Orbexilum, commonly known as leather-root, is a genus of flowering plants in the legume family Fabaceae (order Fabales). The genus comprises roughly ten species native to North America, ranging across the eastern and central United States and Mexico south to Chiapas.
Plants in this genus are herbaceous perennials characteristic of the tribe Psoraleeae. They can be distinguished from closely related genera — notably Psoralea, from which Orbexilum was originally split — by their thick, glabrous pod walls that are distinctively rugose (wrinkled), and by a calyx that is scarcely accrescent (it barely enlarges after flowering).
The genus was established by Constantine Samuel Rafinesque in 1832, with the type species designated as O. latifolia, transferred from Psoralea. That original type species is now treated as a synonym of Orbexilum onobrychis. Rafinesque's primary criterion for separating Orbexilum from Psoralea was calyx morphology.
Notable species include Orbexilum pedunculatum, O. onobrychis, O. lupinellus, and O. virgatum. Two species — O. macrophyllum (bigleaf scurfpea) and O. stipulatum — are considered extinct.
Taxonomy Notes
Orbexilum was described by Constantine Samuel Rafinesque in 1832, carved out of Psoralea primarily on the basis of calyx characters. The type species, O. latifolia, is now treated as a synonym of O. onobrychis. Within Fabaceae tribe Psoraleeae, the genus is set apart by its thick, glabrous, distinctively rugose pod walls and a calyx that barely enlarges in fruit.
Distribution
Orbexilum is native to North America, occurring across the United States and Mexico, with the southern limit of its range reaching Chiapas, Mexico.
