Crossosoma is a small genus of flowering shrubs (rarely small trees) in the family Crossosomataceae, within the order Crossosomatales. It comprises two species native to arid and semi-arid regions of the southwestern United States and northwestern Mexico.
Plants in this genus are deciduous or semi-deciduous shrubs. The leaves are alternate, often clustered on short lateral branches, simple, and entire-margined. The flowers are solitary, terminal or axillary, and relatively showy for the plant's scrubby habit: five sepals (4–5 mm), five white to pinkish petals (9–15 mm), and numerous stamens — usually 15 or more — surround a stipitate gynoecium of 1–5 free carpels with capitate, oblique stigmas. The fruits are pod-like follicles, horizontally wrinkled or reticulately marked, with a cartilaginous suture. Seeds are 4 to many per follicle and bear a conspicuous whitish or yellowish fimbriate (fringed) aril — the feature that gives the genus its name.
Crossosoma bigelovii S. Watson (Ragged Rock-flower) grows in rocky desert washes and canyon walls in the Mojave and Sonoran Deserts of California, Nevada, Arizona, and Baja California. Crossosoma californicum Nutt. is restricted to the coastal sage scrub and chaparral of the Palos Verdes Peninsula and the California Channel Islands (San Clemente, Santa Catalina), as well as Guadalupe Island off Baja California, where it inhabits rocky coastal bluffs and canyon slopes. Together, the two species represent the entire family Crossosomataceae as far as this genus is concerned.
Etymology
The name Crossosoma is formed from the Greek krossoi (fringe) and soma (body), an allusion to the fringed aril that surrounds the seeds — a distinctive feature of the genus.
Distribution
The genus is endemic to the southwestern United States and northwestern Mexico. Crossosoma bigelovii ranges across the Mojave and Sonoran Desert regions of California, Nevada, Arizona, and Baja California. Crossosoma californicum has a much narrower coastal-island range, occurring on the Palos Verdes Peninsula, San Clemente Island, Santa Catalina Island, and Guadalupe Island (Baja California).
Taxonomy Notes
Crossosoma is the type genus of the family Crossosomataceae, which is placed in the order Crossosomatales (class Magnoliopsida). The family is small and entirely New World, and molecular phylogenetic studies place Crossosomatales among the eurosid I clade. GBIF recognizes 4 taxa within the genus when infraspecific ranks are included.
