gnetophyta

Gnetophyta is a division of gymnosperms that comprises three living genera: Ephedra, Gnetum, and Welwitschia. Collectively, they form a distinctive and often relictual lineage found in deserts, tropical forests, and savannas around the world. Gnetophytes are notable for having vessel elements in their xylem, a feature shared with angiosperms and uncommon among other gymnosperms, a characteristic that has influenced discussions of their evolutionary relationships.

Ephedra includes mostly shrubs and small trees with reduced leaves and photosynthesis concentrated in green stems, adapted to arid habitats. Gnetum comprises tropical trees and lianas with broad leaves, primarily in tropical Africa, Asia, and the Americas. Welwitschia mirabilis is a single, largely dioecious species native to the Namib Desert, distinguished by two persistent strap-like leaves and a stout underground or basal shoulder.

In reproductive terms, gnetophytes produce seeds in structures associated with ovules; pollen is released from microsporophylls. The genera vary in habit and reproduction, with Ephedra often dioecious and Gnetum and Welwitschia showing more complex sexual systems. Their pollen and seed morphology, along with their vascular anatomy, have made their phylogenetic placement within gymnosperms controversial.

Systematics and evolution: The APG system recognizes Gnetophyta as a division containing Ephedraceae, Gnetaceae, and Welwitschiaceae. The exact relationship of gnetophytes to other gymnosperms remains debated; molecular studies typically illuminate Gnetophyta as a distinct early-diverging lineage, though its precise affinities with conifers or other gymnosperm groups vary among analyses.

Ecology and human use: Gnetophytes occupy diverse habitats from deserts to tropical forests. Ephedra species have been used in traditional medicine and contain alkaloids such as ephedrine. Some species are threatened by habitat loss and overharvesting, and Welwitschia is of particular conservation concern due to its restricted distribution and slow growth.