draw and label xylem vessel elements, phloem sieve tube elements and companion cells from microscope slides, photomicrographs and electron micrographs

Structure and Function of Plant Transport Tissues (Cambridge IGCSE 9700 – Topic 7)

Learning Objectives

By the end of this unit students will be able to:

1. Identify, label and draw the main components of xylem vessel elements, tracheids and ray parenchyma in transverse sections.
2. Identify, label and draw the main components of phloem sieve‑tube elements and companion cells, including plasmodesmatal connections.
3. Explain how each anatomical feature relates to the mechanisms of water (xylem) and assimilate (phloem) transport.
4. Produce accurate, syllabus‑compliant diagrams from light‑microscope slides, photomicrographs and TEM images.
5. Apply the quantitative concepts of water potential (Ψ = Ψ_s + Ψ_p) and the pressure‑flow equation (flow ∝ ΔP) where required.

Key Anatomical Features

Xylem – Vessel Elements, Tracheids & Ray Parenchyma

• Vessel elements – elongated, dead at maturity; form continuous tubes.
• Secondary walls are thick, lignified and bear pits (bordered or simple) for lateral water movement.
• End walls are replaced by perforation plates (usually a single large opening, sometimes scalariform) that permit unrestricted axial flow.
• Two size classes:

  • Short vessel elements (0.2–0.5 mm) – interspersed among longer elements.
  • Long vessel elements (up to several mm) – give rise to long continuous vessels.

• Tracheids – dead at maturity; long, narrow cells with tapered ends.
• Walls are lignified and contain bordered pits (torus‑margo) but no perforation plates; water moves from pit to pit.
• Ray parenchyma – living axial or radial cells intersecting vessels and tracheids; store starch and provide radial transport of water and solutes.

Phloem – Sieve‑Tube Elements and Companion Cells

• Sieve‑tube elements – living at maturity but highly specialised; thin primary walls.
• End walls are transformed into sieve plates composed of a regular array of pores.
• Callose (β‑1,3‑glucan) may be deposited at the pores to seal the plate after injury.
• In mature sieve‑tube elements the nucleus, vacuole and most organelles are lost; a thin layer of cytoplasm remains at the periphery (the “cytoplasmic sleeve”).
• Companion cells – living parenchyma cells tightly associated (usually on the abaxial side) with each sieve‑tube element.
• Contain dense cytoplasm, a prominent nucleus, numerous mitochondria, abundant rough ER and many plasmodesmata that connect directly to the sieve tube.
• Two common types:

  • Ordinary companion cells – typical of most dicot stems.
  • Transfer (phloem‑parenchyma) cells – larger, with extensive ER and mitochondria; important in long‑distance loading.

Linking Structure to Transport Mechanisms

Water Transport in Xylem – Cohesion‑Tension Theory

• Transpiration creates a negative water potential (Ψ) at the leaf surface, pulling water upward.
• Lignified walls and continuous vessels provide a low‑resistance, unbroken column of water.
• Perforation plates eliminate end‑wall resistance; pits allow lateral movement into ray parenchyma and surrounding tissues.
• Hydrogen‑bonded water molecules transmit the tension throughout the column.

Assimilate Transport in Phloem – Pressure‑Flow (Mass‑Flow) Hypothesis

• Sucrose is actively loaded into the sieve‑tube element at source regions (e.g., mature leaves) by companion‑cell proton‑pump/sucrose‑symporter complexes.
• Loading raises the osmotic potential (more negative Ψ_s), drawing water in from the xylem and generating a high turgor pressure (positive Ψ_p).
• At sink regions (roots, developing fruits) sucrose is unloaded, water exits, and turgor pressure falls.
• The resulting pressure gradient (ΔP) drives bulk flow of solutes through the sieve plates (flow ∝ ΔP).
• Callose deposition can temporarily block sieve‑plate pores to prevent loss of contents after wounding.

Apoplast and Symplast Pathways (Syllabus Terminology)

• Apoplast pathway – movement of water and solutes through cell walls and intercellular spaces; bypasses the cytoplasm. In roots, the Casparian strip forces water to enter the symplast before reaching the xylem.
• Symplast pathway – movement through the cytoplasm of cells, linked by plasmodesmata. Essential for selective loading of sugars into sieve tubes.

