A very distinct order of gym-nosperms containing the three genera Ephedra (joint firs) with about 40 species, Gnetum with about 30 species, and Welwitschia, containing the one species W. mirabilis. Certain species of Ephedra are important as sources of the alkaloid drug ephedrine. Each genus shows certain advanced characteristics reminiscent of angiosperms. For example, their tracheids are arranged in columns and have highly perforated end walls similar to angiosperm vessels, while in Gnetum the sieve cells are closely associated with parenchymatous cells, reminiscent of companion cells. The leaves of Gnetum have a broad oval lamina with reticulate venation, very similar to certain angiosperm leaves. The Gnetales also have compound male and female strobili with superficially sepal- or petal-like structures, while the female gametophytes of Gnetum and Welwitschia lack archegonia. In some species of Ephedra there is a form of double fertilization, whereby one sperm nucleus fuses with the egg cell and the other with the ventral canal cell. However only the zygote undergoes further development. These angiosperm-like characteristics led some to consider the Gnetales as ancestral to the angiosperms but it is now generally considered that the group simply represents a specialized offshoot of the gymnosperms. The three genera of the Gnetales are considered so different from each other that in certain classifications they are raised to the status of orders (Ephedrales, Welwitschiales, and Gnetales) within the subdivision Gneticae (see Pinophyta). |
