leptosporangium

leptosporangium puhekieltä A sporangium formed from a single epidermal cell.
2003, Jill Bailey, Filicinophyta (ferns), entry in The Facts on File Dictionary of Botany, http://books.google.com.au/books?id=QfQ6fQU46EMC&pg=PA84&dq=%22leptosporangium%22%7C%22leptosporangia%22&hl=en&sa=X&ei=Y-D1Usm6D4ickQWQrIDwCA&ved=0CMsBEOgBMB0v=onepage&q=%22leptosporangium%22%7C%22leptosporangia%22&f=false 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.