Leptosporangiate ferns are the largest group of living ferns. They are often considered to be the class Filicopsida, Pteridopsida or Polypodiopsida, although other classifications assign them a different rank. The leptosporangiate ferns are one of the four major groups of ferns, with the others being Marattiopsida, Equisetopsida (horsetails), and Psilotopsida (whisk ferns and ophioglossoid ferns).
There are approximately 9000 species of living leptosporangiate ferns, compared with about 260 for all other ferns put together. Almost a third of leptosporangiate fern species are epiphytes.
These ferns are called leptosporangiate because their sporangia arose from a single epidermal cell and not from a group of cells as in eusporangiate ferns. The sporangia are typically covered with a scale called the indusium, which can cover the whole sorus, but can also be strongly reduced. Many leptosporangiate ferns have an annulus around the sporangium, which ejects the spores.
http://en.wikipedia.org/wiki/Polypodiopsida
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