The gnetophytes have been treated as separate from other seed plants at the ranks of division (Gnetophyta Bessey 1907), subdivision (Gnetophytina Cronquist, Takht. & Zimmerm. ex Reveal 1996), class (Gnetopsida Thom 1886), subclass (Gnetidae Cronquist, Takht. & Zimmerm. ex Reveal 1996) and order (Gnetales Luerss. 1879). Among these, the rank of subclass is perhaps most appropriate in view of the extreme differences between the gnetophytes and the other seed plants, with each of the three subgroups treated as orders; viz.:
The group is a small one, consisting of three families, each with one genus, totalling 68 species in this treatment.
For long, Gnetophytes were thought to represent a sort of primitive angiosperm. Superficially they resemble angiosperms more than gymnosperms, and some authors even suggested that they represented a derived group of angiosperms. A molecular biology study of Gnetum gnemon (Winter et al. 1999), however, found that conifers and gnetophytes are sister groups, and suggested that the evolution of flower-like reproductive structures may have occurred independently in gnetophytes and angiosperms. An independent study of chloroplast DNA from several angiosperm, gymnosperm and gnetophyte taxa (Samigullin et al. 1999) reached a similar conclusion, and subsequent studies have confirmed this interpretation while also showing that the Gnetum+Welwitschia clade is sister to Ephedra (Stevens 2010 and references therein). There remains debate, however, concerning the precise relationship between the gnetophytes and the other principal groups of gymnosperms. Current thinking, summarized by Stevens (2010), tends to indicate that the cycads first diverged from the protogymnosperm line, followed thereafter by Ginkgo, conifers and finally gnetophytes. However, this thinking is based almost entirely upon molecular lines of evidence, and could easily change in response to further study.
I have yet to prepare a proper description of the gnetophytes. See Stevens (2010) for a highly technical description.
The three Gnetophyte genera have largely disjunct distributions. Ephedra occurs mostly in semiarid and arid parts of North America, Mexico, South America (Ecuador to Patagonia and lowland Argentina), S Europe, Asia, and N Africa (including the Canary Islands) (Stevenson 1993). The other genera are exclusively tropical: Welwitschia in Africa: Angola and Namibia, and Gnetum in wet forests of Indo-Malaysia, tropical parts of West Africa, Fiji and the northern regions of South America (Schultes & Raffauf 1990).
Probably Gnetum gnemon.
Probably Welwitschia mirabilis.
Not known to have been done. Most species of Gnetum are vines and thus are probably unsuitable, although the small tree Gnetum gnemon may have potential. Species of Ephedra have rings that are presumably annual and there may be potential to recover useful information, such as oxygen isotope variation, from the rings of this species. Welwitschia does not form rings.
Almost all gymnosperms form a vesicular-arbuscular mycorrhiza, but species in the Gnetales and the Pinaceae form ectomycorrhizas (Brundrett 2008).
Brundrett, Mark. 2008. Mycorrhizal Associations: The Web Resource. mycorrhizas.info, accessed 2009.06.09.
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Martius, C.F.P. von. 1835. Conspectus Regni Vegetabilis: Secundum Characteres Morphologicos Praesertim Carpicos in Classes Ordines et Famillas Digesti, p. 11.
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Samigullin, T.K.; Martin, W.F.; Troitsky, A.V.; Antonov, A.S. 1999. Molecular data from the chloroplast rpoC1 gene suggest a deep and distinct dichotomy of contemporary spermatophytes into two monophyla: gymnosperms (including Gnetales) and angiosperms. Journal of Molecular Evolution 49(3):310-315.
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