Possible Clues:

• Underground network for plants
• 1977 miniseries starring LeVar Burton
• Seeds -- and what they might produce?
• Being synonymous with underground network, I'd be tearing my hair out to get at them
• Origins; Alex Haley novel
• Carrots and daikon
• Background, where tracks heard?
• They may be square or cube
• Parts of trees and teeth
• Emmy-winning 1977 miniseries