How can two plant pieces grow together after grafting?

A grafted apple tree may have roots from one plant and fruit-bearing shoots from another. The upper piece is the scion, and the rooted lower piece is the rootstock. For them to function together, the join must heal into connected living tissue. Pressing two branches together does not by itself join the hidden routes that carry water and sugars.

Near a successful join, living cells divide and produce healing tissue called callus. Tissue from the two pieces can meet in a bridge. Close alignment of their cambium regions helps: cambium is growth tissue whose cells produce new vascular tissues. As the bridge develops, cells specialize to form a continuing connection rather than merely a covering over two cut surfaces.

New xylem, the water-carrying tissue, can connect across the union. New phloem connects the routes used to transport sugars. The union also develops structural support. These changes allow supply from below and resources made above to move through a shared functioning plant. Compatibility and suitable conditions matter, and not every attempted graft makes a lasting union.

The pieces usually retain their own genetic identities. An apple scion generally produces its own variety's fruit, while the rootstock can influence features such as tree size. Grafting combines working tissues and their effects, rather than combining egg and pollen to make a new genetic individual. A tree with two origins can function as one connected transport system without its upper and lower parts becoming the same variety.