Leptosporangium

2003, Jill Bailey, Filicinophyta (ferns), entry in The Facts on File Dictionary of Botany, page 84, In the Filicales the leptosporangium has a specialized spore dispersal mechanism involving an annulus of cells with thick walls that ejects spores explosively as it dries out.

The characteristic that unites the Filicales as a monophyletic clade is the presence of leptosporangia with an annulus-an area of specialized cells involved in dehiscence.[…]Leptosporangia typically produce only a few spores, and are most commonly homosporous.

Leptosporangia are unique among vascular plants in (1) developing from a single cell, and (2) having a single layer of cells making up the sporangium wall.