We study the mechanism of spore discharge in ferns. It consists in the fast released of a spring-like
structure, the sporangium, which contains the spores, after its opening due to dehydratation. Thirteen
cells constituting the annulus surround the capsule containing the spores. Through a thin membrane, the
water inside these cells evaporates. The resulting decrease in volume, along with cohesive forces induced,
lead to a change in curvature of the annulus. We analyze this opening phase for natural, isolated sporangia
dipped into osmotic solutions and we compare our results with a simple theoretical model. For the ¯rst
time, we observe, using high speed imaging, the fast closure motion of the sporangia by releasing the
elastic energy stored, that leads to the spores ejection at a speed around 10 m/s. We ¯nd, in particular,
that the motion presents two steps, an uncommon behavior among fast plant motion which enables a
very e±cient discharge: a wonderful example of an autonomous catapult.