Gnetopsida

Gnetopsida is a class of plants belonging to the seed plants (gymnosperms), whose seeds lack the enclosed ovary characteristic of flowering plants. It comprises three living genera: Gnetum, Ephedra, and Welwitschia. These three genera differ markedly in appearance and habitat, but share anatomical features that have attracted scientific attention.

The group has long served as an important example when studying plant evolution. This is because some of its features—such as water-conducting structures called vessels and a reproductive pattern resembling that of flowering plants—have raised questions about the relationship between Gnetopsida and angiosperms (flowering plants).

Genera

Ephedra is a small shrub with jointed green branches resembling rushes, found mostly in arid and mountainous regions. Gnetum comprises trees and vines living in tropical rainforests, with leaves resembling those of flowering plants. Welwitschia is represented by a single species found in the Namib desert region of southern Africa, characterized by two long-lived leaves that continue to grow.

Biological features

Notable features include specialized tissue in the xylem and a reproductive arrangement of male and female organs within cone-like structures that differ from the typical cones of conifers. Some species reproduce through fragmentation, which differs from the wind-pollination typical of most gymnosperms.

Classification and debate

Various classifications have been proposed: some treat the three genera as a single unit (Gnetophyta or Gnetopsida), while molecular studies have suggested the group may be within or close to the conifers. The idea that Gnetopsida are closely related to flowering plants, supported by earlier hypotheses, has become less favored as molecular data has accumulated.