Categories: Evolution; gymnosperms; Plantae; taxonomic groups
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.
Like most other gymnosperms, the gnetophytes bear their reproductive structures in strobili, or cones. The gnetophytes differ from other gymno-sperms 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.
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