Descurainia

Annuals, biennials, or perennials [shrubs, subshrubs]; not scapose; glabrous, glabrate, or sparsely to densely pubescent, trichomes usually short-stalked, dendritic, rarely also simple, sometimes mixed with unicellular, glandular, clavate papillae. Stems erect or prostrate, unbranched or branched basally and/or distally. Leaves basal and cauline; petiolate or sessile; basal (often withered by flowering), rosulate, petiolate, blade (1–3-pinnate), margins entire or toothed; cauline petiolate or sessile, blade often similar to basal. Racemes (proximally sometimes bracteate), elongated or not in fruit. Fruiting pedicels divaricate or erect, slender. Flowers: sepals erect to spreading, ovate to oblong or linear; petals usually obovate or oblanceolate, rarely oblong, (shorter to longer than sepals), claw obsolete or distinct, (apex obtuse); stamens tetradynamous; filaments not dilated basally; anthers oblong to ovate, (apex obtuse); nectar glands confluent, subtending bases of stamens, median glands present. Fruits siliques or silicles, sessile, usually linear, oblong, clavate, or fusiform, rarely ellipsoid or obovoid, smooth or torulose, terete; valves each often with distinct midvein, usually glabrous, rarely pubescent; replum rounded; septum complete or perforated (membranous, not veined or with 1–3 longitudinal veins); ovules 5–100 per ovary; style usually absent, rarely distinct; stigma capitate. Seeds uniseriate or biseriate, plump, wingless, oblong or ellipsoid; seed coat (minutely reticulate), usually mucilaginous when wetted; cotyledons incumbent. x = 7.

Distribution

North America, Mexico, South America, Eurasia, n Africa, Atlantic Islands (Canary Islands).

Discussion

Species 45–47 (14 in the flora).

Descurainia species are distributed in three major centers: North America (17 species), South America (ca. 20 species), and the Canary Islands (7 species). Excluding D. sophioides, which extends into Siberia from arctic North America, three additional species are found outside those regions: D. kochii (Petri) O. E. Schulz (Caucasus, Turkey), D. sophia (a cosmopolitan weed of Eurasian origin), and D. tanacetifolia (Linnaeus) Prantl (Alps and the Iberian Peninsula).

Descurainia is taxonomically difficult throughout most of the New World, especially the United States. Extensive morphological variation exists within numerous species and many wide-ranging and widely-overlapping taxa appear to intergrade endlessly. Inter- and infraspecific hybridization is probably extensive given the lack of reproductive barriers between the recently evolved New World species, the ready dispersibility of the mucilaginous seeds, and the weedy tendencies of the majority of taxa that readily occupy disturbed habitats. The frequent occurrence of hybrid populations, suggested by the intergrading morphology and widespread polyploidy, is supported by recent molecular evidence (B. E. Goodson 2007). A second factor contributing to the complexity of North American Descurainia is that the number of taxonomically reliable characters is somewhat limited, with a general lack of agreement among various authors on the characters to be emphasized for circumscription of taxa. A taxonomic nightmare has resulted from the recognition of numerous poorly defined species, subspecies, and varieties. Although an extensive molecular systematic study of the genus was recently conducted (Goodson), some relationships and species boundaries remain unclear. Because detailed population-level, morphological, cytological, and molecular studies of North American Descurainia are still needed, this treatment represents an early “pit stop” on the road to full understanding of the complexity in the genus.

The keys and circumscriptions in both L. E. Detling (1939) and R. C. Rollins (1993) are unreliable for the identification of a given collection to an infraspecific taxon. In a genus where interspecific hybridization is so extensive, it is sometimes impossible to identify reliably every single specimen, especially if mature fruits are lacking. Unfortunately, most North American material in major herbaria consists of either specimens without mature fruits or unrecognized hybrids between various taxa. As a consequence, up to 60% of the holdings of Descurainia species in the major North American herbaria are misidentified. Because mature fruits are critical to proper identification, the following key relies primarily on fruiting material.

Almost all taxonomists have used the presence versus absence of glandular papillae as an important character to subdivide a given complex into species, subspecies, varieties, or forms. Both glandular and eglandular forms sometimes occur within a given population of Descurainia incana, D. incisa, D. paradisa, D. pinnata, or D. sophioides. Some authors were inconsistent in according formal recognition to the glandular versus eglandular plants of a given species. For example, R. C. Rollins (1993) treated the glandular and eglandular forms of D. obtusa and D. paradisa, but not of D. incana and D. sophioides, as distinct subspecies; N. H. Holmgren (2005b) recognized such forms as varieties in D. paradisa but not D. incana. Although the presence versus absence of glands appears to be consistent within certain taxa, reliance on the use of glands as a diagnostic character should be applied with extreme caution.

