The seeds of Acanthaceae are dispersed from their capsules by an explosive catapulting action. The mechanism involved is one of energy storage in the septum of the capsule during drying; at seed expulsion this stored energy is converted to kinetic energy. In this paper we will explain the mechanism and explore the relationship between the architecture of the septum and the initial expulsion velocity of the seeds, this latter being important since it determines seed dispersal range. We will see that the geometry of the three tissue layers in the half septum cross-section, as well as the fibril angle in the cells of the active and resistance layers, have been optimized to maximize expulsion velocity. Classical laminate theory and known relationships between expansion coefficients and elastic constants are used in evaluating the efficiency of the system as it depends on fibril angle. The mechanism described above seems to exist in the entire subfamily Acanthoideae, whether the seed capsules are hygrochastic or xerochastic. The specific example considered quantitatively is Ruellia brittoniana Leonard, a cultivated shrub native to Mexico.
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