Dimorphocarpa is a small genus of flowering plants in the mustard family (Brassicaceae), order Brassicales, native to the arid and semi-arid landscapes of the western and central United States and northern Mexico. The genus comprises four species — D. candicans, D. membranacea, D. pinnatifida, and D. wislizeni — and was revised by Reed C. Rollins, who transferred several species from allied genera.
Plants are annuals, biennials, or rarely short-lived perennials, growing erect with stems that may branch at the base or toward the tips. A conspicuous feature is the dense pubescence covering the whole plant, formed by subsessile stellate hairs mixed with minutely stalked, dendritic trichomes. Leaves are both basal and stem-borne; basal leaves are petiolate with dentate, pinnatifid, or lobed margins, while stem leaves are sessile to shortly petiolate and lack auricles at the base.
The small flowers are borne in elongating racemes and display the typical crucifer arrangement: four reflexed to widely spreading oblong sepals and four white or lavender obovate petals with a distinct claw, six tetradynamous stamens, and both lateral and median nectar glands. The most distinctive character of the genus is the fruit: a silicle that is strongly flattened across the septum (angustiseptate), roughly circular and winged, and — critically — breaks apart at maturity into two separate one-seeded units. This breaking or splitting of the fruit into indehiscent segments is the feature that gives the genus its name (di- two, morpho- form, carpa fruit). Seeds are broadly oblong to suborbicular, strongly flattened, with accumbent to obliquely incumbent cotyledons; the base chromosome number is x = 9.
Etymology
The name Dimorphocarpa is derived from the Greek di- (two), morpho- (form or shape), and karpos (fruit), referring to the characteristic two-unit fruit that splits into a pair of indehiscent one-seeded segments at maturity.
Distribution
Dimorphocarpa is native to the western and central United States and northern Mexico, with its centre of diversity in the Sonoran and Chihuahuan desert regions. The four species are concentrated in arid to semi-arid habitats across Arizona, New Mexico, and adjacent Mexican states.
Ecology
Species of Dimorphocarpa occupy arid and semi-arid habitats typical of the southwestern USA and northern Mexico, growing in open, disturbed, or desert scrub communities. The splitting of the fruit into two winged, one-seeded units at maturity is thought to facilitate wind or animal dispersal in these exposed, low-moisture environments.
Taxonomy Notes
The genus was circumscribed and revised by Reed C. Rollins, who transferred species formerly placed in Dithyrea and other allied Brassicaceae genera. GBIF recognises four descendants under the genus. The defining synapomorphy is the didymous, angustiseptate silicle that separates at maturity into two 1-seeded valvate units — a character that distinguishes Dimorphocarpa from the closely related Dithyrea. Base chromosome number is x = 9.
