Gymnosperms are undoubtedly the group from which the angiosperms developed. Gymnosperms have seeds but not fruits or flowers. Gymnos means naked, sperm means seed-gymnosperm = naked seeds. Gymnosperms developed during the Paleozoic Era and became dominant during the early Mesozoic Era. There are 700 living species placed into four divisions-conifers, cycads, ginkgos, and gnetales.
The presence of companion cells in the bast of angiosperms is one of the constant anatomical features which distinguish gymnosperm with the angiosperm. Gnetum belongs to gymnosperm but it has certain feature such as presence of sieve tube (they are small, vertically elongated, parenchymatous cells which have special characteristics) which is exclusive feature of angiospermic plants.
The Gnetales are an odd group they have some angiosperm-like features but are not themselves angiosperms. Cladistic analyses support placement of the gnetales (or some portion of them) as out groups for the flowering plants. Three distinctive genera comprise this group. Welwitschia, Gnetum and Ephedra. Ephedra occurs in the western United States where it has the common name “Mormon tea”.
It is a natural source for the chemical ephedrine, although there is no evidence the Mormons in Utah (where the plant is extremely common) ever used it for tea. Welwitschia is limited to coastal deserts in South Africa, although fossil leaf, cuticle and pollen evidence indicates plants of this type were widespread during the Mesozoic Era. Welwitschia is noted for its two long, prominent leaves. Gnetum has leaves that look remarkably like those in angiosperms, as well as vessels in the xylem, generally considered an angiosperm characteristic.
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Gnetum resembles an angiospermous tree in that the woody stem is frequently and irregularly branched and bears broad leaves, which are fan-shaped with dichotomously branched veins. The leaves of Gnetum look much like those of dicotyledonous angiosperms.