Proximate and ultimate causes of low fruit production in Asclepias exaltata.

In Asclepias exaltata, a hermaphroditic milkweed, fruit production is constrained primarily by the availability of nutrient resources, as shown by the increased survival of fruits after removal of competing fruits and after application of fertilizer. Removal of half the flowers in each inflorescence had little effect on fruit production but decreased pollinarium removals by half, suggesting that one reason why few flowers mature fruits is that many flowers are produced primarily to enhance male reproduction. Fruit abortion may be an adaptation that allows plants to invest resources more profitably in male reproduction. -from Author