Annulus (botany)

As more water evaporates, air bubbles form in the cells causing the contracted annulus to snap forward again, thus dislodging and launching the spores away from the plant.

The type and position of the annulus is variable (e.g. patch, apical, oblique, or vertical) and can be used to distinguish major groups of leptosporangiate ferns.

This is caused by the gradual build up of a high negative pressure (200-300 atmospheres) within annulus cells from water loss.

After reaching a critical threshold cavitation occurs, releasing the tension and causing rapid cell expansion in the annulus, where the potential energy stored in the cells are converted extremely rapidly into kinetic energy, flinging the spores and closing the sporangium.

[2] In mosses, an annulus is a complete ring of cells around the tip of the sporangium, which dissolve to allow the cap to fall off and the spores to be released.