What advantage does being diploid have over being haploid in ferns?
The life cycle of a fern alternates between the sporophyte and gametophyte generation. The sporophyte generation is capable of producing haploid spores by the process of meiosis, or reduction division. These spores then undergo mitotic division, making many more copies of the original spore and becoming a prothallus, a small photosynthetic structure that is part of the gametophyte generation. This structure can produce gametes, or sperms and eggs usually on the same prothallus. When they unite, a diploid sporophyte fern will be the result. An advantage the diploid fern plant of the sporophyte generation has, over the haploid prothallus of the gametophyte generation, is that the prothallus is an easy source of food for herbivores and thus is consumed often in food chains. The diploid fern plant is less easy to consume, thus conferring an advantage to the diploid much larger stage of the fern's life cycle.
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