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Angiosperms
There are an estimated 240,000 species of flowering plants, or angiosperms (from the two Greek words angeion, meaning vessel, and sperma, meaning seed). This makes the angiosperms the largest plant group; it is also the youngest, having first appeared in the fossil record roughly 125 million years ago.
What features have made such diversity possible? While the possession of vascular tissue and the production of seeds, also characteristic of the gymnosperms, have undoubtedly played a part in the success of the flowering plants, it is the flower that may well be the single most important factor responsible for the abundance of species. Many flowering plants have benefited from evolutionary associations with their animal pollinators. This association, better known as coevolution, most likely has driven much of this recent and rapid diversification.
Angiosperm Reproduction [Illustration]
Flowers are where sexual reproduction occurs in angiosperms. The entire sexual reproductive cycle-pollination, fertilization, and seed and fruit production-takes place in flowers. Pollination is the transfer of pollen grains from an anther, the structure where these grains are produced, to a stigma, a structure or surface receptive to pollen. If conditions are right, each pollen grain will produce a tube that grows down through the style and into an egg-containing ovule in the ovary, where it deposits two sperm. Fertilization occurs when a sperm unites with the egg.
In angiosperms, a second remarkable event occurs when the second sperm unites with additional material in the ovary. This is called double fertilization and results in the production of endosperm (a nutritive tissue). The ovule, with its now-fertilized egg, develops into a seed, while at the same time the ovary develops into a fruit. Fruits are the vehicles that disseminate or disperse seeds away from the parent plant. Once a seed is released from its fruit, and conditions are favorable for its germination, it can grow into a mature plant that produces its own flowers.
Although all angiosperms share this basic life cycle, it is affected in various plants by different factors. For example, some plants are annual, producing their flowers and fruits within a single year and then dying; others are biennial, taking two years to produce flowers and fruits before dying; others are perennial, living and producing flowers and fruits for many years. The young plants of some species-biennials, for example -- require a period of exposure to cold in order to flower, a process called vernalization. The onset of flowering in some plants is affected by photoperiodism, an alteration in the length of the periods of light and dark to which the plants are exposed. Some species require exposure to longer periods of light before they flower and are called long-day plants (for example, wheat and cabbage). Others require a longer exposure to dark and are called short-day plants (for example, violets and strawberries).