Energetic Constraints and Adaptive Significance of the Floral Display of a Forest Milkweed

The number of flowers produced by Asclepias quadrifolia, a perennial, woodland milkweed of the Ozarks, is significantly correlated (P < .001) with both aboveground and belowground structural energy at time of flowering. About 3—5% of the total structural energy was devoted to flowers by plants of all sizes. In the year following the production of a pod or major herbivore damage by Rhyssematus lineaticollis or Danaus plexippus, a plant produced a significantly smaller stem with fewer flowers. Individual flowers are relatively inexpensive to produce (63.6 J, equalling 0.2% of the total structural energy of an average plant at time of flowering), but a single seed pod is expensive (10.40 kJ, equalling 21%). Most small flowering individuals lack sufficient resources to mature a pod. A typical seedling passes through a juvenile nonflowering period and a flowering period when it functions solely as a male (all initiated pods are aborted) before it accumulates enough reserves to become functionally hermaphroditic. Since flower production appears to be limited by available energetic reserves, a plant would have a selective advantage if those limited flowers (x = 29.6 flowers per flowering plant) were grouped to maximize genetic contribution to the next generation. Total pollinarium removal increased with increasing umbel size, but pollinarium removal per flower peaked within the natural range of umbel sizes (10—25 flowers). Although mean umbel size in the field was 15—17 flowers, only 5 flowers were necessary to achieve maximal pod initiation per flower. The "extra" flowers cannot be explained adequately in terms of seed production, but are better understood in terms of enhancing pollen contribution through the donation of pollinaria. A cost—benefit analysis suggested that the optimal umbel size for plants growing in typical woods habitat was °12 flowers.