Water‑Potential Components (Quantitative Requirement)

Water potential (Ψ) is the sum of solute potential (Ψ_s) and pressure potential (Ψ_p):

Ψ = Ψ_s + Ψ_p

• Ψ_s = –(RTC) (where R = 0.00831 kJ mol⁻¹ K⁻¹, T = temperature in K, C = solute concentration in mol L⁻¹).
• Ψ_p is the physical pressure exerted on the water column (positive in turgid cells, negative in the xylem under tension).
• Understanding these components helps students estimate the direction of water movement between cells and tissues.

Adaptations in Xerophytic Leaves (Syllabus Requirement 7.2)

Microscopic Observation Guides

Light Microscopy (LM) – Prepared Slides

1. Stain thin transverse sections with safranin O (red) for lignified xylem and fast green (green) for phloem.
2. Locate the vascular bundle:

   • Xylem (inner side) – red‑stained, long empty lumina with perforation plates.
   • Phloem (outer side) – green‑stained, strings of cells with faint sieve plates.

3. Companion cells appear as slightly larger, densely stained cells immediately adjacent to sieve tubes.
4. Use a 40× objective to orient the bundle, then switch to 400–1000× for detailed features.

Photomicrographs (Bright‑field)

Key details to capture and label:

• Perforation plates – central opening flanked by radial thickenings (sometimes scalariform).
• Sieve plates – uniform pores; note any dark callose deposits.
• Bordered pits on vessel walls and tracheid walls.
• Plasmodesmata linking companion cells to sieve‑tube elements (visible as tiny dark dots).

Transmission Electron Microscopy (TEM)

Ultrastructural features essential for accurate labeling:

• Secondary wall thickenings in vessel elements (spiral, annular or pitted).
• Bordered pits with torus‑margo structure (in tracheids).
• Companion‑cell organelles – nucleus, numerous mitochondria, extensive rough ER, and plasmodesmata penetrating the sieve‑tube wall.
• Absence of nucleus and large vacuole in mature sieve‑tube elements.
• Electron‑dense callose layers at sieve‑plate pores.

Diagram Layouts (Syllabus‑Compliant)

What to Label – Checklist (Exam‑Style)

When drawing a transverse stem section, ensure the following structures are labelled (use the exact terminology from the syllabus):

• Lumen of vessel element
• Secondary wall of vessel element
• Perforation plate (single or scalariform)
• Bordered pit (torus‑margo) on vessel wall
• Tracheid (tapered ends) and its bordered pits
• Ray parenchyma cell
• Sieve‑tube element (thin wall, cytoplasmic sleeve)
• Sieve plate (array of pores)
• Companion cell (nucleus, mitochondria, ER)
• Plasmodesma linking companion cell to sieve tube
• Callose deposit (if visible) on sieve plate

Step‑by‑Step Drawing Procedure (AO3 Practical Skill)

1. Locate the vascular bundle at low power (×40). Note that xylem is on the inner side, phloem on the outer side.
2. Switch to high power (×400–×1000). Focus on a single vessel element and sketch:

   • Lumen (empty), secondary wall thickenings, and perforation plate (central opening with radial bars).

3. Move to an adjacent tracheid; indicate tapered ends and bordered pits.
4. Identify a ray parenchyma cell intersecting the vessel wall; draw its thin wall and central vacuole.
5. Shift to the phloem side; outline a sieve‑tube element, showing the thin wall and a sieve plate (array of pores).
6. Draw the companion cell next to the sieve tube; add nucleus, several mitochondria, rough ER and at least two plasmodesmata connecting to the sieve‑tube wall.
7. Label each part clearly; use arrows to indicate direction of water (upward) and assimilate flow (source → sink).
8. Compare your sketch with the provided photomicrograph and TEM image; correct any discrepancies before finalising.

Common Pitfalls and How to Avoid Them

• Perforation plate vs. sieve plate: Perforation plates are large, often a single opening; sieve plates consist of many uniform pores. Look at size and pattern.
• Missing companion cells: Always check the cell immediately adjacent to a sieve tube; a densely stained cytoplasm signals a companion cell.
• Confusing ray parenchyma with vessel elements: Ray cells are short, have thin walls, and lack perforation plates.
• Over‑staining with safranin: Excess red can mask phloem details; use a brief dip in safranin followed by a fast‑green rinse.
• Ignoring tracheids in dicots or conifers: Many species rely heavily on tracheids; include them where relevant.

Assessment Checklist (Cambridge AO3)

Comparison Table (Xylem vs. Phloem)

Quick‑Reference Syllabus Alignment (Topic 7 – Transport in Plants)