REPRODUCTIVE BEHAVIOR OF AMPHICARPAEA BRACTEATA (LEGUMINOSAE), AN AMPHICARPIC ANNUAL

Amphicarpaea bracteata, an annual legume common in woodland communities in the eastern United States, produces three distinct types of flower: subterranean cleistogamous (SCL), aerial cleistogamous (ACL), and aerial chasmogamous (ACH). We sought to quantitatively describe the growth and reproduction of four diverse A. bracteata populations in an effort to explain the adaptive significance of this species’ variable reproductive modes. Virtually all plants develop 1 to 3 SCL seeds on subterranean cotyledonary shoots, starting in late July. Larger plants produce additional SCL seeds later from the tips of axillary shoots which bury. Seeds produced by SCL flowers are large (39 to 134 mg), have restricted dispersal, lack dormancy, have high germination , and produce relatively vigorous seedlings. ACL flowers develop from early August through plant senescence in early October, while ACH flowers appear in a pulse in late August. The number of each aerial flower type was positively correlated with plant size. The ACL flowers take significantly less time to develop mature fruits than do ACH flowers. Both aerial seeds are smaller than the subterranean (mean weights: ACL 12.1, ACH 8.7 mg), have lower germination (ACL 20.2%, ACH 15.3%), appear relatively resistant to environmental extremes, and have the potential for more widespread dispersal. Plants derived from subterranean seeds are much larger than plants derived from aerial seeds, which makes them more likely to produce axillary SCL and aerial seeds. Only larger plants derived from subterranean seeds and growing in favored sites produce ACH flowers. This imposes an alternation of selfing with outcrossing generations. This pattern of reproductive behavior in A. bracteata is similar to that found in several other amphicarpic species.