The bilaterally symmetrical flowers superficially resemble those of the Fabaceae, but the structures are not homologous. Of the 5 sepals, the 2 lateral ones are large and petaloid, usually as large as the petals; the petals are 3, the lower one forming a keel which encloses the stamens and style and usually has a fringe or appendage near the end. The stamens are 6 (in P. paucifolia), 7, or 8. The seeds are pubescent and bear an aril (or aril-like structure), which apparently aids in dispersal by ants. Some species also produce reduced cleistogamous flowers, underground or above ground; except when explicitly stated, the key does not refer to these. The lowermost leaves are often reduced to scales.
1. Flowers 1–4 (–5) on each stem, (12–) 15–20 (–22) mm long, the wings readily deciduous; well-developed leaves few, crowded toward summit of stem.
1. Flowers numerous, ca. 3–7.5 mm long, the wings persistent; well-developed leaves several to many (or often none at anthesis in P. incarnata).
2. Leaves (at least the lower ones) whorled (or opposite, look for scars if necessary).
3. Well-developed inflorescences ca. 10–15 mm broad, sessile or on peduncles at most ca. 5 mm long.
3. Well-developed inflorescences ca. 3–5 mm broad, on distinct peduncles usually 1–5 cm long.
2. Leaves all alternate.
4. Inflorescence a distinct loose raceme, the lower flowers well separated from each other and on pedicels mostly 1–2 (–3) mm long; plant (in common species) producing subterranean cleistogamous flowers or (rare escapes) with chasmogamous flowers at least 6 mm long.
5. Flowers ca. 3.5–4.5 (–5.5) mm long, normally pink-magenta; cleistogamous flowers developing underground.
5. Flowers ca. 6–7.5 mm long, normally blue; cleistogamous flowers none.
4. Inflorescence a dense spike-like raceme, the flowers crowded and on pedicels mostly no more than 1 mm long; plant without cleistogamous flowers, flowers (except in P. incarnata) shorter than 6 mm.
6. Leaves subulate, less than 1 mm wide, soon deciduous; flowers ca. 5–7 mm long, the petals united into a tube (open on one side) much exceeding the wings and other sepals.
6. Leaves broader (mostly 1.2–20 mm), persistent; flowers ca. 3–5.5 (–6) mm long, the petals only very slightly united and scarcely if at all exceeding the wings.
7. Plant annual, with glabrous stem from a poorly developed taproot; inflorescence ca. 8–12 mm broad, at most ca. twice as long when in peak flower (the rachis elongates as lower flowers are shed); flowers ca. 3.5–5.5 (–6) mm long.
7. Plant perennial, with many glandular-puberulent stems from a woody root; inflorescence ca. 4–8 (–9) mm broad, normally at least 3–5 times as long; flowers ca. 3–4 mm long.