Orbexilum onobrychis (Nutt.) Rydb.

French Grass

Orbexilum onobrychis plant

Family - Fabaceae/Faboideae

Habit - Rhizomatous perennial, strongly colonial.

Stems - Erect, to 1 m, sparsely branched above midpoint, reddish toward base, pubescent with +/- antrorse hairs, becoming nearly glabrous with age.

Orbexilum_onobrychis_stem.jpgStem.

Leaves - Petiolate, stipulate, pinnately trifoliate or occasionally appearing palmately trifoliate or simple. Lowermost nodes sometimes leafless, with stipules fused into a single scalelike structure. Petioles to 9 cm long, the terminal leaflet with the stalk 8-25 mm long. Stipules 2-5 mm long. Leaflets to 12 cm long, 5 cm wide, broadly lanceolate to ovate with sharply pointed tips, the upper surface glabrous or sparsely short-hairy along the main veins, the undersurface pale, sparsely to moderately short-hairy, more densely so along the veins.

Orbexilum_onobrychis_leaf.jpgLeaf.

Orbexilum_onobrychis_leaflet.jpgLeaflet abaxial.

Inflorescences - Dense axillary racemes to 24 cm long, elongating with age, the stalk 5-14 cm long, the bracts 2-3 mm long.

Orbexilum_onobrychis_inflorescence.jpgInflorescence.

Flowers - Calyces short-hairy, the tube 1.2-1.6 mm long, the lobes 0.5-1.0 mm long, subequal, usually bluntly pointed or rounded at tip. Flowers papilionaeous, the corollas with the banner 4-5 mm long, the wings 5-7 mm long, the keel 4-5 mm long, pale blue-violet. Stamens 10.

Orbexilum_onobrychis_calyces.jpgCalyces.

Orbexilum_onobrychis_flowers.jpgFlowers.

Fruits - 6-8 mm long, 4-5 mm wide, obliquely ovate in outline, short-tapered to a stout beak, this 2-3 mm long, the surface with a network of cross-wrinkles, also distinctly warty, brown to black. Seeds 4-6 mm long.

Orbexilum_onobrychis_fruits.jpgFruits.

Flowering - May - July.

Habitat - Riverbanks, pond margins, ditches, forest openings.

Origin - Native to the U.S.

Other info. - Of the two species of Orbexilum found in Missouri, this one is by far the less common. It is easily distinguished from O. pedunculatum by its broader leaflets and taller, more strongly colonial growth habit. It is encountered only infrequently, but, where found occurs in large clonal colonies from the widely creeping rhizomes.

These plants were formerly classified in the genus Psoralea, as P. onobrychis or P. latifolia. The inflorescences are structurally similar to those found in the Pediomelum and Psoralidium genera, potentially leading to misidentification. Orbexilum is distinguished from these other genera by its pinnately trifoliate leaves and wrinkled fruits which extend well beyond the calyces.

Photographs taken at Shaw Nature Reserve, Franklin County, MO, 6-20-2014 (SRTurner).



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