Selected References

Bramwell, D. 1977. A revision of Descurainia Webb & Berth. section Sisymbriodendron (Christ) O. E. Schulz in the Canary Islands. Bot. Macar. 4: 31–53.

Detling, L. E. 1939. A revision of the North American species of Descurainia. Amer. Midl. Naturalist 22: 481–520.

Goodson, B. E. 2007. Molecular Systematics and Biogeography of Descurainia Webb & Berthel. (Brassicaceae). Ph.D. dissertation. University of Texas.

Lower Taxa

Descurainia adenophora, Descurainia brevisiliqua, Descurainia californica, Descurainia incana, Descurainia incisa, Descurainia kenheilii, Descurainia longepedicellata, Descurainia nelsonii, Descurainia obtusa, Descurainia paradisa, Descurainia pinnata, Descurainia sophia, Descurainia sophioides, Descurainia torulosa

Key

1	Fruits sparsely to densely pubescent at least when young	> 2
1	Fruits glabrous	> 4
2	Perennials; fruiting pedicels 1.5-3 mm; fruits subappressed to rachises; styles sparsely pubescent; ovules 10-18 per ovary; Wyoming.	Descurainia torulosa
2	Biennials; fruiting pedicels 6-31 mm; fruits not appressed to rachises; styles glabrous; ovules 16-64 per ovary; Arizona, California, Nevada, New Mexico	> 3
3	Seeds biseriate; ovules 48-64 per ovary; fruits 1-1.3 mm wide; fruiting pedicels 13-31 mm.	Descurainia adenophora
3	Seeds uniseriate; ovules 16-40 per ovary; fruits 0.7-1 mm wide; fruiting pedicels 6-15 mm.	Descurainia obtusa
4	Fruits usually fusiform, obovate, clavate, or broadly ellipsoid, rarely broadly linear (wider distally)	> 5
4	Fruits linear (sometimes oblong in D. brevisiliqua)	> 7
5	Fruits usually clavate, rarely broadly linear (wider distally); seeds biseriate; ovules 16-40 per ovary; valves each with distinct midvein.	Descurainia pinnata
5	Fruits fusiform, obovate, or broadly ellipsoid (sometimes clavate in D. paradisa); seeds uniseriate (sometimes biseriate in D. paradisa); ovules 4-12 per ovary; valves each with obscure midvein	> 6
6	Fruits long-acute basally and apically; styles (0.2-)0.3-0.6(-0.8) mm; stems (1.3-) 2-10.5(-13.5) dm, unbranched basally, branched distally.	Descurainia californica
6	Fruits acute basally, obtuse apically; styles 0.05-0.3 mm; stems (1-)1.5-3.2(-4.1) dm, branched basally and distally.	Descurainia paradisa
7	Perennials (short-lived); stems 0.1-0.15 dm; racemes not elongated in fruit; ovules 4-8 per ovary.	Descurainia kenheilii
7	Annuals or biennials; stems (0.5-)0.9-12(-18) dm; racemes elongated considerably in fruit (sometimes slightly elongated in D. sophioides); ovules 10-62 per ovary (6-12 in D. nelsonii)	> 8
8	Fruits often strictly appressed to rachises; fruiting pedicels erect to erect-ascending.	Descurainia incana
8	Fruits not appressed to rachises; fruiting pedicels horizontal, divaricate, or ascending	> 9
9	Leaf blades 2- or 3-pinnate; fruit septums appearing 2- or 3-veined.	Descurainia sophia
9	Leaf blades usually 1-pinnate (rarely 2-pinnate in D. sophioides); fruit septums not veined	> 10
10	Plants eglandular; fruits 3-8(-10) mm; petals 0.7-1.2 mm; seeds 0.5-0.8 mm	> 11
10	Plants glandular or eglandular; fruits 8-30(-34) mm; petals 1.7-2.8 mm; seeds 0.9-1.5 mm	> 12
11	Biennials; stems 6-11 dm, unbranched; ovules 10-28 per ovary; seeds biseriate.	Descurainia brevisiliqua
11	Annuals; stems (0.7-)0.9-3.2(-4.5) dm, usually branched at or near base, rarely unbranched; ovules 6-12 per ovary; seeds uniseriate.	Descurainia nelsonii
12	Flowers overtopped by developing fruits; ovules 30-62 per ovary.	Descurainia sophioides
12	Flowers not overtopped by developing fruits; ovules 14-32 per ovary	> 13
13	Plants canescent or not; cauline leaf blades: distal segments oblong to lanceolate, or linear, margins dentate, denticulate or entire; fruits straight or strongly curved inward.	Descurainia incisa
13	Plants not canescent; cauline leaf blades: distal segments linear or oblong, margins entire; fruits straight or slightly curved inward.	Descurainia longepedicellata