Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal-tube campanulate to tubular, sometimes oblique, the lobes setaceous to triangular, often unequal; pet all separate (spp. 15–17) or ± united into a tube, usually withering and persistent after anthesis; standard ovate to oblong or obovate, often folded about the wings or with only its summit outwardly curved; stamens 10, diadelphous, filaments dilated below the anthers; fr short, straight, often included in the persistent cal, dehiscent or indehiscent, 1–6-seeded; herbs with 3-foliolate, serrulate lvs and small fls in heads, spikes, or head-like racemes or umbels. In the absence of fr, our spp. may be distinguished from Medicago by their strongly bilabiate cal. 250, widespread.