Flora of North America species comparison

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.

(Discussion copyrighted by Flora of North America; reprinted with permission.)