[support] Permissions Question

I have what I think must be a common permissions question, but I can't seem to find the answer. I want to set my Drupal site up in a tree structure. I want to grant create/edit/delete permissions by selecting one or more nodes in the tree and granting a role the ability to edit those nodes as well as any child nodes. A simplified example would be a site with two primary sections and each primary section has two subsections. I want to be able to give rights to the primary section, which would include rights to the relevant subsection. However, I also want to be able to grant rights just to a particular subsection. How can I do this?