The Mechanics of Seed Expulsion in Acanthaceae

The bilocular seed capsules of species on the Acanthaceae subfamily Acanthoideae are either hygrochastic or xerochastic, but in both cases the mechanism for seed expulsion is similar; only the “trigger” differs in the two instances. The drying of the capsule results in the storage of elastic energy in the capsule valves. The failure of the seam joining the two values precipitates the conversion of the elastic potential energy stored in the valves and seeds. In the hygrochastic case the failure is due to moisture absorption on wetting of the capsule beak which weakens the pectic “glue”; in the xerochastic case the seam failure is due simply to the high stress in the bonding layer at some degree of desiccation. This paper explains quantitatively how the anatomy of the capsule efficiently imports high initial expulsion velocity to the seeds in order to maximize their range. The specific example considered inRuellia brittonianaLeonard, a cultivated shrub native to Mexico, but the situation is similar for the entire Acanthoideae subfamily.