The 70 known species of gnetophytes are distributed among three distinctive genera. They are unique among the gym-nosperms in having vessels in the xylem. More than half of the gnetophytes are species of joint firs in the genus Ephedra. These shrubby plants inhabit drier regions of southwestern North America. Their tiny leaves are produced in twos and threes at a node and turn brown soon after they appear. The stems and branches, which are often whorled, are slightly ribbed; they are photosynthetic when they are young (Fig. 22.13).
Before pollination, the ovules of Ephedra produce a small tubular extension resembling the neck of a miniature bottle extending into the air. Sticky fluid oozes out of this extension, which constitutes the micropyle, and airborne pollen catches in the fluid. Male and female strobili may be produced on the same plant or on different ones, depending on the species.
Most of the remaining species in this division are in the genus Gnetum (Fig. 22.14), which has not been given an English common name. Its members occur in the tropics of South America, Africa, and Southeast Asia. Most are vinelike, with broad leaves similar to those of flowering plants. The best-known species of Gnetum, however, is a tree that grows up to 10 meters (33 feet) tall.
The third genus, Welwitschia, has only one species, which is confined to the temperate Namib and Mossamedes deserts of southwestern Africa. Here the average annual rainfall is only 2.5 centimeters (1 inch),
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