Lathyrus palustris L. (vetchling, marsh pea)

L. palustris var. myrtifolius (Muhl. ex Willd.) A. Gray

Pl. 398 g, h; Map 1762

Plants perennial, with rhizomes, the roots not producing tubers. Stems 30–100 cm long, trailing or climbing, unbranched or few-branched from the base, glabrous, angled or narrowly winged, the wings to 1 mm wide. Leaves with mostly 6 leaflets, some of them occasionally with 4, 5, or 8 leaflets, the petiole 0.8–3.0 cm long, unwinged, the tendrils branched or some of them unbranched. Stipules 6–18(–30) mm long, 4–10(–18) mm wide, ovate to broadly ovate, the basal lobe 4–8(–20) mm long, triangular to rounded-oblong and sometimes toothed. Leaflets 2–8 cm long, 3–20 mm wide, elliptic to narrowly elliptic, narrowly lanceolate, or linear, angled at the base, angled to a bluntly or sharply pointed tip, the midvein sometimes extended into a minute sharp point at the very tip, glabrous. Inflorescences open racemes of 2–6(–9) flowers, the stalk 3–6 cm long, the flower stalks 2–6 mm long. Calyces with the tube 3–4 mm long, glabrous or hairy, the lobes 1–6 mm long, the upper 2 short and triangular, the lowermost lobe 2–3 times as long as the upper 2, narrowly lanceolate-triangular, the lateral lobes of the lower lip intermediate in size and shape. Corollas 12–20 mm long, reddish purple, to purple, lavender, or pink (rarely white elsewhere). Filaments with the fused portion 9–10 mm long, the free portion 3–4 mm long. Ovary glabrous. Fruits 4–6 cm long, 4–5 mm wide, linear to narrowly oblong, flattened, glabrous or with sparse, minute glandular hairs, 5–10-seeded. Seeds 3.0–3.5 mm long, more or less globose to slightly angular, not or only slightly flattened, the surface smooth, reddish brown to greenish brown or nearly black, sometimes faintly mottled. 2n=14. May–June.

Uncommon in the eastern half of the state (eastern U.S. west to North Dakota and Texas, also Alaska to California; Canada, Europe, Asia). Fens, acid seeps, and bottomland forests; rarely also roadsides.

Lathyrus palustris is variable in leaf size and shape, stem size, and degree of pubescence, and several varieties have been described (Fernald, 1911; Hitchcock, 1952; Steyermark, 1963). Plants with slender, wingless stems and leaves with four or six broad, obtuse leaflets have been called var. myrtifolius. Plants with narrowly winged stems and six or eight narrow leaflets are var. palustris. However, Isely (1998) concluded that these varieties have no ecological or geographic basis and intergrade completely.