Northern Prairie Wildlife Research Center
4. Rorippa palustris (L.) Bess. -- Bog yellow cress
Stuckey (op. cit.) recognizes several subspecies within this polymorphic species of which two occur in our region. R. palustris subsp. glabra (Schulz) Stuckey is characterized by leaves glabrous to sparingly hirsute on the upper surface, glabrous below, and stems glabrous to sparingly hirsute near the base, glabrous upward. This is the most common form in our area. R. palustris subsp. hispida (Desv.) Jonsell is somewhat less common, with leaves hirsute on the lower surface and stems hirsute usually up to the terminal raceme.
Stuckey has further divided these subspecies into varieties mainly on the basis of fruit characters. In our area there are two varieties of subsp. glabra. In R. palustris subsp. glabra var. fernaldiana (Butt. & Abbe) Stuckey, the style is expanded in fruit and the silique is constricted at the middle so that the replum has concave margins and becomes twisted with age and upon drying. Our other variety is R. palustris subsp. glabra var. glabrata (Lunell) Stuckey, which differs from var. fernaldiana in that the style is unexpanded in fruit and the silique is not constricted at the middle, thus the replum has straight margins and remains flat with age and upon drying. There often seems to be little if any correlation between the condition of the stigma in fruit and the silique/replum shapes.
Both varieties of R. palustris subsp. hispida occur in our range. R. palustris subsp. hispida var. hispida is the common phase, with siliques globose to subglobose, mostly 2.5-6 mm long, 1.3-3 mm wide, ca. 1-2X longer than wide; replum circular to elliptic in outline; petals 1-1.5(2) mm long; pedicels usually divergent to ascending; stem usually densely hirsute below. Of much lesser occurrence is R. palustris subsp. hispida var. elongata Stuckey. This differs from var. hispida in having siliques cylindrical, mostly 5-8 mm long, 2.1-3.1 mm wide, ca. 2X or more longer than wide; replum oblong in outline; petals 1.5-2(2.8) mm long; pedicels usually divergent to recurved; stem usually sparingly hirsute below.