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Copyright  2001 McGraw-Hill
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Student Center Principles of Botany
First Edition
Gordon Uno, Richard Storey, Randy Moore
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Chapter 18: Gymnosperms and Angiosperms: The Seed Plants

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Seed plants possess several characteristics that have allowed them to dominate the land. These include seeds, which protect the embryo until environmental conditions are favorable for germination and growth, and pollen grains, which transport sperm cells from plant to plant without the need for water. In addition, seed plants have roots, which absorb water, and cuticles, which help maintain the water inside their bodies.

Pollen arose more than 150 million years after the origin of seeds. Seed plants diverged and flourished throughout the Carboniferous period as seed ferns; diversification of ancestral pines and cycads followed. The Jurassic period was dominated by cycads, cycadeoids, and primitive pines. This period is also called the Age of Cycads.

Most of the features used for classifying living gymnosperms involve reproductive structures. Most gymnosperms bear seeds produced in cones. Pollination requires that pollen be transferred from microstrobili to ovules. Ovules are generally exposed (i.e., naked) or temporarily enclosed by sporophylls or other branches from the main axis of the strobilus.

The Pinophyta, Cycadophyta, Ginkgophyta, and Gnetophyta are four divisions of gymnosperms with living representatives. Pines are abundant in northern coniferous forests and are planted throughout the Southern Hemisphere. Bristlecone pines are long-lived. Other conifers dominate similar habitats in the Southern Hemisphere. Pine leaves form on short shoots called fascicles, which usually consist of two to five needles with deciduous, nonphotosynthetic, scalelike leaves at the base. The fascicles themselves are intermittently deciduous, usually lasting for less than 5 years.

Cycads are slow-growing gymnosperms of warmer climates. Their reproduction is similar to that of the pines, except that their sperm are motile by means of many flagella and their megastrobili can be massive. All cycads are dioecious.

There is only one living species of Ginkgo, a tree with fan-shaped, dichotomously veined leaves. Ginkgo is dioecious and produces seeds that stink.

There are three distinct genera of gnetophytes. Ephedra species are monoecious or dioecious and native to northern drier areas. These plants are mostly shrubby and otherwise resemble horsetails in having whorled branches and small, essentially functionless leaves. Their cones consist of paired bracts, some of which contain microsporangia. Their life cycle is similar to that of conifers. Gnetum species are tropical vines or trees with broad leaves; their reproduction resembles that of Ephedra. Welwitschia is a bizarre, dioecious plant of southwest African deserts. It produces two large, strap-shaped leaves with basal meristems. The leaves arise from a concave, bark-encrusted, trunkless stem from which a taproot extends into the ground.

Gymnosperms, especially pines and their relatives, are an important source of lumber, wood pulp, and resin. Resin is used to make turpentine and rosin. Wood pulp is used as raw material for making paper.

Angiosperms, which are the dominant plants on earth, have a flower that includes seeds in a carpel. The main force behind the rapid evolutionary radiation of angiosperms may have been pollination by insects and the availability of habitats left open by the disappearance of many gymnosperms. The first flowers were probably pollinated by beetles; later angiosperms attracted butterflies and bees.

Another hypothesis that explains the rapid evolution of angiosperms involves the influence of dinosaurs. Large, high-browsing dinosaurs mysteriously gave way to smaller, low-browsing dinosaurs by the end of the Mesozoic era. The low-browsing dinosaurs probably devastated gymnosperms by eating young seedlings, which allowed low-growing, herbaceous angiosperms to survive and diversify.

Flowering plants provide us with food, clothing, shelter, and medicine. Cereals such as corn, wheat, and rice are members of the grass family, whose edible grains can be stored for long periods. Modern wheats contain two, four, or six sets of chromosomes, and they probably evolved from a natural cross between ancient einkorn wheat (a diploid) and a wild grass (also a diploid), followed by chromosome doubling in some germ cells thousands of years ago. This eventually yielded tetraploid emmer wheat. Emmer wheat also crossed with a wild grass to produce the first hexaploid wheats, which today are used to make bread and other baked goods.

Corn probably evolved from an ancient grass relative, teosinte. Both corn and teosinte have 20 chromosomes, they sometimes grow in the same fields, and they can crossbreed. About 7,500 years ago, an environmental stress may have selected teosinte plants with large kernels, and early farmers may then have cultivated these individual plants. Breeding experiments led to the development of hybrid corn, which results from crosses between separate inbred lines to yield bountiful crops.

Rice is perhaps the most widely consumed modern cereal crop, and fossils of the plants date back some 130 million years. Today, rice grows in a wide range of environments. In 1961, a rice seed bank was started in the Philippines to preserve different varieties and prevent reliance on a few types. Seed and pollen banks have since been founded for other valuable plant species.

Although the Green Revolution increased food production in many countries, overpopulation and poor distribution of food still remain problems around the world.


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