Two Species of Yellow Wood-sorrel

Two species of wood-sorrel (Oxalis) with yellow flowers grow in my neighborhood. The most common is Yellow Wood-sorrel Oxalis stricta which has bright green foliage and grows relatively erect. The other species seems to be Creeping Wood-sorrel Oxalis corniculata. This plant has a distinct purplish cast to its foliage and has a lower growth habit. Oxalis corniculata also appears to be more densely hairy, especially the fruits (pods). Although Weeds of the Northeast (Uva et al., 1997) say that leaf color is variable in both species and cannot be used as a diagnostic characteristic in my experience wood-sorrel plants with low growth habit always seem to have a purplish cast to the foliage. Gleason and Cronquist also mention this feature under Oxalis corniculata but not under O. stricta, so perhaps the purplish foliage color is diagnostic for Creeping Wood-sorrel O. corniculata in spite of Uva et al.