Vascular tissues in plants form an integrated transport network that supports water, mineral and assimilate movement. Their structure is closely related to efficient transport and mechanical support.

Structure and Function of Vascular Tissues

Overview of Plant Vascular Systems

Plants possess two main vascular tissues — xylem and phloem — organised into vascular bundles that extend through the roots, stems and leaves. These tissues transport essential substances and provide structural integrity. The precise arrangement within organs is crucial for transport efficiency and mechanical stability.

Xylem: Water and Mineral Transport

Composition and Structure

Xylem is responsible for the transport of water and dissolved mineral ions from the roots to aerial parts of the plant. It also provides mechanical support due to lignin deposition in its walls.

The main components of xylem include:

• Xylem vessels – elongated, hollow tubes formed by the fusion of vessel elements end-to-end. The end walls are completely or partially removed to form a continuous column for water flow.

• Tracheids – spindle-shaped cells with tapered ends and pits allowing lateral movement of water. These are more prominent in non-flowering plants.

• Xylem parenchyma – living cells that store starch and facilitate lateral transport of water and minerals.

• Fibres – elongated cells with thick lignified walls that add tensile strength.

Adaptations for Efficient Transport

• Lignification strengthens vessel walls, preventing collapse under the tension created during transpiration.

• Pits in vessel walls enable lateral movement of water between adjacent vessels.

• Narrow lumens maintain capillary action for water ascent.

• Continuous water columns facilitate cohesive water flow without obstruction.

Phloem: Translocation of Organic Solutes

Composition and Structure

Phloem is responsible for translocating organic molecules, particularly sucrose and other assimilates, from sources (photosynthetic tissues) to sinks (growing tissues, roots, storage organs).

Phloem is composed of several cell types:

• Sieve tube elements – elongated living cells that join end-to-end to form sieve tubes. The end walls contain sieve plates, perforated with pores that allow cytoplasmic continuity between cells.

A labelled diagram of phloem showing a sieve tube element and its adjacent companion cell, connected by plasmodesmata. The sieve plate and reduced organelles in the sieve element illustrate how structure facilitates mass flow. This precisely matches the OCR requirement to describe sieve tubes and companion cells. Source.

• Companion cells – metabolically active cells closely associated with sieve tubes, connected via plasmodesmata. They regulate metabolic functions and provide energy for active transport during loading and unloading of solutes.

• Phloem parenchyma – involved in storage and lateral transport of nutrients.

• Phloem fibres – provide additional structural support.

Functional Adaptations

• Reduced cytoplasmic contents and absence of a nucleus in sieve elements allow unimpeded flow of phloem sap.

• Mitochondria-rich companion cells actively pump sucrose into sieve tubes, maintaining concentration gradients for efficient transport.

• Bidirectional flow within sieve tubes allows simultaneous transport to multiple sinks.

Organisation of Vascular Tissues in Plant Organs

Roots

In roots, vascular tissues are centrally located within the stele to resist pulling forces during water absorption.

• Xylem forms a cross or star shape at the centre, with phloem occupying the spaces between the arms of the xylem.

A schematic transverse section of a dicot root showing central xylem in a star-like pattern with phloem in between. The pericycle and endodermis surrounding the stele are shown to orient students when drawing. This reflects the typical organisation used to resist tensile forces during water uptake. Source.

• Pericycle surrounds the vascular bundle and contributes to the formation of lateral roots.

• Endodermis with the Casparian strip regulates the selective movement of water and solutes into the xylem.

This central arrangement maximises support against tension generated by transpiration pull.

Stems

In stems, vascular bundles are arranged in a ring near the periphery, supporting flexibility and upright growth.

• Xylem is positioned towards the inside of each bundle, providing structural rigidity and water transport.

• Phloem lies towards the outside, transporting assimilates from photosynthetic tissues.

• The cambium, a layer of meristematic cells between xylem and phloem, allows secondary thickening through production of secondary vascular tissue.

This configuration provides both mechanical support and efficient transport pathways for long-distance movement.

Leaves

In leaves, vascular tissues form a branching network of veins essential for photosynthesis and transpiration.

• The midrib contains the main vascular bundle, with smaller veins branching into the lamina.

• Xylem occupies the upper side of the vein, ensuring a direct water supply to mesophyll cells for photosynthesis.

A high-quality leaf cross-section highlighting a vascular bundle with xylem on the upper side of the vein and phloem below. The figure also shows mesophyll and supporting tissues (sclerenchyma/collenchyma) — helpful but beyond the strict syllabus; students can ignore those labels if focusing only on vascular tissues. The clear labelling makes the positional relationship within the vein unambiguous. Source.

• Phloem lies on the lower side, exporting photosynthetic products from the leaf to other parts of the plant.

• This arrangement ensures close proximity between vascular tissues and mesophyll cells, maximising efficiency of gas exchange and transport.

Microscopic Observation of Vascular Tissues

Stained Sections

When viewing stained sections under a microscope:

• Xylem stains brightly due to lignin, often appearing red or purple depending on the stain (e.g., toluidine blue or safranin).

• Phloem typically stains a paler colour as its cells are living and non-lignified.

• The pattern of vascular bundles helps identify the organ:

  • Root: star-shaped xylem at the centre.

  • Stem: vascular bundles arranged in a ring.

  • Leaf: network of veins with xylem uppermost.

Students should be able to identify and draw labelled diagrams of these structures, following OCR’s expectations for accurate biological drawings.

Functional Integration of Xylem and Phloem

Although xylem and phloem perform distinct roles, they are closely associated within vascular bundles, allowing coordination between water supply and assimilate distribution. The continuous vascular network ensures that water, minerals and organic solutes can move efficiently throughout the plant, maintaining physiological balance and supporting growth, photosynthesis, and reproduction.