SEX EXPRESSION IN A NATURAL-POPULATION OF THE MONOECIOUS ANNUAL, AMBROSIA-ARTEMISIIFOLIA (ASTERACEAE)

Sex allocation in the monoecious annual, Ambrosia artemisiifolia L., was investigated in an old field of southeastern Pennsylvania. I tested the hypothesis that floral sex ratio (male:female) changes with plant size (height and weight) and with light availability. The results do not agree with the traditional expectations of the resource limitation model of sex allocation, and support predictions of increasing maleness in plants that presumably grow under favorable conditions. The floral sex ratio was determined for each plant axis (primary, secondary and tertiary) and did not differ significantly among them. More female flowers on the primary axis resulted in a lower sex investment (male plus female flowers) in the secondary and teritary axes, indicating that there might be a resource allocation constraint in plant architecture. The possible adaptiveness of the floral architectural arrangement in this wind-pollinated species is discussed.