Family Oxalidaceae (Wood-Sorrel Family).
Also called Lady’s Sorrel. These plants closely resemble the common Sourgrass (O. stricta), but the stems of the seedpods do not have the pronounced angle usually seen in their close cousin. This species is more urban than the other, not found in many rural areas, and it is often found, as here, in a beautiful purple-leaved variety, which seems to favor the sunniest spots (or perhaps sunlight brings out the purple color).
Gray describes the genus and the species.
ÓXALIS L. WOOD SORREL. Sepals 5, persistent. Petals 5, sometimes united at base, withering after expansion. Stamens 10, usually monadelphous at base, alternately shorter. Styles 5, distinct. Pod prismatic, cylindric, or awl-shaped, membranaceous; valves persistent, being fixed to the axis by the partitions. Seeds pendulous from the axis, anatropous, their outer coat loose and separating. Embryo large and straight in fleshy albumen; cotyledons flat. —Several species produce small peculiar flowers, precociously fertilized in the bud and particularly fruitful; and the ordinary flowers are often dimorphous or even trimorphous in the relative length of the stamens and styles. (Name from oxys, sour.)
O. corniculàta L. (LADY’S SORREL.) Erect or decumbent, apparently flowering the first year but perennial by numerous slender pale runners; leaflets green or often purplish; pedicels subumbellately or at length cymosely arranged at the summit of the peduncle, ascending, sparingly pubescent, the hairs spreading; petals yellow. (O. cymosa, Bushii, & rufa Small; O. stricta of many auth., not L.) —Dry or moist open soil, a very common weed. (Eu.)