Many seedless fruits are the product of two related, yet different, plants being crossed. The offspring of these plants is itself sterile and has some of the attributes of both of the contributing parents, just as a mule, a cross between a donkey and a horse, is sterile and has attributes from both parents. One such seedless fruit that was developed in this way is the banana. Seedless oranges, though, did not come about by deliberate human intervention. Instead, they are the result a random mutation that took place in the 1800s in Brazil. Recognizing the benefits of an orange without seeds, growers cultivated new plants from material from the original tree. In time, the plant was brought to and cultivated in California, where it was named the navel orange, owing to the resemblance of one end of the fruit to a navel.
Because there is no seed for a seedless orange tree to grow from, a shoot from a mature seedless orange tree must be grafted onto an immature citrus fruit tree of another kind. The shoot from the seedless orange tree is called the scion, while the other plant is called the rootstock, as that is the other plant's only purpose, to serve as a base for the shoot to grow off of. The scion is tied or taped into the rootstock during dormant periods. Over the course of several weeks, the two plants fuse, the scion determining the fruit that will be produced by the plant.
After the scion and rootstock have fused, the new seedless orange tree, a clone of the tree from which the scion was taken, grows like any other orange tree. The tree develops and produces orange blossoms. From the blossoms come the seedless oranges. The mutation changes the cosmetic appearance of the tree itself in no way. Instead, the mutation causes a twin to grow inside the peel of each fruit produced at the end opposite to the stem, instead of the normal seeds. Thus, inside each peel, there are really two oranges growing.