Plant Vascular Tissue (Leaving Cert Biology): Revision Notes

Plant Vascular Tissue

Plant vascular tissue forms the transport system that moves water, minerals, and food throughout the plant body. Understanding the structure and function of these specialised tissues is essential for grasping how plants maintain themselves and grow effectively.

Plant tissues overview

Plants contain three main types of tissue that work together to support plant life. Dermal tissue forms the protective outer layer, ground tissue carries out photosynthesis and provides support, whilst vascular tissue handles the crucial job of transporting materials around the plant.

Xylem

Xylem tissue is responsible for transporting water and dissolved mineral salts from the roots up to all parts of the plant. This upward movement of water is essential for photosynthesis, maintaining plant structure, and cooling the plant through transpiration.

Structure and composition

Xylem is made up of two main types of cells: vessels and tracheids. Both cell types are dead when mature, forming hollow tubes that allow water to flow freely through them.

Xylem vessels are tubular structures that form when multiple cells join end-to-end. The end walls between these cells break down completely, creating continuous tubes that are wider than tracheids. These vessels are particularly efficient at transporting water and are commonly found in flowering plants.

Xylem tracheids are longer, narrower cells with tapered ends that overlap with neighbouring tracheids. Unlike vessels, tracheids retain their end walls, so water must pass through small openings called pits to move from one tracheid to the next.

Adaptations for water transport

Xylem has several key adaptations that make it perfectly suited for moving water efficiently:

• Narrow tube structure - creates strong capillary forces that help pull water upward
• Hollow interior (lumen) - provides an unobstructed pathway for water flow
• No end walls in vessels - allows continuous water movement without barriers
• Pit structures - enable horizontal water movement between adjacent cells
• Lignin reinforcement - prevents collapse under negative pressure and provides mechanical support

Function and location

The primary function of xylem is to transport water and mineral salts from the roots to the leaves, where they are needed for photosynthesis. Xylem also provides mechanical support to the plant due to its lignified structure.

Xylem tissue is found in roots, stems, leaves, and flowers, often grouped in vascular bundles alongside phloem tissue. This arrangement ensures efficient distribution of materials throughout the plant.

Phloem

Phloem tissue transports the products of photosynthesis (mainly sugars) from the leaves to all other parts of the plant where they are needed for growth, respiration, or storage.

Structure and composition

Sieve tube elements are long, tubular cells that join end-to-end to form continuous tubes called sieve tubes. The end walls between these cells, known as sieve plates, contain numerous perforations (holes) that allow materials to flow from one element to the next.

Companion cells are found alongside each sieve tube element and play a crucial role in maintaining the sieve tubes. Since sieve tube elements lose most of their cellular contents as they mature, including their nucleus, the companion cells provide the metabolic support needed to keep the sieve tubes functioning.

Appearance and key features

Sieve tubes are living structures that lack lignin, making them more flexible than xylem vessels. The cytoplasm of each sieve tube element remains intact, although the nucleus degenerates during maturation. The characteristic sieve plates with their numerous pores are clearly visible under microscopic examination and are essential for the transport function.

Function and location

The primary function of phloem is to transport food materials, particularly sugars produced during photosynthesis, from the leaves to other parts of the plant. This process, called translocation, can move materials both upward and downward depending on where they are needed.

Companion cells control the activities of the sieve tube elements, ensuring that transport occurs efficiently and that the sieve tubes remain functional despite lacking a nucleus.

Phloem is found in vascular bundles throughout the plant, typically positioned alongside xylem tissue in roots, stems, leaves, and flowers.

Key differences between xylem and phloem