Unstable patterns of flower structure and development in the almost dioecious species Acer negundo (Sapindaceae, Sapindales).

This article focuses on the floral morphology, development, and sexual expression of Acer negundo L., a wind-pollinated, dioecious maple species native to North America and invasive in Eurasia. It presents detailed observations showing that male and female flowers each have only two whorls (sepals and stamens or carpels), with male flowers exhibiting unstable merism and variable numbers and positions of stamens and sepals, while female flowers display more stable floral structures. The study documents the presence of occasional bisexual flowers containing rudimentary organs of the opposite sex—staminodes in female flowers that can produce some viable pollen, and sterile carpellodia in male flowers—indicating potential functional bisexuality, particularly in female trees. These findings challenge the notion of strict dioecy in A. negundo and suggest that floral development and sexual expression may be influenced by environmental factors and inflorescence architecture.