rainfall is very rare in the desert – that’s why many species of desert plant have developed special mechanisms that help them to increase the offspring’s chances of survival.
cheiridopsis sp. (family aizoaceae) is one of these species: it has dry woody fruits that protect the seeds during dry periods.
when it starts to rain – sometimes it takes years from one rainfall to another – time is of the essence: as soon as a drop of rain lands on the capsules, they absorb the water and hydrostatic pressure opens them.
within minutes, seeds are released onto humid ground and find a good base to germinate.
while in south africa last september, i tried it myself: a drop of water sufficed, and the seed capsule was completely open just 10 minutes later.
i’ve also created an animated time-lapse gif that shows the process:
(click for bigger version)
some species of aizoaceae even have little catapult-like structures that shoot out the seeds when another raindrop hits.