Flower:

Clusters of 2 to 6 stalked flowers on long naked stems that arise from the leaf axils. Individual flowers are ¾ inch long and a typical shape for a member of the pea family. Flower color varies from deep pink to purple to blue. A 2-tone purple and white or purple and blue flower is not unusual. The calyx holding the flower is bell-shaped with irregular teeth around the tip, longer teeth on the underside about the same length as the tube. The cluster's stem is typically as long as or longer than the compound leaf stem.

Leaves and stem:

Leaves are compound in 2 to 4 pairs; attachment is alternate with the leaflets attached oppositely. Each leaflet is generally elliptical, up to 2½ inches long and ½ inch wide, though they can range from short and wide to long and narrow, tapering to a point at both ends. There is a tendril at the end of the leaf stem that entwines around other plants.

The pair of leafy appendages (stipules) attached to the stem at the leaf joint are small and narrow, pointed at both ends with the upper portion nearly twice as long as the lower, in outline shaped like half of an arrowhead. Stems are usually, but not always, “winged”, making it look and feel ribbon-like. Stems and leaves may be hairless or hairy to varying degrees, and often tangles itself in the surrounding vegetation.

Notes:

There are 4 varieties of Lathyrus palustris but it is not clear where the ranges are for each, nor does the DNR or Bell Herbarium make any distinctions at this time. According to Gleason and Cronquist's Manual of Vascular Plants, they should all be represented somewhere in Minnesota. Marsh Vetchling is very similar to Veiny Pea (Lathyrus venosus), which has distinctly angled and wingless stems, 4 to 8 pairs of leaflets, more flowers per cluster, and prefers drier habitats.