The Angiosperm Revolution
Of all the kinds of modern land plants one group dominates: the angiosperms, or flowering plants. With over 250,000 living species, they are the majority of plants of most habitats- except marine environments, which are still habitats for the more primitive algae. But angiosperms were a comparatively recent development in plant evolution. They arose in the mid-Mesozoic (approximately 200 million years ago to 145 million years ago), but by about 100 million years
ago they had pushed the conifers (plants having cone-shaped reproductive structures rather than flowers for reproduction) into the background. Even earlier types of plants such as ferns are now restricted to certain wet habitats, and many of the dominant gymnosperms (plants with exposed seeds, such as conifers) of the early Mesozoic have now been largely replaced by angiosperms; the formerly dominant gymnosperms that did not become completely extinct now survive in comparatively few places.
Why were the angiosperms so successful? A major advantage they have over more primitive plants is their efficient mode of reproduction- -the flower and all of its complex reproductive mechanisms that ensure success. Instead of the inefficient wind-pollinated gymnosperm seed, which wastes a huge amount of pollen and is dependent on random breezes, angiosperms have evolved flowers specifically as devices to attract pollinators- -mainly insects (especially moths, butterflies, and bees) but also birds, bats, and other flying creatures. The pollinators ensure that the pollen is carried directly from one flower of the same species to another, which is more efficient than relying on the wind. This process of delivery is called cross-pollination. The reproductive cycle is highly modified: the ovules (egg-producing organs) are fully enclosed within protective covers called carpels, which form the core of the flower. The carpel protects the ovule from drying out, from fungal infection, and from predation by plant-eating insects. Pollen-producing organs called stamens are surrounded by petals (which serve to attract the pollinator and guide it to the ovules in many cases) and an outer covering of sepals for protection. Typically, a pollinator gets pollen stuck onto it as it climbs into a flower, seeking the nectar that is generated to lure it. The ovule is usually pollinated by the sperm carried from a different flower, thus minimizing self-fertilization (but angiosperms can also self-fertilize if cross-pollination is not possible).
Once the pollen has been delivered, a pollen tube transports the sperm to the ovules. Here angiosperms have another advantage: double fertilization. The pollen carry two sperm nuclei, one of which fuses with the egg nucleus to form the embryo, the other of which fuses with two other nuclei to form a food supply for the embryo. This means that angiosperms don’t need to invest a lot of energy creating food stores for each seed until it is fertilized (unlike gymnosperms, which create food even for infertile seeds).
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