Shape Matters: Hofmeister’s Rule, Primordium Shape, and Flower Orientation

Abstract

Hofmeister's rule is an empirical heuristic derived from the observation that new leaf primordia are formed in the largest space between the existing flanks of the older primordia. These observations have been repeatedly validated in studies of leaf arrangement, but there has been little attempt to extend them to inflorescence and floral organs. This investigation demonstrates the validity of Hofmeister’s observations to cincinnus and early flower development in Phenakospermum guyannense (Strelitziaceae) and Heliconia latispatha (Heliconiaceae) and relates these results to Paul Green’s work on the biophysics of organ formation. The cincinni of Phenakospermum and Heliconia arise in the axils of primary bracts and produce a prophyll, continuation apex, and flower in regular succession. The shapes and orientations of the apical regions of the cincinni are correlated with the placement of these organs, which in turn effect the positions of the sepals and their sequence of formation. The result is two rows of mirror‐image flowers. The mirror‐image symmetry of the flowers is a direct result of Hofmeister’s rule in connection with the shape of the apical region. These two factors create a self‐sustaining developmental system that produces prophylls, continuation apices, flowers, and sepals in regular succession. Although the cincinni and flowers of these species are built on a common plan, slight differences in apical shape and orientation produce differences in mature floral orientation. Understanding these orientations, and identifying the sequence of sepal formation, allows a proper identification of organ homologies. A study of Green’s results and theories shows that Hofmeister’s rule can be considered as an empirical condensation of the biophysical factors that influence organ position. These biophysical factors are widely applicable to organ formation in many species.