How carpels and perianth affect stamens: evidence from stochastic meristic changes

When floral organs are arranged in whorls, merism represents the number of organs per whorl. Calyx and corolla are usually isomerous (with an equal number of organs), whereas the carpel merism often differs. Staminal number may be isomerous either with the perianth members or with the carpels. Isomerous whorls typically alternate, whereas the relative positions of anisomerous whorls may be labile. The stochastic floral variation was examined in 13 species from eight eudicot families. In Oleaceae (two species), the staminal number seems to be defined by the gynoecial merism. This suggests bipolar patterning in the floral development of this family, i.e. carpels define the position of stamens, and petals are guided by sepals. In Cucurbitaceae (two species), the androecium tends to be isomerous with the perianth, but stamens are localized, either singly or in pairs, in the sites the number of which correlates better with the gynoecial merism. In Sapindaceae (four species), the correlations between all organ numbers are weak, and the androecium is supposedly in two whorls, the inner trimerous and the outer pentamerous. The positions of both are affected by the gynoecium. In other families (five species), the correlations among floral numbers were high except for the cases when the gynoecium was anisomerous with other floral whorls and independent of them. If the androecium is isomerous with the perianth, not the gynoecium, it suggests the perianth-dependent patterning of stamens. Various patterns of floral architecture regulation may coexist in the same taxon, with flexible boundaries of different whorls’ influence.