Why pendulum symmetry is absent from the cymose partial inflorescences of Cannaceae? Insights into the essential characteristic of cincinni.

In a typical cincinnus, the neighboring two flowers are generally enantiomorphic, which leads to the pendulum symmetry of the entire cyme. While in a two-flowered Cannaceae cincinnus, the flowers develop the same chirality. In this study, we observed several abnormal cincinni of Canna indica that extended longer than their normal form, which presented a second enantiomorphic flower, thus reflecting a typical pendulum symmetry. The chirality change of the second flower was strongly associated with the position of the lateral cincinnus meristem, which determines the angle size of the cincinnus zigzag shift and may serve as a key factor controlling the formation of pendulum symmetry. We propose that alternating floral chirality and the concomitant pendulum symmetry are the essential characteristics of a typical cincinnus. Accordingly, Canna flowers with the same chirality are arranged in modified cincinni.