When the coal swamps dried up, the pterido-sperms and the cordaites were replaced by the cy-cads, cycadeoids, and conifers. The cycads and cy-cadeoids evolved from the medullosan seed ferns. The cycads and cycadeoids were among the dominant plants during the age of the dinosaurs. The conifers are related to the cordaites.
Some cycadeoids had slender, branching trunks, while others were short and stumpy. Both types had compound leaves. Cycadeoid cones contained both male and female reproductive structures. Earlier researchers thought that the cones of the beehivelike cycadeoids resembled primitive angio-sperm flowers, but detailed reinvestigation of the cones showed that this was not true. The cycadeoids became extinct about sixty-five million years ago.
The cycads were more abundant in the past than they are now. Eleven genera and 160 species exist worldwide. They are dispersed in the modern tropics—in Africa, Cuba, Mexico, Australia, India, China, and Japan. Zamia floridana (coontie) is the only cycad native to the United States. Some cycads
Welwitschia: The Strangest Gymnosperm
There are many unusual plants in Africa, but one of the most unusual is Welwitschia mirabilis, a resident of the Namib Desert. It has a short, swollen stem only about 4 inches (10 centimeters) high, which terminates in a disc-like structure. Coming off the top of the stem are two straplike leaves. These two leaves last for the lifetime of the plant and continue to grow very slowly. As they grow, they twist and become shredded, so that an individual plant appears to have many leaves. The reproductive structures rise from the center of the stem, and, instead of flowers, Welwitschia has small cones.
Welwitschia is such a successful survivor of the Namib that it easily lives for hundreds of years. Some specimens have been dated to about two thousand years old. Older plants can reach tremendous sizes, with the top of the stem sometimes reaching 5 feet (1.5 meters) in diameter. Specimens of Welwitschia are extremely difficult to grow in cultivation, requiring special desert conditions and room for the deep taproot.
are small, unbranched trees that grow to 18 meters (59 feet) tall and resemble palm trees. Others have subterranean stems, and only their leaves and cones show above the ground. All the cycads possess stiff, leathery, compound leaves, often with very sharp tips on each leaflet. The male and female reproductive structures are borne at the end of the stem in separate cones on different plants. Cycad cones are the largest cones that have ever been produced. Cycad ovules are also very large. Some reach lengths of 6 centimeters. Their ripe seeds are often brightly colored.
The dominant group of living gymnosperms is the conifers. About 550 species are divided among
51 genera. Conifers are most abundant in temperate areas, such as the northern parts of North America, Europe, and Asia, and New Zealand and southern Australia. The conifers are divided into seven families— the Araucariaceae (examples are the kauri pine and Norfolk Island pine), Podocarpaceae (typified by the yellow woods), Pinaceae (pine, spruce, hemlock, and fir), Cupressaceae (juniper and arborvitae), Taxaceae (yew), Cephalotaxaceae (cultivated in the United States as plum-yew or cow's-tail pine, it also has at least one Asian genus), and Taxodi-aceae (bald cypress, redwoods). Some researchers separate the Taxaceae from the conifers on the basis of their arillate ovule (an ovule that forms an additional seed coat to the normal one). Several extinct conifer families are also known.
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