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Gnetopsida

(Class)

Overview

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The division Gnetophyta or gnetophytes comprise three related families of woody plants grouped in the gymnosperms. The gnetophytes differ from other gymnosperms in having vessel elements as in the flowering plants.

The living Gnetophyta comprise three genera:

Gnetum species are mostly woody climbers in tropical forests. However, the most well-known member of this group, Gnetum gnemon, is a tree. Its seeds are used to produce a crispy krupuk snack known as emping or krupuk belinjo.

Welwitschia comprises only one species, Welwitschia mirabilis. It grows only in the deserts of Namibia and Angola. The plant is strange in having only two large strap-like leaves for all its life. These grow continuously from the base, and are usually tattered at the ends by flapping in the winds.

Plants of the genus Ephedra are known as jointfirs because they have long slender branches which bear tiny scale-like leaves at their nodes. Ephedra has been traditionally used as a stimulant, but is a controlled substance today in many jurisdictions because of the risk of harmful or even fatal overdosing.

In some classifications all three genera are placed in a single order (Gnetales) but in others distributed among three orders, each containing a single family and genus.

Gnetum and Welwitschia diverged from each other more recently than from Ephedra.1]

Knowledge of fossils of the gnetophytes has increased greatly since the 1980s.[1] There are fossils from the Permian,[2] the Triassic, and the Jurassic which may belong to the gnetophytes, but this is uncertain.[3] The fossil record is richer starting in the early Cretaceous, with fossils of plants as well as seeds and pollen which can be clearly assigned to the gnetophytes.[3]

Molecular phylogenies of extant gymnosperms have conflicted with morphological characters with regard to whether the gymnosperms as a whole (including gnetophytes) comprise a monophyletic group or a paraphyletic one that gave rise to angiosperms. At issue is whether the Gnetophyta are the sister group of angiosperms, or whether they are sister to, or nested within, other extant gymnosperms. Numerous fossil gymnosperm clades once existed that are morphologically at least as distinctive as the four living gymnosperm groups, such as Bennettitales, Caytonia and the glossopterids. When these gymnosperm fossils are considered the question of gnetophyte relationships to other seed plants becomes even more complicated.

Photos

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Taxonomy

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The Class Gnetopsida is further organized into finer groupings including:

Orders

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Gnetales

Gnetum is a genus of about 30-35 species of , the sole genus in the family Gnetaceae and order Gnetales. They are tropical evergreen trees, shrubs and lianas. Unlike other gymnosperms they possess vessel elements in the xylem. [more]

At least 232 species and subspecies belong to the Order Gnetales.

More info about the Order Gnetales may be found here.

References

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  1. ^ a b Peter R. Crane, Patrick Herendeen and Else Marie Friis (2004). "Fossils and plant phylogeny". American Journal of Botany 91: 16831699. doi:10.3732/ajb.91.10.1683. http://www.amjbot.org/cgi/content/full/91/10/1683
  2. ^ Zi-Qiang Wang (2004). "A New Permian Gnetalean Cone as Fossil Evidence for Supporting Current Molecular Phylogeny". Annals of Botany 94 (2): 281288. doi:10.1093/aob/mch138. PMID 15229124. http://aob.oxfordjournals.org/cgi/c ontent/full/94/2/281
  3. ^ a b Catarina Rydin, Kaj Raunsgaard Pedersen, Peter R. Crane and Else Marie Friis (2006). "Former Diversity of Ephedra (Gnetales): Evidence from Early Cretaceous Seeds from Portugal and North America". Annals of Botany 98 (1): 123140. doi:10.1093/aob/mcl078. PMID 16675607. http://aob.oxfordjournals.org/cgi/content/full/98/1/123

Sources

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Last Revised: April 26, 2010
2010/04/26 07:06:09