Seed plants have reduced gametophytes, which are then nurtured by the sporophytes. The gametophytes are protected from stressful conditions such as drought and UV radiation. Pollen grains of seed plants have thick, protective walls. They can also be carried for long distances, which means that they can be spread away without an essential need for water. Seeds also have an internal supply of food, allowing the seeds to live longer than spores. This also provides nourishment for any developing embryos, which spores cannot provide. These features have allowed seed plants to survive and flourish on land.
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More protection, the ability to travel farther (and without water), and an internal supply of food: these are factors that have helped seed plants to be successful on land, and seedless plants do not have these factors