FLORAL DEVELOPMENT AND HOMEOSIS IN ACTAEA-RUBRA (RANUNCULACEAE)

Actaea rubra is a member of the Ranunculaceae in which the number and positions of petals are variable. To understand the process by which t his variation arises, we used epi-illumination microscopy to study flo ral development in the species. We found that four sepal primordia ini tiate in two pairs, one outer and one inner. Then four primordia form in alternation with the sepal primordia, followed by the initiation of six primordia in opposition to the sepal primordia (one primordium op posite each outer sepal primordium, and two opposite each inner sepal primordium). These first 10 small, rounded primordia can develop into either petals or stamens, but they cannot be discerned in early stages because the primordia are very similar in size and shape. Subsequentl y, numerous primordia that usually develop into stamens initiate in al ternation with previously formed primordia, and the floral apex termin ates in a crescent-shaped gynoecial primordium. Petal and stamen primo rdia can be distinguished in later stages when petal primordia are sma ll and flattened in appearance, whereas stamen primordia have anther s acs. At maturity petals are small, white, and spatulate-shaped, and st amens have long, slender filaments and oval anthers. Examination of th ese later stages showed that the number and positions of petals are no t constant. We therefore mapped petal and stamen positions in more mat ure floral buds and found that petals most frequently occupied the pos itions that alternate with the sepals. However, a comparison of the po sitions that petals and stamens occupy in different flowers showed tha t petals could form in what were usually stamen positions, and stamens could form in what were usually petal positions. We interpret this as ao example of homeosis occurring within and among plants of A. rubra. When the floral morphology of Actaea is compared with that of related taxa, various phylogenetic interpretations of homeosis can be hypothe sized, depending on the putative outgroup condition.