Quite a number of fruits snap open is such a way that the seeds are expelled ballistically. The basic principle is the fruit in maturing and drying is constructed such that a force is built up and when the force gets great enough to overcome the anatomical constraints, themselves often weakening with maturation of seams and sutures, and the whole thing snaps open and the seeds are thrown several meters. Cool. However the detailed features of the mechanism have not been much studied, and here's a pretty neat image of one of the two fruit walls of the hairy bittercress (a mustard, Cardamine hirsuta) after the two halves have split and thrown the enclosed seeds. Many such fruits (siliques) just open with maturity and seeds drop out (yawn). The study featured in the latest American Journal of Botany documents the type and shape of cell wall thickenings that result in the rapid shape change of the fruit wall that forcibly expells the seeds. The image is from the cover of the August 2011 issue.
The mechanism for explosive seed dispersal in Cardamine hirsuta (Brassicaceae). 2011. K. C. Vaughn, A. J. Bowling, and K. J. Ruel. AJB 98: 1276-1285.
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