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| Gnetophytes |
The gnetophytes are a small group of vascular seed plants composing the phylum Gnetophyta, which is one of four phyla of gymnosperms that have living representatives.
The Gnetophyta include only three genera Ephedra, Gnetum, and Welwitschia—each of which belongs to a separate family, in a single order, the Gnetales. The gnetophytes have a number of features in common with the flowering plants (phylum Anthophyta, the angiosperms), which has sparked scientific interest in the evolutionary relationships between the two groups; they are the only gymnosperms, for example, in which vessels occur.
There are about ninety species of gnetophytes. They are diverse in form and size, and their distribution varies widely, from moist, tropical environments to extremely dry deserts. Most gnetophytes are shrubs or woody vines. The leaves occur oppositely or in whorls of three.
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Like most other gymnosperms, the gnetophytes bear their reproductive structures in strobili, or cones. The gnetophytes differ from other gymnosperms in that both the seed-producing (ovulate or female) cones and the pollen-producing (male) cones are compound; that is, they are, in turn, composed of cones.
Both male and female cones contain oppositely arranged bracts, or modified leaves, which bear short, fertile shoots at the axil (the angle between the bract and the stem that bears it). Most gnetophytes are dioecious, meaning that they bear their pollen and ovulate cones on separate plants.
Angiosperm-like Features
The gnetophytes share with the angiosperms a number of structural and developmental characteristics. One of these is the presence of water conducting tubes, called vessels, in the secondary xylem, orwood.
Vessels, although present in angiosperms, do not occur in gymnosperms other than gnetophytes. Another similarity is that archegonia structures that protect the egg—which are typical of gymnosperms but absent from angiosperms, are not found in either Gnetumor Welwitschia (although they are present in Ephedra).
In addition, the cones of gnetophytes bear some resemblance to angiosperm flower clusters, and the leaves of Gnetum are similar in form, structure, and venation to those of the Eudicotyledones of the angiosperms.
Another feature common to angiosperms and gnetophytes—but not found in gymnosperms other than gnetophytes—is double fertilization. In double fertilization, there is union of each of two sperm nuclei with a nucleus in the female gametophyte (the gamete-producing generation in plants), rather than just a union of a single sperm and egg nucleus.
Further, in at least some species of all three gnetophyte genera, the reproductive structures produce nectar that attracts insects, as in many angiosperms. Insects play a role in the pollination of gnetophytes, in contrast to the typical gymnosperm’s reliance on the wind.
The similarities between gnetophytes and angiosperms have led scientists, who have long thought that the ancestor of the angiosperms is a gymnosperm, to look closely at the gnetophytes.
The living gnetophytes are considered too specialized to include the angiosperm ancestor. In addition, evidence suggests that such shared features as similar appearing vessels, similar-appearing reproductive structures, and the absence of archegonia were derived independently in the two groups.
Scientists have still not determined, however, whether the gnetophytes and angiosperms share a close, common ancestor. Unfortunately, the fossil record for gnetophytes is too sparse to shed much light on this question.
Although the gnetophytes were once more diverse than they are today, there is no indication that the group was ever rich in genera or abundant in individuals. The earliest known gnetophyte fossils date back 140 million years, to the Early Cretaceous period of the Mesozoic era, which is about as far back in time as the angiosperm fossil record goes.
Researchers have turned to cladistics (phylogenetic analyses) to study fossil, structural, and molecular evidence in an attempt to determine evolutionary relationships.
The results of these studies have been inconsistent, and additional research will be needed. There is strong evidence, however, that, within the gnetophytes, Ephedra is the closest to the common ancestor of the group, and that Gnetum and Welwitschia are derived sister clades.
Ephedra
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| Ephedra fruit |
The genus Ephedra includes about sixty species, most of them adapted to semiarid and desert conditions. Ephedra is the only genus of gnetophyte that occurs in the United States, with twelve species growing in the desert Southwest, some of them ranging into Mexico.
Another twelve species occur over a wide area in South America. The rest grow in the Eastern Hemisphere, from central Asia westward across southwest Asia and into Mediterranean Europe and North Africa.
