Гнетофиттер: үш туыстың ерекше тобы

Division of plants containing three genera of gymnosperms

Gnetophyta (/n//ɛ/'/t//ɒ//f//ᵻ//t//ə//, /'/n//ɛ//t//oʊ//f//aɪ//t//ə/) is a division of plants (alternatively considered the subclass Gnetidae or order Gnetales), grouped within the gymnosperms (which also includes conifers, cycads, and ginkgos), that consists of some 70 species across the three relict genera: Gnetum (family Gnetaceae), Welwitschia (family Welwitschiaceae), and Ephedra (family Ephedraceae). The earliest unambiguous records of the group date to the Jurassic, and they achieved their highest diversity during the Early Cretaceous. The primary difference between gnetophytes and other gymnosperms is the presence of vessel elements, a system of small tubes (xylem) that transport water within the plant, similar to those found in flowering plants. Because of this, gnetophytes were once thought to be the closest gymnosperm relatives to flowering plants, but more recent molecular studies have brought this hypothesis into question, with many recent phylogenies finding them to be nested within the conifers. Though it is clear they are all related, the exact evolutionary inter relationships between gnetophytes are unclear. Some classifications hold that all three genera should be placed in a single order (Gnetales), while other classifications say they should be distributed among three separate orders, each containing a single family and genus. Most morphological and molecular studies confirm that the genera Gnetum and Welwitschia diverged from each other more recently than they did from Ephedra.

Unlike most biological groupings, it is difficult to find many common characteristics between 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, known as "jointfirs" in the United States, have long slender branches which bear tiny scale like leaves at their nodes. Infusions from these plants have been traditionally used as a stimulant, but ephedrine is a controlled substance today in many places because of the risk of harmful or even fatal overdosing.

With just three well defined genera within an entire division, there still is understandable difficulty in establishing an unambiguous interrelationship among them; in earlier times matters were even more difficult, with Pearson in the early 20th century discussing about 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. Recent research by Lee, Cibrian Jaramillo, et al. (2011) suggests 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.

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. 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.