Angiosperms Study Documents

ANGIOSPERMS The Mesozoic is a transitional period, especially for plants. They show little resemblance at the beginning of the era to those at the end, when they more closely resembled present-day vegetation. Gymnosperms, including conifers, ginkgo, and palm-like cycads, originated in the Permian and produced seeds without fruit cover. True ferns prosper in higher latitudes, whereas today they live only in the warm tropics. Cycads, which resemble palm trees, are also very successful. They were scattered across all major continents, possibly contributing to the diet of plant-eating dinosaurs. Ginkgo, of which the maidenhair tree in eastern China is the only living relative, is probably the oldest genus of seed plant. Also dominating the landscape are conifers up to 5 feet tall and 100 feet tall. Its petrified trunks are numerous in Yellowstone National Park (Fig. 163). About 110 million years ago, vegetation in the early Cretaceous underwent radical changes with the introduction of angiosperms, flowering plants that evolved alongside pollinating insects. Plants offer pollinators, such as honeybees and birds, brightly colored and fragrant flowers, and sweet nectar. The unwary intruder is sprinkled with pollen, which is transported to the next flower he visits for pollination. Many angiosperms also rely on animals to disperse their seeds, which are encased in delicious fruit that passes through the digestive tract and falls some distance. The sudden appearance of angiosperms and their eventual dominance over other plant life remain a mystery. They may have originally exploited the skinny rift valley formed when Pangaea broke apart. The earliest angiosperms living in the fertile rift valley appear to have been large plants, growing as tall as magnolia trees. However, fossils found in Australia suggest that the first angiosperms there and possibly elsewhere were small, herb-like plants. Within a few million years of their introduction, efficient flowering plants crowded out ferns and gymnosperms that were once abundant. Angiosperms have water-conducting cells called blood vessel elements that allow advanced plants to cope with drought conditions. Before such vessels emerged, plants were restricted to moist areas such as wet shrubs of the rainforest. Angiosperms were distributed throughout the world at the end of the Cretaceous. Today, they include about a quarter of a million species of trees, shrubs, grasses, and herbs. All major groups of modern plants (Figure 164) are represented by the early Tertiary. Angiosperms dominate the plant world, and all modern families have evolved about 25 million years ago. Grass is the most significant angiosperm, providing food for hoofed mammals called ungulates. Their grazing habits evolved in response to the widespread availability of grasslands, which triggered the evolution of large herbivorous mammals and ferocious carnivores to prey on them. Towards the end of the Cretaceous, the forest expanded into polar regions far beyond the current tree line. The most remarkable example is the well-preserved fossil forest on Alexander Island, Antarctica.To survive in harsh conditions, trees must develop means of protection against cold because plants are more sensitive to lack of heat than to the absence of sunlight. They probably adapted mechanisms to intercept the maximum amount of sunlight during periods when global temperatures were much warmer than today. Tropical vegetation that was widespread during the Mesozoic withdrew to narrow areas around the equator in response to a cooler, drier climate, resulting from the general uplift of the continents and the drying up of the interior oceans. Forests of giant hardwood trees that grow as far north as Montana are now occupied by coarse conifers, a further indication of a colder climate. The emergence of angiosperms may have even contributed to the death of dinosaurs and certain marine species in late Cretaceous.By absorbing large amounts of carbon dioxide from the atmosphere, causing angiosperms to cause a drop in global temperatures. The forests of broadleaf trees and shrubs that were dinosaurs' favorite food apparently disappeared shortly before the end of Cretaceous.This therefore also contributed to the downfall of the dinosaurs.