Vestal, Paul A. A. G. Bush Science Center; Department of Biology, Rollins College, Winter Park, Florida.
- Alternation of generations
- Related Primary Literature
- Additional Reading
The largest order of modern ferns, commonly called the true ferns; also known as Filicales. The order Polypodiales contains approximately 250 genera and 9500 species. Although members of the Polypodiales are well represented in the temperate regions, they reach their greatest development in the moist tropics. They vary in habit (Fig. 1 and Fig. 2) from small filmy structures to large treelike plants. Many are epiphytic (living perched on other plants) and a number are climbing species. A few are aquatic. Perhaps the most striking species are the tropical tree ferns with their upright, unbranched stems and terminal clusters of large graceful leaves. Members of the Polypodiales differ from members of the other fern orders in being leptosporangiate—that is, their sporangium, or spore sac, arises from a single surface cell—and in having small sporangia with a definite number of spores. The wall of the sporangium is almost encircled with a ring of cells having unevenly thickened walls. This ring is called the annulus. When the sporangium is mature, the annulus, acting as a spring, causes the sporangium wall to rupture, thus discharging the spores.
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