The part of a desired plant inserted into rootstock during grafting is called the

The answer is; scion

Successful grafting ensures a proper vascular connection between the scion and rootstock tissue, therefore, plants lacking a vascular cambium, such as monocots, cannot normally be grafted.   Grafting, a type of asexual reproduction, is particularly important in the horticultural industry. The two plants being grafted must be close genetically for a successful graft union.

i would say it is