Most species of Ephedra are scraggly, profusely branched shrubs. Some are vinelike, commonly climbing over other vegetation. The leaves of most species are small, dry, brown scales.
Their reduced size may be related to the plants’ need to minimize evaporative water loss in their dry environments. Photosynthesis is carriedon mostly in the branches, which remain green while young. The branches are jointed, giving rise to the genus’s common name, joint fir.
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Both the branches and the leaves are arranged two or three to a node. With its jointed stems and small leaves, Ephedra superficially resembles the horsetail, Equisetum. The stems of Ephedra form secondary xylem, or wood, as do the stems of conifers and many angiosperms. Ephedra wood is extremely hard.
The cones of Ephedra are borne in the leaf axils and are very tiny—the smallest in the gnetophyte group. The female cones have at their tips one or two ovules borne on very short stalks. The male cones are in spikelike clusters. Male and female cones are produced on the same plant or on different ones, depending on the species. The mature female cones are fleshy and berrylike and often brightly colored.
Stem extracts of Ephedra have a long history of use as folk medicines. Many Eurasian Ephedra species, especially Ephedra sinica, contain ephedrine, an alkaloid chemical that the Chinese have used for more than five thousand years as a decongestant drug, called Ma-huang, to treat asthma and hay fever.
Ephedrine alkaloids have not been found in New World species of Ephedra, but Western countries manufacture synthetic ephedrine, which is used in cough medicines. European settlers in the American Southwest and Mexico brewed a beverage known as Mormon tea or Mexican tea from stem fragments of Ephedra.
Gnetum
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| Gnetum gnemon |
The genus Gnetum includes approximately thirty species, which grow throughout the moist tropics. Most of these are woody vines that climb on trees in the rain forests of central Africa, Asia, and northern South America and on some Pacific islands. The best-known species, Gnetum gnemon, however, is a tree native to Indonesia that grows to 10 meters. It is cultivated for its edible seeds and tender young leaves.
Gnetum stems characteristically bear two broad, leathery leaves at each node and produce secondary xylem, or wood. In all Gnetum species, male and female reproductive structures are borne on separate plants. The cones, like those of Ephedra, look like berries, and the seeds may be brightly colored.
Welwitschia
The genus Welwitschia includes a single species, Welwitschia mirabilis. This low-growing, perennial plant is restricted to a 150-kilometer-wide strip of coastal desert in Angola, Namibia, and South Africa.
In this extremely arid environment, where there may be no precipitation for several years at a time, Welwitschia may survive, at least in part, by using dew and condensate from fog that rolls in off the ocean at night. Young plants seem to become established mainly during rare wet years. Some living Welwitschia plants have been dated at fifteen hundred years old.
Described by some as the strangest living plant, Welwitschia bears little resemblance to other gymnosperms. Most of the plant is taproot, buried in sand to a depth of 1 to 1.5meters. At its tip, the taproot divides into smaller roots that probably absorb water unavailable to less deeply rooted plants.
The exposed part of Welwitschia includes a massive, squat stem that emerges only a short distance above the ground. The stem widens with age, becoming up to a meter across, and may develop a crusty, barklike covering on its surface. This broad, woody, concave disk, having ceased elongating from the tip, produces only two leaves during the plant’s lifetime.
The wide, strap-shaped leaves continue to grow from their bases at a rate of 8 to 15 centimeters per year, for the life of the plant. Battered by wind and hot sand, the leaves break off at their tips and split lengthwise, giving older plants the appearance of having numerous leaves. With their worn tips, the leaves seldom exceed 2 meters in length, although they may reach 6 meters.
The only real branches that Welwitschia produces are the branch systems of the pollen and ovulate cones, which are borne on separate plants. These branch systems develop in the axils of the leaves, although they appear to grow from around the rim of the stem cup.
The pollen cones, which are red, are produced in groups of two or three at the end of each branch. Ovulate cones are also red. Droplets of nectar lure flies to pollinate the plant. The seeds, generally only one to a cone, have winglike extensions that may aid dispersal by the wind.