Eckenwalder, James E. Department of Botany, University of Toronto, Toronto, Ontario, Canada.
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The only order of the class Gnetopsida in the division Gnetophyta. There are three living families, each with a single genus: Ephedraceae (Ephedra; 65 species in arid regions), Gnetaceae (Gnetum; 29 species in the tropics), and Welwitschiaceae (Welwitschia; 1 species in Namibia). Among the gymnosperms, Gnetales are usually considered the closest living relatives of the angiosperms. This relationship is supported by such morphological features as wood vessels, netted venation, and double fertilization, but contemporary molecular evidence is ambiguous, sometimes favoring this relationship and sometimes implying a closer affinity with the conifers. The fossil record is sparse but increasing, perhaps extending back to the Triassic Period. Each modern genus is quite distinctive. Species of Ephedra (Mormon tea) somewhat resemble the spore-bearing horsetails (Equisetum), with scale leaves on photosynthetic, jointed stems. Most species of Gnetum are lianas, woody vines closely resembling the Australian flowering plant family Austrobaileyaceae. The only species of Welwitschia is unlike any otherliving plant. From its stumplike trunk, just two leaves grow ever wider and longer [up to 7 m (23 ft)], tattering at the tips. All three genera are dioecious, with separate male and female individuals. The pollen and seed cones are compound, with flowerlike buds in the axils of bracts. See also: Cycadopsida; Pinophyta; Plant kingdom
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