Autorotating dispersal units of <i>Getonia floribunda</i> Roxb. (Combretaceae): morphology, aerodynamics and geometrical significance

Dispersal units that continuously rotate while falling purely due to aerodynamic forces are called autorotating dispersal units. Single‐ to multi‐winged forms of autorotating dispersal units occur in nature. The dispersal unit of Getonia floribunda Roxb. (Combretaceae) is an example of a multi‐winged whirling diaspore that performs efficient autorotation in the air during wind dispersal. The present study aims to investigate the diaspore morphology, geometry, underlying principles, and reasons for the specificity of its autorotation during the dispersal and to find the morphological variations and geometrical significance. Plants and their diaspores from four locations in northern Kerala, South India, were selected for this study. Detailed macromorphological, micromorphological, developmental, anatomical, aerodynamic, and experimental studies were performed on the collected diaspores. The results show that fruit diaspores with distinct spatial configuration of persistent cyclically arranged tepals (perianth whorl) and distribution of the centre of mass along the middle vertical axis are the reasons for vertical autorotation or helicopter motion. Wing modification and paper model experiments prove that the wings have an intrinsically predetermined geometrical right‐side leading‐edge curvature, which is the reason for its distinct anticlockwise direction during autorotation. A significant variation in morphological parameters of these diaspores was observed across locations. Still‐airdrop tests and further statistical analysis show that wing fold angle developed due to the distinct spatial configuration of the wings exhibited a significant, predictable relationship with descent rate and terminal velocity in this diaspore across locations. Thus, wing geometry has an impact on flight in these diaspores and demonstrates the relationship between the shape and fitness of natural flight organs.