Organization, available space and organ morphology within floral buds of Iris (Iridaceae)

Before anthesis, older floral organs enclose younger ones forming a bud, within which the shape and relative size of floral organs are largely determined. The floral diversity in Iris, including sepal ornamentation, limited development of petals or sepals in some species and presence of petaloid style branches in all but one species, provides a unique opportunity to compare organ development within the confined space of a bud. Using transverse serial sections, light microscopy and measurement software, we investigated floral packing geometry (relative size and spacial relationships of floral organs) in seven species focusing on five species where we studied changes across three developmental stages. In this study, we found that floral packing geometries are diverse even among species with similar floral organ morphologies. The “filling law” proposed for vegetative buds is not applicable during floral bud development with empty space increasing as bud size increased for each of the five species that were examined. Other key findings include the presence of space between anther thecae in some species, the enlargement of sepal outgrowths and petal margins into space between thecae, differences in the relative growth of anthers versus petaloid style branches and the curvature of connectives, petals and style branches in some species. This study clarifies the integration of floral bud growth and illustrates a largely ignored yet important aspect of flower development: coordinated growth of floral organs within the bud.