Other
Cartage.org: Mature Root Cross Section
This thorough lesson provides a wealth of root related information ranging from the anatomy of a root to the different types of roots that exist. Many photos and drawings provide clarity to the text.
CK-12 Foundation
Ck 12: Biology: Roots
[Free Registration/Login may be required to access all resource tools.] Covers the structure, function, and growth of roots.
University of Illinois
University of Illinois Extension: Great Plant Escape: Plant Parts: Roots
Site provides a brief explanation of how roots help support a plant.