A Sporangium Revealed.

This series of photos shows the sporangium revealed. After removing the fibrous calyptra you can see the elf cap shaped operculum. Upon removal of the operculum the really interesting structure, to me, is revealed. The periostome teeth surround the opening of the sporangium and the opening is sealed by a membranous epiphragm. When the humidity decreases the epiphragm ruptures and the peristome teeth regulate the dispersal of the spores from the sporangium. When the humidity increases the teeth close the peristome keeping the spores safe inside the sporangium, when the humidity decreases the teeth bend outward allowing the spores to be dispersed by the dry breezes. What allows the teeth to act this way is that the inside and the outside surfaces of the teeth are structurally different and therefore respond differently to humidity.
There is a good micro photograph of the cross section of a similar sporangium on this webpage.
http://www3.botany.ubc.ca/bryophyte/poly1.htm