When I sat down in an idle moment to watch a sporangium of Angiopteris opening, I was not expecting to see anything more interesting than grass growing. After all, eusporangia are just dull objects, without all the flashy movements with which the leptosporangium ejects the spores.
I was in for a surprise: the Angiopteris spores eject themselves! It appears to be due to a subtle interplay between the stiffness of exo- and endospore and the adherence between the two spore wall layers.
What surprises do other eusporangiate ferns in store? When you have a ripe frond at hand, and can spare a few hours, it might pay to watch.