Floral ontogeny and anatomy in Koelreuteria with special emphasis on monosymmetry and septal cavities

The floral ontogeny and anatomy of Koelreuteria paniculata have been investigated to understand the developmental basis for the occurring monosymmetry and the origin of the septal cavities. Petals arise sequentially and one petal is missing between sepals 3 and 5, or rarely between sepals 2 and 5. The eight stamens arise sequentially before petal initiation is completed. The last formed petal and one stamen arise on a common primordium. Two stamen positions are empty (opposite the petal between the sepals 2 and 5, and the petal between sepal 1 and 3); consequently two antesepalous stamens have cebome displaced. The derivation of octandry from a diplostemonous ancestry, and reduction of the petal are discussed. The triangular gynoecium has a strong impact in obliquely reorganizing the symmetry of the flower, loss of organs, and shifts of stamens. The so-called septal slits occurring within the style are a deep-reaching non-nectariferous extension of the stigma. Alternating locular furrows are present which could play a role as pollen transmitting tissue and in the loculicid dehiscence of the capsule.