rootstock

root +‎ stock

rootstock (plural rootstocks)

1. (agriculture) A healthy and vigorous-rooted plant that is used in grafting, most commonly as a sound base to support a scion that bears desirable fruit in orchard culture.
2. (by extension) The necessary basis for something to develop
   • 2009 January 30, Simon Jenkins, “For real secrets we already have the one-and-a-half-year memoir rule”, in The Guardian‎:

     We know more - vastly more - about how we are governed, and that knowledge is the rootstock of consent.

• rootstock on Wikipedia.Wikipedia