Ruellia tweediana

Nature drops surprises on you when you least expect it- as with what happened to me the other day while working in our flower gardens.  While watering I began to hear a faint "popping" sound coming from the garden area, almost sounding like an electrical arc.  Puzzled I shut off the water to try and figure out what it was that I was hearing.  After a minute or so of silence, I began watering again.  Seconds later the popping noise returned.  What helped me discover what was happening, was when I felt something lightly strike the front of my shirt.  I looked down to discover several small, flat seeds affixed to my chest.  The popping noise I heard was coming from the seed pods of the Ruellia (aka Mexican petunia, wild petunia, Mexican bluebell) that has taken over one area of our yard.  These pods, when wetted were popping open, sending the seeds within flying in all directions.

This plant uses "explosive dispersal" as a method of spreading its seed, allowing each seed to be separated from all of the others giving it the room it needs in order to mature.  Many other plants distribute their seeds through this means of dispersal such as wisteria, witch hazel, jewelweed, okra.  My son was able to capture a nice video of this:

What is it about the contact with water that causes the pods to burst open?  I'm assuming that as the seed pods dry, tension is built up within the pod.  Then when they come in contact with water by way of rain or garden hose, this somehow releases that tension allowing for the rapid explosive dispersal of the seeds.  Note how the walls of the spent pod are turned outward, where they were once straight before release.

Once I figured out what was happening I couldn't wait to show this to my granddaughter.  I removed one of the pods and placed it in her hand.  I then wetted it and told her to cup her hands together.  Seconds later we heard a light popping sound and then a wide grin spread across her face.  Opening her hands she found the seeds that had been released.