The Ontogenetic Basis for Floral Diversity in the Baeckea Sub-Group (Myrtaceae)

Abstract

Androecial morphology differs widely amongst flowers of the Australasian Myrtaceae. Differences occur in the number, position and arrangement of stamens. Mature flowers of the Baeckea sub-group genera are superficially similar at maturity, but are very diverse in terms of their androecial structure. Floral development of six taxa in the informal Baeckea sub-group is described, and three developmental factors are identified as major contributors to mature androecial pattern. These are: differences in (i) timing of primordia initiation, (ii) duration of primordia initiation, and (iii) enlargement of the hypanthium subsequent to stamen initiation. Late or early initiation of stamens affects where the first stamens are formed, and how many stamens are produced, primarily due to available space on the floral meristem. Total number of stamens produced is influenced by the duration of stamen production. How the hypanthium enlarges following stamen initiation determines the final arrangement of the stamens in the mature flower if the bud expands more in some areas than others, the stamens will be shifted into different positions around the flower. Minor changes in any of these developmental factors result in significantly different flower morphology. Understanding these potentially variable and reversible developmental processes is important for assessing mature flower characters for phylogenetic analysis, and investigating mechanisms of evolutionary change.