3.2 - Organ for Water and Mineral Salts Uptake

• Roots usually grow below the soil surface towards the centre of the earth and watery areas.
• Main functions of roots:
  • Provide support and strength and anchor plants in the soil.
  • Absorb water and mineral salts from the soil and transport them to the stem and leaves.

External Structures of Roots

• Root collar: part of the root connected to the base of the plant stem.
• Primary root: main root from which lateral roots grow.
• Lateral roots: branches of the primary root.
• Root hairs:
  • Adapted from epidermal cells of the roots.
  • Increase the total surface area of the roots to increase the uptake of water and mineral salts.
• Root cap:
  • Located at the tip of the root.
  • Protects the root from damage when going through the soil.

Root Adaptations for Water and Mineral Salts Uptake

Epidermis

• Epidermal cells are closely arranged.
• Thin cell walls and water-permeable cell membranes facilitate water movement in the roots.
• Some epidermal cells form root hairs by lateral elongation of their outer walls.
• Root hair cells have no cuticle, allowing water absorption.
• Large vacuoles store water and mineral salts, increasing water absorption.

Cortex

• Located below the epidermal layer.
• Thin cell walls facilitate water movement in the roots.
• Loosely arranged cells facilitate gaseous exchange.
• Consists mainly of parenchyma cells.

Endodermis

• Separates the cortex from the vascular cylinder.
• Cells are closely arranged in a single layer.
• Most cell walls are thickened with suberin or lignin, forming the Casparian strip.
• Allows water and mineral salts absorbed from the soil to enter the vascular cylinder, but prevents air bubbles from entering.

Vascular Cylinder

• Forms the root core.
• Consists of xylem and phloem tissues surrounded by a one-cell-thick pericycle.
• The pericycle is involved in secondary growth and lateral-root formation.
• Xylem and phloem are usually arranged in a star-shaped pattern.
• Xylem transports water and mineral salts.
• Phloem transports organic substances such as sucrose and plant hormones.

Monocot and Eudicot Root Cross-sections

• Identify root hairs, epidermis, cortex, endodermis, pericycle, xylem and phloem in root cross-sections.
• Compare monocot and eudicot roots in terms of cortex, endodermis and Casparian strip, and the arrangement of xylem and phloem.