrow scales; our spp. perennial herbs, usually simple to the infl; lvs alternate, in our spp. sessile or subsessile and serrate or entire; heads in ours ± numerous, medium-sized, in corymbiform cymes. 1000, N. Amer., S. Amer., Afr., Madagascar, and se. Asia. Our spp. bloom in late summer and fall. Hybrids abound. A hybrid swarm in the Midwest, derived chiefly from V. missurica and V. gigantea, but also involving V. baldwinii and V. fasciculata, has been called V. ×illinoensis Gleason.