a subdivision of gymnospermous plants containing three orders: Ephedrales, Welwitschiales, and Gnetales. Each of these has one family and one genus, Ephedra, Welwitschia, and Gnetum, respectively. All three genera have common features: vessels in the secondary xylem and dichasial branching of strobile clusters (racemes), similar to the perianth cover around strobiles. They also have long microserrated tubes, dicotyledonous buds, and opposing leaves. There are no sap flows in the stems.
Many scientists recognize common origins for these three genera (probably from Bennettitales). Others believe Ephedra, Welwitschia, and Gnetum have different origins. Electronic-microscope research of the microspore layer of gneticae confirms their relationship to one another.
REFERENCETakhtadzhian, A. L. Vysshye rasteniia, vol. 1. Moscow-Leningrad, 1956.
A. L. TAKHTADZHIAN