This causes the annulus to peel back and the sporangium to open. As more water is lost, the tensile strength of water is overcome and air comes out of solution forming a bubble within each cell of the annulus. This results in a violent snapping back of the annulus, dislodging spores. Fern spores are about 50 m in diameter. Dehiscence flicks them only a couple of cm from the leaf surface but this is enough.
Air currents do the rest.
Fern spores travel far and wide and are very resistant to desiccation and uv light. This comprises photosynthetic cells full of chloroplasts and a few colourless, thread-like rhizoids, as seen at right. Initially, this is dependant on the gametophyte but it soon begins to photosynthesize and soon the gametophyte withers and dies. A key point to note about the ferns is that the sporophyte is limited by the gametophyte. This gametophyte is really much like that of a leafy liverwort.