The New International Encyclopædia/Gnetaceæ
|←Gnesen||The New International Encyclopædia
|Edition of 1906. See also Gnetum, Ephedra (genus) and Welwitschia on Wikipedia, and the disclaimer.|
GNETACEÆ, nḗ-tā'shḗ-ē (Neo-Lat. nom. pl., from Gnetum, from Malay gnemon, gnemo, the native name). One of the great groups of gymnosperms, which comprises at present three genera, that differ remarkably in habit and habitat. The genera are Ephedra, with about 30 species, from the arid regions of both hemispheres; the very peculiar Tumboa (Welwitschia), from the extremely arid regions of Western South Africa; and Gnetum, with about 15 species, from the tropics of both hemispheres. The species of Ephedra are low, straggling shrubs, with long-jointed and fluted green stems, and opposite, scale-like leaves connate into a two-toothed sheath. The body of Tumboa has the shape of a gigantic radish, which rises little above the surface of the ground, and whose crown is sometimes 12 to 15 feet in circumference. From the edge of the crown two enormously long, parallel-veined leaves arise, which extend upon the ground sometimes for 10 to 15 feet, and become split into numerous ribbons. This single pair of opposite leaves, the only pair produced, grows continually at the base, and lasts through the lifetime of the plant, which is said to reach more than a hundred years. The species of Gnetum are either small trees or woody climbers, and are among the prominent lianas of tropical forests. The foliage is leathery in texture and suggests dicotyledons, as the well-developed opposite leaves are lanceolate to ovate in outline and pinnately net veined. See Plate of Gymnosperms.
The group is of special interest to the botanist on account of the display of certain angiospermous characters that have suggested that Gnetaceæ may have given rise to the angiosperms; but such a theory has been abandoned by most morphologists. The characters that distinguish the group from other gymnosperms are the occurrence of true vessels in the secondary wood, and the presence of a so-called perianth. In addition to these two distinguishing characters, the group has the following four characters in common, but not peculiar to it: (1) Opposite leaves; (2) dicotyledonous embryos; (3) cauline ovules; and (4) no resin-ducts. Some fossil forms of plants referable to the Gnetaceæ have been found; the majority from Tertiary deposits seem to belong to the modern genus Ephedra. The stems and female flowers of a species of Ephedra are found commonly in finely preserved condition in the Tertiary amber of the Baltic provinces. Stems and leaves of still earlier age, resembling those of Ephedra, and hence called Ephedrites, have been obtained in the Jurassic rocks of Eastern Siberia.