Sphagnum mosses (peat mosses) have a capsule with a really interesting manner of spore release. They open with an explosive *pop* that can be heard when this moss is close by. The mechanism by which this occurs has been described as internal air pressure building up as the capsules dry, shooting the lid off the top. This is the only type of moss that has an explosive opening, thus it has attracted some interest.
In lab group we recently read a paper that explored this mechanism to try to figure out the processes involved in their explosive opening.
Duckett, JG, S Pressel, KMY P'ng, KS Renzaglia. (2009) Exploding a myth: the capsule dehiscence mechanism and the function of pseudostomata in Sphagnum. 183:1053-1063.
I don't have any photos that illustrate the cool little capsules, but you can link here to the image gallery at the British Bryological Society to check them out (listed under Sphagnum fimbriatum).
In the above research study, they carried out a seemingly obvious, but previously unexamined experiment. They poked holes in the capsules, dried them out and then observed how many of the capsules opened explosively. With holes in the capsule air pressure should not be able to build up inside and you would predict that the lid would not pop off.
Despite poking holes in the capsules the lids still popped off. So the explosive nature is probably not due to a buildup of pressure. The researchers propose that the mechanism to explode off the lid is due to shrinkage of the capsule walls. The side walls of the capsule buckle in and the rigid lid (operculum) flies off.
I think that it is super cool when simple experiments can be used to test seemingly established biological ideas. It is even more fun when the results turn the established idea upside down. Boy, science is cool!