Leptosporangiate sporangia develop from
Ontogenetic difference fundamental to fern systematics distinguishes sporangial types by initial cell number. Leptosporangium initiates from single epidermal cell that divides periclinally to form stalk cell and capsule mother cell, later differentiating wall, tapetum from inner wall layer, and sporogenous tissue producing thin-walled capsule with few spores. Eusporangium arises from group of initial cells including both archesporial and parietal layers from several cells resulting in massive structure. Single-cell origin explains small size and presence of annulus-mediated catapult mechanism
Ref: Bower 1881 Leptosporangiate development; Foster & Gifford Comparative Morphology