8.1 Xylem and phloem

2026 Syllabus Objectives

  1. State the functions of xylem and phloem:

    • (a) xylem – transport of water and mineral ions, and support
    • (b) phloem – transport of sucrose and amino acids

  2. Identify in diagrams and images the position of xylem and phloem as seen in sections of roots, stems and leaves of non-woody dicotyledonous plants

  3. Relate the structure of xylem vessels to their function, limited to:

    • (a) thick walls with lignin (details of lignification are not required)
    • (b) no cell contents
    • (c) cells joined end to end with no cross walls to form a long continuous tube

Transport Systems in Plants 🌿

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Plants need to transport substances throughout their structure. Unlike animals, plants have a branching shape that provides a large surface area relative to their volume, meaning most cells are close to the surface.

Why Plants Need Transport Systems

  • Carbon dioxide diffuses easily into leaves through stomata and air spaces, reaching photosynthesising mesophyll cells
  • Water comes from the soil and must be transported from roots up to leaves
  • Mineral ions must also be transported from roots to other parts of the plant
  • Sugars and amino acids made in leaves must be transported to other parts like roots and flowers

Plants have two transport systems to move these substances:

  • Xylem – transports water and mineral ions
  • Phloem – transports sucrose and amino acids

Xylem Tissue 💧

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Functions of Xylem

Xylem has two main functions:

  1. Transport of water and mineral ions from roots to stems and leaves
  2. Support – helps hold the plant upright

🔑 Key Definition: Xylem is a plant tissue made up of dead, empty cells joined end to end; it transports water and mineral ions and helps to support the plant.

Structure of Xylem Vessels

Xylem vessels are highly specialized structures with unique features:

Key structural features:

  • Cells are joined end to end with no cross walls between them
  • Each cell is dead with no cell contents (no cytoplasm or organelles)
  • The continuous tube formed allows water to flow freely from roots to leaves
  • Cell walls are thickened with lignin, a hard, strong, waterproof substance
  • The diameter of vessels ranges from approximately 15 μm15 \, \mu\text{m} to 200 μm200 \, \mu\text{m}

🔑 Key Definition: Lignin is a hard, strong, waterproof substance that forms the walls of xylem vessels.

Structure-Function Relationship in Xylem

The structure of xylem vessels is perfectly adapted to their functions:

FunctionStructural FeatureExplanation
SupportWalls contain ligninLignin is very strong and can support the great weight of even a heavy tree. In leaves, xylem vessels in vascular bundles help hold the leaf flat to provide a large surface area to absorb sunlight
TransportCells are dead with no contentsWater can flow easily through the tube without obstruction
TransportNo cross walls between dead cellsCreates a continuous tube for water to flow through from roots to leaves
TransportWalls contain ligninLignin keeps vessels open and prevents collapse, allowing water to flow through easily
TransportDiameter is 1515-200 μm200 \, \mu\text{m}Narrow enough to prevent the water column from breaking; wide enough to allow substantial water flow

Xylem in Tree Trunks 🌲

The world's tallest trees, coastal redwoods (Sequoia sempervirens), can grow up to 116 m116 \, \text{m} tall. Scientists believe trees cannot grow taller than about 130 m130 \, \text{m} due to limitations of the xylem transport system.

Wood is made of xylem tissue – the long xylem vessels run through the entire trunk and branches, providing both structural support and a pathway for water transport from roots to leaves.

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