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Gnetophyta consists of only three genera of woody plants grouped in the gymnosperms. The living gnetophytes are a handful of survivors of a group that was diverse and dominant in the Tertiary,〔 and fossilized pollen attributed to a close relative of one of them, ''Ephedra'', has been identified in the Early Cretaceous.〔("Morphology and affinities of an Early Cretaceous Ephedra" ).〕 They now comprise some 70 species across the three relict genera ''Gnetum'' (family Gnetaceae), ''Welwitschia'' (family Welwitschiaceae), and ''Ephedra'' (family Ephedraceae). The gnetophytes differ from other gymnosperms (i.e. conifers, cycads, and ginkgos) in having vessel elements (which transport water within the plant) like those found in flowering plants. In the past, the Gnetophyta were thought to be the gymnosperms' closest relations to the flowering plants; however, molecular studies have largely contradicted this hypothesis. Among themselves, the gnetophytes' evolutionary inter-relationships remain unclear, however: in some classifications, all three genera are placed in a single order (Gnetales), while in other classifications they are distributed among three orders, each containing a single family and genus. Most morphological and molecular studies confirm that ''Gnetum'' and ''Welwitschia'' diverged from each other more recently than from ''Ephedra''.〔 ==Ecology and morphology== The three genera of the gnetophytes are highly specialized to their respective environments, making it difficult to identify homologous characters. The three extant genera of gnetophytes, a "bizarre and enigmatic" trio, are likely aberrant members of the group. Some synapomorphies of the gnetophytes include enveloping bracts around the ovules and microsporangia, and a micropylar projection of the outer membrane of the ovule that produces a pollination droplet.〔Judd, W.S.; Campbell, C.S.; Kellogg, E.A.; Stevens, P.F.; and Donoghue, M.J. (2008) Plant Systematics: A Phylogenetics Approach. 3rd ed. Sunderland, Massachusetts, USA: Sinauer Associates, Inc.〕 ''Gnetum'' species are mostly woody climbers in tropical forests. However, the best-known member of this group, ''Gnetum gnemon'', is a tree. In western Malesia, where it is native, its seeds are used to produce a crispy cracker snack known as ''emping'' or ''krupuk belinjo''. ''Welwitschia'' comprises only one species, ''Welwitschia mirabilis''. It grows only in the extremely dry deserts of Namibia and Angola. This strange ground-hugging species has only two large strap-like leaves that grow continuously from the base throughout the plant's life. Plants of the genus ''Ephedra'' are known as "jointfirs" in the western United States because they have long slender branches which bear tiny scale-like leaves at their nodes. Infusions from ''Ephedra'' have been traditionally used as a stimulant, but ephedrine is a controlled substance today in many jurisdictions because of the risk of harmful or even fatal overdosing. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Gnetophyta」の詳細全文を読む スポンサード リンク
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