Flora of North America species comparison

Species ca. 85 (22 in the flora).

Rorippa coloradensis Stuckey was based on material collected in 1875 by T. S. Brandegee from Colorado. Although R. C. Rollins (1993) did not examine the holotype (Brandegee 1180, PH), he recognized the taxon as a distinct species. An examination of the holotype immediately reveals that it is the introduced European weed Sinapis alba Linnaeus. Although the holotype has no fruits, the presence of retrorse trichomes along the stems and on the pistil, and the spreading sepals and flower size match S. alba.

The infraspecific variation in some species of Rorippa, especially in R. curvipes, R. curvisiliqua, and R. palustris, has led some authors (e.g., F. K. Butters and E. C. Abbe 1940; R. L. Stuckey 1972; R. C. Rollins 1993) to recognize varieties or subspecies so poorly defined that they cannot be distinguished with some (or any) degree of confidence. I take the position of recognizing infraspecific taxa only if it leads to a practical taxonomy and if the infraspecific taxa have some delimited morphological and geographical attributes.

Rorippa cantoniensis (Loureiro) Ohwi, cited by R. L. Stuckey (1972) and R. C. Rollins (1993) as R. microsperma (de Candolle) L. H. Bailey, is known from a single collection made by J. Macoun from Vancouver Island in 1893 (NY). Rorippa globosa (Turczaninow ex Fischer & C. A. Meyer) Hayek is known from collections made over 120 years ago by Walter Dean from Cambridge, Massachusetts (GH). Finally, R. islandica (Oeder ex Murray) Borbás was collected from western Greenland by A. E. and M. Porsild in 1925 (GH). To my knowledge, no additional collections of any of these species have been made in the past 80 or more years, and they are not treated here.

Hybridization has been documented between species of Rorippa in both Europe and North America (B. Jonsell 1968; G. A. Mulligan and A. E. Porsild 1968; R. L. Stuckey 1972; W. Bleeker and H. Hurka 2001). At least in North America, the occurrence of these interspecific hybrids is rather rare.

Rorippa prostrata (Berteret) Schinz & Thellung apparently is an interspecific hybrid involving R. amphibia and R. sylvestris (R. L. Stuckey 1972); it is known from sporadic collections made in Connecticut, Massachusetts, New Jersey, New York, and Pennsylvania. Most of the specimens that I have examined were collected 60–70 years ago, but occasional ones have been collected recently from Massachusetts; it is not known if these hybrids were introduced or originated in the United States. Rorippa prostrata has not become established as a weedy member of our aquatic flora and therefore is not treated here.

The number of ovules per ovary is an important taxonomic character in Rorippa, and some closely related species can easily be separated using this feature. It is misleading to give the number of mature seeds per fruit because not all ovules develop into mature seeds, and the difference between ovule and seed number can be substantial. For example, R. L. Stuckey (1972) reported in R. columbiae 20–30 seeds per fruit, but the ovule number in this species is 48–64.

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

Of conservation concern.

Rorippa subumbellata appears to be restricted to Tallac and Truckee lakes and Lake Tahoe in eastern California (El Dorado and Placer counties) and western Nevada (Douglas County).

The record by Rollins of n = 5 for the species (see S. I. Warwick and I. A. Al-Shehbaz 2006) is highly unlikely; no such count is known in the entire tribe Cardamineae, to which Rorippa belongs.

Rorippa subumbellata is in the Center for Plant Conservation’s National Collection of Endangered Plants.

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