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The flower is the reproductive structure of angiosperms or flowering plants. Compared to the reproductive structures of other plants, the flower is unique in several ways. It consists of four kinds of modified leaves, two of which (stamens and carpels, the latter sometimes called pistils) bear pollen and seeds. Several non-flowering plants also produce pollen and seeds on modified leaves, but in angiosperms the modified leaf called the carpel forms an ovary that completely encloses the ovule, which becomes the seed. In the gymnosperms, ovules are borne on open modified leaves, such as the scale of a pinecone. The term angiosperm, derived from the Greek, means "seed in a vessel." Gymnosperm means "naked seed".

According to the fossil record, flowering plants appeared only about 140 million years ago, although some recently found fossil evidence suggests that they appeared 80 million years before that. (The earliest land plants, blue-green algae, appeared perhaps 1.2 billion years ago.) The angiosperms now dominate the world's vegetation. Only the gymnosperms offer any substantial competition. There may be more than 250,000 angiosperm species, compared to fewer than 1,000 gymnosperm species and fewer than about 40,000 other types of vascular plants (ferns and their relatives) and bryophytes (liverworts, mosses, hornworts). There are fewer than 15,000 species of algae and perhaps more than 100,000 species of fungi and bacteria.

More than any other of the major plant groups, flowering plants are ecologically related to animals. Modern animals, including humans, and flowering plants are equally dependent upon each other. Most flowering species rely on animals for reproduction. Insects carry pollen from the stamens to the carpels; bats and birds participate in Pollination of some species.

Animals attracted to their colorful and aromatic flowers and tasty fruits further ensure the dispersal and growth of the seeds. Many fruits and seeds (the exclusive products of angiosperms) are also collected and consumed by humans, and the seeds are planted in extensive systems of agriculture. Almost all plants used in agriculture are angiosperms. (Mushrooms, fern fiddleheads, and pine nuts are exceptions.) In another relationship between plants and animals, only the special growing cells at the base of a grass (angiosperm) leaf seem well adapted to animal grazing.

 

 

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