Genus Psorothamnus in Subfamily Papilionoideae
In botanical taxonomy, a genus (plural genera) is a rank used to group closely related species within a family. In the hierarchy, genus sits below family and above species.
Genera are defined by shared morphological, anatomical, and genetic characteristics (for example, features of flowers, fruits, seeds, or leaves) that indicate a close evolutionary relationship among the species they contain.
Each genus can include one or more species. Examples include Rosa (roses) and Solanum (nightshades, including tomato and eggplant).
Do you wish to read more about plant taxonomy? Click here!
Genus Description
Suggest a correction!The genus Psorothamnus Rydb. (family Fabaceae, subfamily Faboideae) includes about nine species of deciduous or evergreen shrubs endemic to the warm deserts of the southwestern United States and northwestern Mexico (POWO & WFO, 2024). Plants occupy Sonoran, Mojave and Chihuahuan Desert scrub on limestone outcrops or sandy washes from sea level to about 2 000 m. The type species, designated by Rydberg (1905), is Psorothamnus spinosus (A.Gray) Rydb., originally Dalea spinosa (Rydberg, 1905).
Morphologically Psorothamnus is a low, spreading shrub with pinnately compound leaves bearing three to eleven glandular‑punctate leaflets that give a warty surface. Stipules are minute or absent. Flowers form terminal or axillary racemes; the five‑lobed, heavily glandular calyx persists. The papilionaceous corolla has a reflexed standard, slightly curved wings and a keel enclosing the stamens. The superior ovary bears one or two ovules, and the fruit is a dehiscent legume pod that may be narrowly winged (LPWG, 2017).
Species richness is highest in the Sonoran Desert of Arizona, California and Sonora, where endemics such as P. spinosus (Mojave) and P. fremontii (Colorado Plateau) are confined to particular basins (POWO & WFO, 2024). P. thurberi is restricted to the Baja California peninsula, while P. arborescens spans the Great Basin. The plants inhabit arid shrublands, desert washes and rocky slopes on limestone or sandy substrates and tolerate saline soils (Piper et al., 2015).
Field observations show P. spinosus is pollinated by native solitary bees, especially halictids such as Diadasia and Melissodes (Koch & Cane, 2020). Seeds disperse by gravity and rodent caching; hard seed coats enable long‑term soil persistence. Chromosome counts indicate a base number x = 8 (Koch & Cane, 2020).
Phylogenomic analyses place Psorothamnus in the tribe Amorpheae as a monophyletic clade sister to Dalea (LPWG, 2017). Molecular work (Piper et al., 2015) supports generic status, overturning earlier treatments that merged it with Dalea or Parosela; the current circumscription follows the nine species delimited by Rydberg (1905) (POWO & WFO, 2024). Alternative circumscriptions have largely been abandoned.
Some species, especially P. spinosus, are cultivated as drought‑tolerant ornamentals for silvery foliage. The shrubs also provide browse for livestock and wildlife and are used in restoration plantings to curb erosion on desert rangelands.
While most taxa remain stable, narrow endemics such as P. thurberi may be vulnerable to habitat loss and climate change. Future research should clarify conservation priorities and integrate phylogenomic data to guide management of desert‑legume diversity.
-
Psorothamnus emoryi ((A.Gray) Rydb.)
2 -
Psorothamnus polydenius ((Torr.) Rydb.)
2 -
Psorothamnus scoparius ((A.Gray) Rydb.)
-
Psorothamnus thompsoniae ((Vail) R.L.Welsh & N.D.Atwood)
2