Subdivision of the plant kingdom, comprising the classes Lycopsida (club mosses), Sphenopsida (horsetails), Psilopsida (but see psilophytales), and Pteropsida (the various families of ferns). They first enter the fossil record in the Silurian. They are flowerless plants exhibiting an alternation of two 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 (sporophylls) grouped into cones (as in horsetails and club mosses), or in the axils of leaves on unspecialized stems (as in some club mosses). 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.
Subjects: Earth Sciences and Geography.