This section is from the book "Nature's Garden", by Neltje Blanchan. Also available from Amazon: Nature's Garden; An Aid To Knowledge Of Our Wild Flowers And Their Insect Visitors.
Flowers - Fragrant, chocolate brown and reddish purple, numerous, about 1/2 in. long, clustered in racemes from the leaf-axils. Calyx 2-lipped, corolla papilionaceous, the broad standard petal turned backward, the keel sickle-shaped; stamens within it 9 and 1. Stem: From tuberous, edible rootstock; climbing, slender, several feet long, the juice milky. Leaves: Compounded of 5 to 7 ovate leaflets. Fruit: A leathery, slightly curved pod, 2 to 4 in. long.
Preferred Habitat - Twining about undergrowth and thickets in moist or wet ground.
Flowering Season - July - September.
Distribution - New Brunswick to Ontario, south to the Gulf States and Kansas.
No one knows better than the omnivorous "barefoot boy " that
" Where the ground-nut trails its vine " there is hidden something really good to eat under the soft, moist soil where legions of royal fern, usually standing guard above it, must be crushed before he digs up the coveted tubers. He would be the last to confuse it with the Wild Kidney Bean or Bean Vine
(Phaseolus polystachyus) - P. perennis of Gray. The latter has loose racemes of smaller purple flowers and leaflets in threes; nevertheless it is often confounded with the ground-nut vine by older naturalists whose knowledge was "learned of schools."
Usually a bee, simply by alighting on the wings of a blossom belonging to the pea family, releases the stamens and pistil from the keel; not so here. The sickle-shaped keel of the ground-nut's flower rests its tip firmly in a notch of the standard petal, nor will any jar or pressure from outside release it. A bee, guided to the nectary by the darker color of the under side of the curved keel which spans the open cavity of the flower, enters, at least partially, and so releases by his pressure, applied from underneath, the tip of the sickle from its notch in the standard. Now the released keel curves all the more, and splits open to release the stigmatic tip of the style that touches any pollen the bee may have brought from another blossom. Continuing to curve and coil while the bee sucks, it presently dusts him afresh with pollen from the now released anthers. A mass of pulp between anthers and stigma prevents any of the flower's own pollen from self-fertilizing it. These little blossoms, barely half an inch long, with their ingenious mechanism to compel cross-fertilization, repay the closest study.
At midnight the leaves of the ground-nut and wild bean "are hardly to be recognized in their queer antics," says William Hamilton Gibson. "The garden beans too play similar pranks. Those lima bean poles of the garden hold a sleepy crowd."