Floral development of Bougainvillea spectabilis Willd., Boerhaavia diffusa L. and Mirabilis jalapa L. (Nyctaginaceae)

Three morphological problems were investigated in three species of the Nyctaginaceae: epiphylly, phyllotaxis and placentation. Epiphylly, which occurs in Bougainvillea spectabilis, is the result of ontogenetic displacement resulting from the activity of an intercalary meristem at the base of the floral bract and the floral bud. Floral development of Bougainvillea spectabilis was compared with that of Boerhaavia diffusa and Mirabilis jalapa. Considerable variation occurs with regard to the number and arrangement of stamens. Five stamens are initiated simultaneously, alternate to the petals, in Mirabilis. In Bougainvillea, eight stamens arise sequentially at divergence angles suggestive of a 3/8 spiral. No developmental evidence was found to support the derivation of the eight stamens from a two whorled pentamerous androecium. Boerhaavia normally has only two stamens which most frequently are initiated toward opposite sides of the floral apex, but may also be formed in a 2/5 to 3/8 divergence. In some flowers only one or three stamens are formed. The gynoecium is formed in the same way in all three species: growth occurs in a crescent-shaped zone at the periphery of the floral apex thus producing the gynoecial wall. The single ovule, which is basal in the mature gynoecium, is formed from the gradual upgrowth and transformation of the floral apex and is developmentally terminal. Even the two-layered tunica is maintained as the floral apex is transformed into the ovule primordium. If ‘carpel' is defined traditionally as a folded megasporophyll which bears and encloses ovule(s) then carpels are not present in the gynoecia of the three species studied. If ‘carpel' is re-defined as an appendage which encloses ovule(s), then the gynoecia of the Nyctaginaceae are carpellate. However, the ovules remain cauline regardless of which definition is adopted.