Biology Semester II |
 |
|
| Sections: |
Introduction | Section 1 | Section 2 | Section 3 |
| Section Two: |
Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 |
 |
Biology: Flowering Plant Reproduction: Part One Flowering Plant Reproduction Like most other eukaryotes, plants reproduce sexually. They use meiosis to reduce their chromosome number by half and use fertilization to restore that number. Unlike most animals, though, plants can reproduce by asexual methods as well. Flowering plants are the most recently evolved major plant group, first appearing around 140 million years ago during the Cretaceous Period. Imagine the world before flowers. Imagine the time when dinosaurs roamed Earth and our remote mammalian ancestors were no bigger than shrews. Ferns and a number of now-extinct gymnosperm groups played a more dominant role in many of the world’s floras. There were not yet bees flitting from flower to flower—there were no flowers! Once they appeared, flowering plants underwent an amazing diversification. Today, they dominate the world’s floras. What made them so successful that they could out-compete and overthrow established plant species? Chief among angiosperm advances was a new wrinkle on sexual reproduction: seeds enclosed within an ovary. Insects delivered pollen from one flower to another. A fertilized flower grew into a fruit that encased the seed, and the seed was nourished by a unique tissue within it. Life Cycles Animal life cycles have a dominant diploid phase. Meiosis is followed immediately by gamete production. Gametes are produced directly by meiosis. Male gametes are sperm and female gametes are eggs (or ova). The plant life cycle consists of two stages, or phases. In the sporophyte phase, the plant is genetically diploid (2n). Meiosis produces haploid (1n) reproductive cells, called spores. In the spores, mitosis occurs. The spores germinate and grow to form the gametophyte (gamete-producing) phase of the plant. The size of the haploid gametophyte form can range from three cells (in angiosperm pollen) to several million cells (in a nonvascular plant such as moss). The gametophyte produces gametes by mitosis. The gametes will fuse during fertilization to produce the diploid sporophyte phase of the next generation. This continual switching between sporophyte and gametophyte forms is called “alternation of generations.” The sporophyte phase produces spores within a specialized structure known as a sporangium. The gametophyte phase has special gamete-producing structures as well. The antheridium produces sperm and the archegonium produces eggs. Within the plant kingdom, the dominance of phases varies. In nonvascular plants (the mosses and liverworts), the gametophyte phase is dominant. From the ferns and fern allies to the angiosperms, vascular plants show a progression toward an increasingly dominant sporophyte phase.
|
|
© 2009 Aventa Learning. All rights reserved. |
