The floral ontogeny of three species of Tropaeolum was studied using scanning electron microscopy to find morphological evidence for discussing the systematic position of the family. The initiation of the androecium is highly unusual: there are always eight stamens which arise (1) either in a spiral sequence starting with the stamen opposite sepal four, running in a direction opposite to the sequence of the sepals, and with reversals in the direction of the spiral, or (2) as a sequence of paired and unpaired stamens. The floral symmetry changes twice during the development of the flower, from polysymmetrical at sepal and petal initiation, through oblique monosymmetry at stamen initiation, and ending with median monosymmetry in later developmental stages. The occurrence of median monosymmetry is a late-developmental event and is caused by the initiation of a hypanthial spur, and the unequal growth of the petals and styles. The origin for the unusual sequence of stamen initiation reflects a trend affecting the whole flower which is linked with the changing patterns of floral symmetry. Octandry is enhanced by multiple causes, such as the loss of two stamens in an originally diplostemonous androecium and the regulating pressure of the gynoecium. The change in symmetry during ontogeny is significant for discussing the systematic position of Tropaeolaceae in comparison with the glucosinolate-producing taxa and the Sapindales. The combination of an androecium with eight stamens and oblique monosymmetry is either a single event in evolution and links Tropaeolum with the Sapindales, or it has evolved at least twice, once in the Sapindales, and once in a clade comprising Tropaeolaceae, Akaniaceae and Bretschneideraceae. Morphological data support a sister group relationship of the three latter families, which is in line with macromolecular studies. (C) 2001 Annals of Botany Company.