The mechanics of explosive dispersal and self-burial in the seeds of the filaree, Erodium cicutarium (Geraniaceae)

Abstract

Fracture at minimum energetic cost is needed to maximize dispersal distance. However, a premature fracture risks launching the seed before it has stored sufficient energy to reach appreciable range, resulting in a short range 'dud'. The two-sided constraint, in which the work of fracture must be high enough to prevent premature fracture but low enough to maximize ejection distance, is in sharp contrast to classical fracture mechanics problems where avoidance SUMMARYThe filaree (Erodium cicutarium), a small, flowering plant related to geraniums, possesses a unique seed dispersal mechanism: the plant can fling its seeds up to half a meter away; and the seeds can bury themselves by drilling into the ground, twisting and untwisting in response to changes in humidity. These feats are accomplished using awns, helical bristles of dead but hygroscopically active tissue attached to the seeds. Here, we describe the kinematics of explosive dispersal and self-burial based on detailed high-speed and time-lapse videos. We use these observations to develop a simple mechanical model that accounts for the coiling behavior of the awn and allows comparison of the strain energy stored in the awn with the kinetic energy at launch. The model is used to examine tradeoffs between dispersal distance and reliability of the dispersal mechanism. The mechanical model may help in understanding the invasive potential of this species and provides a framework for examining other evolutionary tradeoffs in seed dispersal mechanisms among the Geraniaceae. Supplementary material available online at

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