Gymnosperms are heterosporous seed plants that produce naked seeds. Angiosperms appeared during the Mesozoic era and have become the dominant plant life in terrestrial habitats. Fruits offer additional protection to the embryo during its development, and also assist with seed dispersal. Flowers expand the possibilities for pollination, especially by insects, who have coevolved with the flowering plants. In the gymnosperms, which appeared during the drier Permian period and became the dominant group during the Triassic, pollen was dispersed by wind, and their naked seeds developed in the sporophylls of a strobilus. The gametophytes of seed plants shrank, while the sporophytes became prominent structures and the diploid stage became the longest phase of the life cycle. Pollen allows seed plants to reproduce in the absence of water. Seeds are also equipped to delay germination until growth conditions are optimal. Seeds protect the embryo from desiccation and provide it with a store of nutrients to support the early growth of the sporophyte. Two major innovations were seeds and pollen. Seed plants appeared about 350 million years ago, during the Carboniferous period.
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