With the ozone layer complete, harmful ultraviolet rays were reduced, and some green algae advanced onto land for the first time. Finally, giant ferns created forests along the water's edge, but water is necessary for ferns to fertilize. On the other hand, seed plants have their pollen carried by the wind or insects to their pistils, so they do not require much water. Therefore, they could also thrive under dry conditions, and have now adapted to various environments, from arid to polar-regions.