Gnetidae, commonly known as gnetophytes, is a subclass of gymnosperms within the class Pinopsida. The group was previously considered the distinct class Gnetopsida within the distinct division Gnetophyta. However, nuclear phylogenomic studies recover gnetophytes as the sister group of the Pinaceae, which means that gnetophytes are nested within Pinopsida; separating them renders Pinopsida paraphyletic. Accordingly, Yang et al. (2022) treat Gnetidae as one of three subclasses of Pinopsida, together with Pinidae and Cupressidae. The group comprises three morphologically distinct extant genera, each the sole living genus of its family, Ephedra (Ephedraceae), Gnetum (Gnetaceae), and Welwitschia (Welwitschiaceae). It has also been treated as a broadly circumscribed order, Gnetales, though Yang et al. (2022) and WFO Plant List divide Gnetidae into three orders (Ephedrales, Gnetales sensu stricto, and Welwitschiales) each containing a single extant family and genus. Within the subclass, Ephedra is sister to a clade formed by Gnetum and Welwitschia. The earliest unambiguous fossils attributable to gnetophytes date to the Late Jurassic, and the group reached its greatest known diversity during the Early Cretaceous. Gnetophytes are distinguished from other living gymnosperms by the presence of vessel elements in the secondary xylem. This and other angiosperm-like features once supported hypotheses of a close relationship with flowering plants, but recent phylogenomic analyses indicate that these similarities evolved independently.
Ecology and morphology Unlike most biological groupings, it is difficult to find many common characteristics among all of the members of the gnetophytes. The two common characteristics most commonly used are the presence of enveloping bracts around both the ovules and microsporangia as well as a micropylar projection of the outer membrane of the ovule that produces a pollination droplet, though these are highly specific compared to the similarities between most other plant divisions. L. M. Bowe refers to the gnetophyte genera as a "bizarre and enigmatic" trio because the gnetophytes' specialization to their respective environments is so complete that they hardly resemble each other at all. Gnetum species are mostly woody vines in tropical forests, though the best-known member of this group, Gnetum gnemon, is a tree native to western Malesia. The one remaining species of Welwitschia, Welwitschia mirabilis, native only to the dry deserts of Namibia and Angola, is a ground-hugging species with only two large strap-like leaves that grow continuously from the base throughout the plant's life. Ephedra species, with a wide distribution in dry climate zones, have long slender branches which bear tiny scale-like leaves at their nodes.
Classification With just three well-defined genera within an entire division, there has been difficulty in establishing an unambiguous interrelationship among them; in earlier times matters were even more difficult, with Pearson in the early 20th century discussing the class Gnetales, rather than the order. G. H. M. Lawrence referred to them as an order, but remarked that the three families were distinct enough to deserve recognition as separate orders. Foster & Gifford accepted this principle, and placed the three orders together in a common class for convenience, which they called Gnetopsida. In general the evolutionary relationships among the seed plants still are unresolved, and the Gnetophyta have played an important role in the formation of phylogenetic hypotheses. 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. Several hypotheses, illustrated below, have been presented to explain seed plant evolution. Some morphological studies have supported a close relationship between Gnetophyta, Bennettitales and the Erdtmanithecales. Other authors have suggested that the Gnetophyta are a sister group to the rest of the gymnosperms, contradicting the anthophyte hypothesis, which held that gnetophytes were sister to the flowering plants.
Gnetifer hypothesis In the gnetifer hypothesis, the gnetophytes are sister to the conifers, and the gymnosperms are a monophyletic group, sister to the angiosperms. The gnetifer hypothesis first emerged formally in the mid-twentieth century, when vessel elements in the gnetophytes were interpreted as being derived from tracheids with circular bordered pits, as in conifers. It however only gained strong support with the emergence of molecular data in the late 1990s. Although the most salient morphological evidence still largely supports the anthophyte hypothesis, some more obscure morphological commonalities between the gnetophytes and conifers lend support to the gnetifer hypothesis. These shared traits include tracheids with scalariform pits with tori interspersed with annular thickenings, absence of scalariform pitting in primary xylem, scale-like and strap-shaped leaves of Ephedra and Welwitschia; and reduced sporophylls.
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