Aerodynamics and right-left symmetry in wind dispersal of maple, dipterocarps, conifers and some genera of apocyanaceae and magnoliaceae

The wind dispersing seeds have evolved remarkable aerodynamic optimisation to minimise the speed of descent on detachment from the tree, so that they could be carried away by the wind and deposited a distance away from the parent. An efficient physical mechanism that enables slowing down the descent is rotation, which reduces the translational kinetic energy. Rotation and translation always define a right or left-handedness. The present work examines the aerodynamics of the dispersal of seeds of many species in relation to right-left asymmetry, giving examples to show that in nature this symmetry is broken either spontaneously or intrinsically. In the former situation, seeds have no geometrical right-left asymmetry, but an initial instability chooses one sense of rotation (right or left). Whereas in the latter, geometrical asymmetry dictates the sense of rotation. Seeds of the maple familiy belongs to the first category and a theortical model is presented to explain the motion. Seeds of dipterocarps, conifers and some genera of apocyanaceae and magnoliaceae are found to possess a handedness, which determines the rotation during seed fall. In dipterocarps, conifers and magnoliaceae, both right-handed and left-handed seeds are produced in the same tree but correlated to the handedness of the sprial phyllotaxy of the branch that bears the fruit. Apocyanaceae is found to be unique and seeds of all genera have the same handedness. The details of extensive field observations, experiments and theoretical interpretations are presented to illustrate the relationship of right-left asymmetry to the aerodynamics of seed wind dispersal, pointing out the implications of evolutinary optimisation in practical aerodynamics.