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.

P. paucifolia

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.

P. cruciata

3. Well-developed inflorescences ca. 3-5 mm broad, on distinct peduncles usually 1-5 cm long.

P. verticillata

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.

P. polygama

5. Flowers ca. 6-7.5 mm long, normally blue; cleistogamous flowers none.

P. vulgaris

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.

P. incarnata

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.

P. sanguinea

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.

P. senega