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October 2019: Going Ballistic- Ballistochory

Ballistochory is used to describe a form of dispersal where seeds are forcefully ejected. Many of our native plant species  (e.g. Doveweed (Croton sp.), Spanish Clover (Acmispon sp.), some species of the California lilacs (Ceanothus sp.) and most Lupines (Lupinus sp.)) use this form of explosive mode of dispersal by sending the seeds several feet away from the plant sometimes with an audible pop. In fact, if you are standing next to Lupines whose mature pods are drying in the sun you can hear the pops and sometimes be sprinkled by the seeds. While trying to collect seeds for our restoration efforts, we sometimes have to design creative ways to catch and retain the seeds that would otherwise be launched away from the plant. And if we collect the ripe but unopened pods, it is best to cover them so seeds will not be distributed through out the room when they finally do dry out- this was learned from experience :).
 
The tropical SandboxTree, (Hura crepitans), can disperse its seeds at up to 70m/s (over 150mph) and reportedly sounds like a gun shot when they explode. At that speed, the seed (~ the size of a  $0.25 cent coin USD) would almost certainly cause injury if it was close enough.
 
All of these plants rely on differential drying of the fruit wall to generate tension. The walls of a ballistic fruit will have at least two layers in which the woody fibers are usually oriented in different directions, often at right angles. As the fibers dry, the layers begin to pull against each other. Fractures distributed along predetermined seams form weak spots that eventually rip dramatically, sometimes throwing the seeds a considerable distance. In legumes, the bean pod splits along two natural seams, and the two halves curl into a corkscrew shape (like those of the Spanish Clover shown in the photograph), flinging out seeds in the process.

The capsules of the yellow wood sorrel (Oxalis sp.), a common greenhouse weed, hops from pot to pot by launching seeds from its exploding pods. As a youngster, I delighted in inviting my unsuspecting friends to pinch a ripe Oxalis capsule and the pod would startle them when it exploded between their fingers spraying us with seeds.