Streptanthus, commonly known as jewelflowers or twistflowers, is a genus of flowering plants in the mustard family (Brassicaceae). The genus comprises approximately 56–61 accepted species, depending on the taxonomic treatment, and was first described by the English-American botanist Thomas Nuttall in 1825. Members of the genus are annuals, biennials, or perennials, and are united by their often strikingly colored and contorted floral structures — the common names "twistflower" and "jewelflower" both reflect the characteristic twisted, urn-shaped, or pouch-like calyces and petals that distinguish the genus.
The center of diversity for Streptanthus lies in California, which hosts 24 species and eleven additional infraspecific taxa, of which 32 are California endemics. The genus extends across the western and south-central United States — from Washington and Oregon east to Kansas and Oklahoma, and south through Nevada, Arizona, New Mexico, and Texas — and into northern Mexico, including the Pacific coast and northeastern regions.
Streptanthus has a taxonomically complex history. At least 14 former genera, including Caulanthus, Guillenia, Cartiera, and Stanfordia, have been subsumed into Streptanthus under modern circumscriptions, though some earlier treatments maintained them separately.
Conservation is a notable concern within the genus: seventeen California taxa are classified as rare plants, and two subspecies of Streptanthus glandulosus hold federal Endangered status under the US Endangered Species Act — the Metcalf Canyon jewelflower (subsp. albidus) and the Tiburon jewelflower (subsp. niger), both restricted to small serpentine outcrops in the San Francisco Bay Area.
Etymology
The genus name Streptanthus derives from Greek: streptos (twisted, contorted) and anthos (flower), a direct reference to the distinctive twisted petals and contorted calyces that characterize the flowers. The English common names "twistflower" and "jewelflower" reflect the same morphological feature — the ornate, often brightly colored, urn-shaped or pouch-like flowers — as well as the gem-like visual quality of the blooms.
Distribution
Streptanthus is native to the western and south-central United States and adjacent northern Mexico. Its US range spans 18 states: Arizona, Arkansas, California, Colorado, Idaho, Kansas, Louisiana, Nevada, New Mexico, Oklahoma, Oregon, Texas, Utah, Washington, and Wyoming. In Mexico it occurs in the northwest, northeast, and Pacific island regions.
California is by far the center of diversity, with 24 species and eleven additional infraspecific taxa; of these, 32 are California endemics. The genus reaches its eastern limits in the Great Plains (Kansas, Oklahoma, Arkansas) with species such as Streptanthus maculatus.
Ecology
Many Streptanthus species are strongly associated with serpentine (ultramafic) soils — geologically unusual substrates high in magnesium, iron, and heavy metals, and low in calcium, which exclude most plants. This edaphic specialization has driven high endemism in the genus, particularly in California's Coast Ranges and Sierra Nevada foothills where serpentine outcrops are common. The Tiburon jewelflower (S. glandulosus subsp. niger) is restricted to a single serpentine hillside on the Tiburon Peninsula, exemplifying the extreme habitat fidelity found in several taxa.
Streptanthus polygaloides is documented as a heavy metal (nickel) hyperaccumulator, able to concentrate nickel in its tissues at levels toxic to most plants — a trait of interest to researchers studying phytoremediation and the evolutionary ecology of metal tolerance.
Conservation
Two subspecies of Streptanthus glandulosus are listed as federally Endangered under the US Endangered Species Act: subsp. albidus (Metcalf Canyon jewelflower), restricted to serpentine soils in Santa Clara County, California, and subsp. niger (Tiburon jewelflower), confined to a single serpentine outcrop on the Tiburon Peninsula in Marin County, California.
Seventeen California taxa in the genus are classified as rare plants, reflecting the combination of narrow endemism, serpentine soil dependency, and vulnerability to habitat loss from development and invasive species encroachment.
Taxonomy
Streptanthus was described by Thomas Nuttall in 1825 and belongs to the family Brassicaceae, order Brassicales. It is currently accepted as a valid genus in the GBIF backbone taxonomy. The genus has accumulated at least 14 synonyms reflecting a history of taxonomic splitting and lumping: former genera Caulanthus, Guillenia, Cartiera, and Stanfordia have all been reduced to synonymy under Streptanthus in modern treatments. The primary North American taxonomic treatment is Ihsan A. Al-Shehbaz's account in Flora of North America volume 7. The type species is Streptanthus maculatus Nutt., native to Arkansas, Oklahoma, and Texas.
