Pteridophytes are the division of plants that include the ferns and so-called fern allies. This is an extremely diverse group of approximately 12,000 species of plants, so divergent that in some classifications, they have been placed in four divisions (e.g., Cronquist et al. 1966). However, three common features unite the group:
1) They are not flowering plants, but instead produce and are dispersed by spores, rather than seeds;
2) They feature a complicated life cycle that includes an alternative of generations, with germination of spores into a gametophyte generation, which is haploid (containing half the normal chromosome number, n) and usually short-lived and inconspicuous and cannot themselves produce spores, but are essential to the reproductive cycle and that exists in a separate stage from the spore-producing plants, sporophytes, which are usually perennial and conspicuous, and have roots, stems (often rhizomatous), and leaves, and are diploid, with 2n chromosomes.
3) They require free (standing) water in order to reproduce, because their flagellate sperm swim to fertilize the eggs; for this reason, many of the species live in moist habitats.
In addition to sexual reproduction through the alternation of generations, many pteridophytes reproduce extensively through vegetative (clonal) propagation, typically from rhizomatous stems, but also from leaves and roots. Because of this, sterile hybrid forms that arise may persist and become common in local regions.
In all but a couple of genera, modern pteridophytes lack secondary growth, including cambium tissue (which produces cork cells and bark on trees). Their characteristics remain similar those found in many of the earliest land plants. However, in contrast to mosses (Bryophyta), they are vascular plants, containing vessels (xylem and phloem) to transport water and nutrients through the stem tissues.
Although no single fern species is of widespread economic importance, over 700 species from 124 genera are grown as ornamentals, either indoors or outdoors for landscaping, and some species are increasingly used in North Amerian gardens where browsing by white-tailed deer (Odocoileus virginiana) is a problem. (Ferns in general are less likely to be browsed by deer than grasses and flowering species, but cultivars of fern species including Athyrium, Dryopteris, and Osmunda are particularly promoted as deer resistant.) Ferns are also sometimes used as a food plant--the emerging stems of some species are gathered in the wild and eaten as a vegetable (fiddlehead ferns, actually the unfurling leaves of various fern species, including Pteridium aquilinum (bracken fern), Matteuccia struthiopteris (ostrich fern), Osmunda cinnamomea (cinnamon fern or buckhorn fern), Osmunda regalis (royal fern), and Athyrium esculentum (vegetable fern), although some of these species are reported to contain potential carcinogens. Many fern species also have traditional medicinal uses.
(Cronquist et al. 1966, Hoshizaki and Moran 2001, Moran 2004, Wagner and Smith 1993, Wikipedia 2012.)
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