The sperm of seedless vascular plants must be able to swim through a few centimeters of water. They must do this in order to reach the eggs at all. However, the sperm of seed plants are produced within pollen grains, which can be transported very long distances by wind or animals. These sperm do not require water. While some seed plants’ sperm do have flagella, they do not directly need mobility since pollen tubes move them straight to the eggs.
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Since the sperm of seedless vascular plants don’t have the assistance of pollen, they need the ability to be motile and water in order to have any chance of reaching the eggs. The sperm of seed plants have pollen grains, which are delivered directly to the eggs by way of the pollen tubes. Therefore, they do not require motility.