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Angiosperm

Angiosperm is the name given to those plants that produce flowers during sexual reproduction. The term literally means "vessel seed" and refers to the fact that seeds are contained in a highly specialized organ called an ovary.

Flowering plants are the most recently evolved of the major groups of plants, arising only about 130 million years ago. Despite their geological youthfulness, angiosperms are the dominant plants of the world today: about 80% of all living plant species are flowering plants. Furthermore, they occupy a greater variety of habitats than any other group of plants. The ancestors of flowering plants are the gymnosperms (e.g., pine and fir), which are the other major group of plants that produce seeds. The gymnosperms, however, produce their seeds on the surface of leaf-like structures, which makes the seeds vulnerable to mechanical damage when winds whip the branches back and forth, and to drying out. Most importantly, conifer seeds are vulnerable to insects and other animals, which view seeds as nutritious, energy packed treats. In angiosperms, the margins of the seed-bearing leaves have become inrolled and fused, so the seeds are no longer exposed but are more safely tucked inside the newly evolved "vessel," which is the ovary.

The other major advance of the angiosperms over the gymnosperms was the evolution of the flower, which is the structure responsible for sexual reproduction in these plants. The function of sexual reproduction is to bring together genetic material from two individuals of differing ancestry, so that the offspring will have a new genetic makeup. The gymnosperms dealt with their immobility by packaging their male component into tiny pollen grains, which can be released into the wind to be blown to the female component of another individual of the same species. Although this method of pollination succeeds, it is wasteful and inefficient because most of the pollen grains land somewhere other than on a female, such as in one's nose, where they may cause hay fever. Furthermore, pollen grains are rich in fixed energy and nutrients such as nitrogen, so they are costly to make (native North Americans used to make pancakes out of the pollen of cattail). By evolving bright colors, scents, and nectar, the flowers of angiosperms served to attract animals. By traveling from one flower to another, these animals would accidentally move pollen as well, enabling sexual reproduction to take place. Because flower-seeking animals such as bees, butterflies, and hummingbirds can learn to recognize different types of flowers, they can move pollen from flower to flower quite efficiently. Therefore, animal-pollinated species of flowering plants do not need to produce as much pollen as gymnosperms, and the resources they save can be put into other important functions, such as growth and greater seed production. Therefore, the flower and its ovary have provided angiosperms with tremendous advantages, and have enabled them to become rapidly dominant over their gymnosperm ancestors.

Les C. Cwynar

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