A band or circle of tissue, especially: 1. The ring of differentially thickened cells that encircles the sporangium of certain ferns. It aids spore dispersal by inducing tension in the spo-rangial wall as the water in its cells evaporates. This causes the rupture of the *stomium
and the top of the sporangium gradually curls back. As the tension increases a certain stage is reached when the water in the cells vaporizes, so releasing the tension and causing the top of the sporangium to spring back. Such movements serve to release the spores.
2. (velum) The remnants of the ruptured partial veil that encircle the stalk of a mushroom or toadstool.