Reproductive Biology of Diploid and Triploid Apios Americana (Leguminosae)

Apios americana Medicus (Leguminosae; Phaseoleae), a tuberous, herbaceous vine, consists of diploid and triploid clones which are morphologically nearly identical. Although A. americana is widespread in eastern North America and flowers profusely throughout its range, fruit set is low. In part, this is because populations north of Connecticut consist almost exclusively of sterile triploid individuals. Although triploid populations occur south of this region, diploids predominate. Fruit set is also low in diploids. We attribute this to partial self‐incompatibility and to a low rate of floral visits by leaf‐cutter bees ( Megachile spp.), the only legitimate pollinator. Visitation is easily measured because flowers have a nonrepetitive explosive tripping mechanism which must be triggered by insects. In two Connecticut populations only 10% of 299 flowers were tripped, and on average only about one‐half of these set fruit. We conclude that A. americana maintains populations by vegetative propagation, but expands its range and retains variability through sexual reproduction of diploids.