What causes joining up of stock and .scion grafting technique of

What causes joining up of stock and .scion grafting technique of vegetative propagation in plants ?
A. 1. The stock and scion unite due to cambial cavity.
2. Stock is the portion on which the grafting is done and it provides the roots.
3. Scion is the portion of the plant which is grafted on the other plant and it contributes the stem.
4. The plant contributing scion should have large sized fruits and the plant contributing stock should have deep root system.