leptosporangiate

Leptosporangiate is a term used for a large clade of ferns whose sporangia, or spore cases, develop as leptosporangia. Leptosporangia arise from a single epidermal cell and typically form a small, stalked capsule with an annulus, a ring of specialized cells that dehisces to eject spores. This mode of sporangium formation distinguishes leptosporangiate ferns from eusporangiate ferns, whose sporangia develop from multiple initial cells and lack a dedicated annulus.

The group includes the vast majority of living ferns and encompasses a variety of familiar lineages, such as filmy ferns, typical woodland ferns, and many large tree ferns. Most leptosporangiate ferns are homosporous, producing a single type of spore, though the aquatic order Salviniales is notable for heterospory, bearing distinct megaspores and microspores.

Life cycle proceeds with spore germination into a free-living, photosynthetic gametophyte called a prothallus, which requires water for the motile sperm to reach the egg. Fertilization produces a diploid sporophyte, the familiar fern plant, which grows and eventually bears the sporangia that release spores.

In evolutionary terms, leptosporangiate ferns likely originated in the late Devonian to early Carboniferous and diversified substantially during the Mesozoic and Cenozoic eras. Today they occupy a wide range of habitats worldwide, from tropical forests to temperate woodlands, and represent the dominant form of fern diversity.