Pteridophyta

Pteridophyta (pteridophytes) A division of the plant kingdom, comprising the vascular cryptogams. They are flowerless plants exhibiting an alternation of 2 distinct and dissimilar generations. The first is a non-sexual, spore-bearing, sporophyte generation. It usually appears as a relatively large plant, with stems containing vascular tissue that conducts water and dissolved solutes through the plant, and usually bears the leaves and roots. Spores are produced in sporangia that are either attached to the leaves (as in ferns) or are on specialized scales grouped into cones (as in horsetails and club-mosses), or in the axils of leaves on unspecialized stems (as in Psilotaceae and some clubmosses). The second is a sexual, gametophyte generation, in which the plants generally are relatively small, and without differentiation of stem, leaves, or roots. These plants bear male (antheridia) and female (archegonia) sex organs, together or on separate plants. When the eggs in the archegonia are fertilized by sperms from the antheridia, an embryo results: this can grow into a new sporophyte generation. The cells of the sexual, gametophyte generation each contain a single chromosome set in their nuclei (the haploid condition). Those of the sporophyte generation each contain a double chromosome set (the diploid condition), reduced to a single set in the spores. The Pteridophyta, in the wide sense as it is usually understood, includes the classes Lycopsida (families Lycopodiaceae, Selaginellaceae, and Isoetaceae), Sphenopsida (or Arthropsida; family Equisetaceae), Psilopsida (family Psilotaceae), and Filicopsida (the various families of ferns). They first enter the fossil record in the Silurian.