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Phylum Gnetophyta

The genus Ephedra includes about sixty species, most of them adapted to semiarid and desert conditions. 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.

EPHEDRA

by: Arjun Rustria

8-Uranus

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.

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.

The Gnetophyta include only three genera:

  • Ephedra
  • Gnetum
  • Welwitschia

Gnetophyta

Welwitschia

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.

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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.

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. 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.

Gnetum

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