Overview of Dicotyledonous Roots
- Dicotyledonous plants (commonly called dicots), such as beans, sunflowers, and oak trees, have roots with a distinct tissue organization.
- Unlike dicot stems, where vascular bundles are scattered near the periphery, the vascular tissues in dicot roots are concentrated at the center.
- This centralized arrangement reinforces the root, allowing it to push through the soil effectively.
Root Tissue Layers
1. Epidermis: The Outer Shield
- Single-layered, thin-walled cells without cuticle (allows water absorption).
- Root hairs extend from some cells to massively increase surface area for absorption.
- Root hairs are short-lived but vital for maximizing water and nutrient uptake.
- Be sure to identify them when examining microscope slides of root sections.
2. Cortex: The Storage and Transport Zone
- Made of parenchyma cells (loosely packed for gas exchange).
- Stores starch for energy.
- Transports water & minerals to vascular tissue.
- Ends in the endodermis, a single-cell layer controlling water entry into the vascular cylinder.
- Contains the Casparian strip – a waterproof barrier that forces water through cell membranes, ensuring selective uptake.
- The Casparian strip acts like a security checkpoint, ensuring that only essential nutrients enter the vascular tissue while harmful substances are kept out.
- This is critical for maintaining the plant’s health.
3. Vascular Cylinder (Stele): The Transport Hub
- Pericycle (outer ring of cells) – generates lateral roots.
- Xylem (center, star-shaped) – thick-walled water transport vessels.
- Phloem (between xylem arms) – thin-walled sugar transport cells.
Think of the xylem as a network of highways carrying water and minerals upward, while the phloem acts like a delivery service, distributing sugars and nutrients wherever they are needed in the plant.
Plan Diagram of a Dicot Root (Transverse Section)
- When drawing a plan diagram, ensure to clearly mark the following key structures:
- Epidermis - Outermost protective layer.
- Root Hairs - Extensions of the epidermis for nutrient absorption.
- Cortex - Between the epidermis and vascular bundle.
- Endodermis - A layer of tightly packed cells surrounding the vascular bundle (may or may not be visible depending on magnification).
- Vascular Bundle (Central Cylinder) - Contains the xylem and phloem in a star-shaped arrangement.
- Xylem - The central part of the vascular bundle, responsible for water transport.
- Phloem - Surrounds the xylem, responsible for transporting sugars.
How to Draw a Root Plan Diagram
- Use low magnification to see the tissue layout.
- Identify epidermis, cortex, endodermis, pericycle, xylem, and phloem.
- Draw clear tissue boundaries, not individual cells.
- No shading or excessive detail – just structure.
- Students sometimes confuse the xylem and phloem in plan diagrams.
- Xylem is located at the center of the root and forms a star-like structure, while phloem is found between the arms of the xylem.
- How do the structural adaptations of dicot roots reflect the plant’s need to balance water uptake with protection against environmental stress?
- Consider this in the context of ecosystems with varying soil conditions.
- What is the role of root hairs in the root structure?
- What is the arrangement of xylem and phloem in the root's vascular bundle?
- Draw and label a plan diagram of a dicot root cross-section.
- Why is the Casparian strip essential for nutrient regulation?
