Romans 11:17But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree.
One of the reasons we graft is that some plants go through a juvenile stage that takes several years. So an apple from seed may take 4 or 5 years to bear, but grafting an apple can get you fruit in 2 or 3 years. The "scion" i.e. part grafted, takes advantage of a root that is already established. The mature root (Old Testament among other things? How would we be without the recorded history of God's dealing with His people?) serves as a nurturing base for the newly grafted shoot.
With rare cactus, we often graft to get flowers, and therefore seed years earlier than with plants grown on their own roots. Often a rare plant will only be found on one or two mountains in the whole world, so seed is really limited. This plant, Eriosyce heinrichii came from a packet of seeds a friend from Chile sent me. This is two years old, and half again as big as the other seedlings. And the other seedlings are at least a year away from blooming.
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PS the rootstock in this case is the common Prickly Pear cactus.