AskNature

“Some seed capsules have a particularly neat way of achieving motility by varying water content. Witztum and Schulgasser (1995) showed that the two halves of the capsule (as in fig. 22.1) store energy as they dry; at a certain dryness (or, in some, when the top of the capsule is wetted) the seam joining them gives way suddenly. The capsule halves spring outward, which not only releases the seeds within, but expels them explosively at speeds up to 12 meters per second (27 miles per hour); they travel up to about 3 meters. The springing mechanism, as they point out, matches that of the bimetallic strips we use for thermometers and thermostats–differential expansion of longitudinally joined layers produces the stress that springing relieves.” (Vogel 2003:444)

“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.” (Witztum and Schulgasser 1995:531)