Pollination
The apple tree needs the help of insects, mainly bees, for pollination, because its pollen is too heavy to be carried by the wind, as occurs in other plants. The aroma and colour of flower petals attract bees, which land on them to collect or feed on their nectar, at which point they also collect the pollen that covers their bodies. When the next flower is sucked, some pollen grains are deposited on the pistil, leading to pollination.
This video shows how the bees transport the pollen on their foots.