| MadSci Network: Botany |
One possible reason that stands out is that the sporophyte phase has an advantage over the gametophyte phase because it is diploid rather than haploid. Having two sets of genes can avoid expression of deleterious traits. The seed plant pattern of large sporophyte is a very effective system. Remember the key features: 1. Large sporophyte nurtures the tiny female gametophytes. 2. There are millions of tiny male gametophytes (pollen) that can be carried great distances, either by wind or by animals. 3. New sporophytes arise from very mobile, usually very tough, seeds that contain energy supply and tiny plant in suspended animation. Can you imagine an equally efficient or more efficient system where the gametophyte could reach tree size and nurture tiny sporophytes? A dominant gametophyte will have some major disadvantages. Its major mobile phase would be the tiny spores, which are much more delicate and demanding in their germination conditions compared to seeds. Dominant gametophytes would either be stuck with a very inefficient method of transferring gametes externally using free water or would have had to evolve a more efficient method. Can you envision a more efficient method of gamete transfer with a dominant gametophyte? Seaweeds are various types of algae that are no longer classified as plants for several reasons. A main reason algae are placed in the Kingdom Protista is that they lack the alternation of heteromorphic generations found in members of the Plant Kingdom. Other general differences are that algae are aquatic while plants are mainly terrestrial, and that algae are often unicellular while all plants are multicellular. The majority of plant species have roots designed for anchorage and absorption of water and mineral nutrients. Algae may have holdfasts to anchor themselves but the holdfasts are not specialized for absorption. References Plant Kingdom Kingdom Protista Seaweeds
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