rootstock -
A healthy plant that is used as the base for grafting a scion.
rootstock -
The necessary basis for something to develop.
Wikipedia
A rootstock is a plant, and sometimes just the stump, which already has an established, healthy root system, used for grafting a cutting or budding from another plant. The tree part being grafted onto the rootstock is usually called the scion. The scion is the plant which has the properties desired by the propagator, and the rootstock is the working part which interacts with the soil to nourish the new plant.