Hofmeister’s Rule’s Paradox: Explaining the Changeable Carpel Position in Caryophyllaceae

Abstract

Premise of research. In Caryophyllaceae with a gynoecium isomerous with the other floral whorls, carpels can be in either antesepalous or antepetalous position. Morphologists have long been unable to find a unifying hypothesis to explain the different carpel positions in Caryophyllaceae, as variation in carpel position breaks with Hofmeister’s rule of alternating whorls.Methodology. In this study, we investigate the floral development of six species of Caryophyllaceae with isomerous flowers using scanning electron microscopy to clarify the differences between carpel positions and whether developmental modes are responsible. Special attention is paid to the correlation between the development of carpels and other floral organs.Pivotal results. In Spergula arvensis, Agrostemma githago, and Myosoton aquaticum, carpels are in antepetalous position, while the antesepalous stamens develop more centrally in a sequence inverse to the sepals. In Cerastium glomeratum and Silene diocia, carpels are in antesepalous position, while the antesepalous stamens do not show an inverse development and are inserted almost at the same level as antepetalous stamens. In Lychnis flos-cuculi, the carpels are not clearly in an antesepalous or antepetalous radius, and the development of antesepalous stamens shows a slight unequal growth.Conclusions. Developmental evidence rejects the hypothesis that the developmental modes of carpels are the reason for the different carpel positions. Carpel position is correlated with the developmental sequence of antesepalous stamens and the level of emergence of antepetalous stamens on the floral apex. Space availability and pressure exercised by the sepals is responsible for the unequal development and a more central position of antesepalous stamens, with those opposite the inner sepals developing faster, causing the carpels to shift to an antepetalous position. With greater availability of space, antesepalous stamens initiate simultaneously or continue the sequence of sepal initiation, and all stamens are more or less equidistant from the apex; the antesepalous stamens do not display an inverse development, and the carpels initiate in antesepalous position, which is consistent with Hofmeister’s rule.