Structure of transport tissues

Xylem is the plant tissue responsible for the upward transport of water and dissolved mineral ions from roots to leaves. The main conducting cells are xylem vessel elements, which fuse end-to-end to form long continuous xylem vessels.

Key structural features and their functions:

• Dead at maturity. The cells lose their cytoplasm, nucleus and all organelles during development. Only the lignified cell wall remains. This leaves the lumen completely hollow, reducing resistance to water flow.
• End walls broken down. The end walls between adjacent vessel elements disappear completely, producing a continuous hollow tube that may extend many metres up a tree without interruption.
• Lignified walls. The cell walls are thickened with lignin, deposited in characteristic patterns (rings, spirals or networks). Lignin has two key roles:
  • It is waterproof, so water does not leak out through the walls.
  • It is rigid and strong, preventing the vessel from collapsing inwards under the tension generated by transpiration pulling water up.
• Pits. Small unlignified regions of the wall — pits — are aligned between adjacent vessels and between vessels and surrounding parenchyma. They allow water to move laterally out of the xylem to bypass blockages or to enter leaf mesophyll.
• Narrow diameter. The narrow lumen aids capillary action and helps maintain an unbroken column of water, essential for the cohesion-tension mechanism.

Other xylem tissues include tracheids (narrower, with tapering ends and pits but no continuous tube — common in conifers), xylem fibres (provide additional mechanical support) and xylem parenchyma (storage). The 9700 syllabus focuses on vessels.

In woody plants, dead xylem accumulates over years to form the wood of the stem — heart wood (central, no longer conducting) and sap wood (outer, still conducting).

Phloem is the tissue that transports dissolved organic solutes — mainly sucrose and amino acids, plus plant hormones — from regions of production (sources) to regions of use or storage (sinks). The two key cell types are sieve tube elements and their associated companion cells.

Sieve tube element

• Living, but stripped down. Sieve tube elements are alive at maturity but have lost their nucleus, vacuole (tonoplast) and most other organelles during development. This leaves the lumen mostly empty, reducing resistance to flow.
• Sieve plates. The end walls between adjacent sieve tube elements are perforated to form sieve plates. The pores in the sieve plate allow phloem sap to flow rapidly from one element to the next, while the plate still provides some structural support.
• Thin cellulose walls. The cell walls are made of cellulose and are not thickened with lignin, unlike xylem. The thin walls give some flexibility but cannot withstand large tensions — phloem operates under positive pressure, not negative.
• No nucleus. Without a nucleus, the sieve tube cannot make its own proteins; it depends entirely on the companion cell for metabolic support.

Companion cell

• Nucleus and dense cytoplasm. Each sieve tube element is paired with a smaller companion cell that has a normal nucleus and a dense cytoplasm rich in ribosomes — for protein synthesis.
• Many mitochondria. The cytoplasm contains a high density of mitochondria, producing large amounts of ATP.
• ATP for active loading. The ATP is used to power active transport of sucrose into the phloem against its concentration gradient, via an H⁺/sucrose co-transport mechanism.
• Plasmodesmata to sieve tube. Numerous plasmodesmata connect the companion cell to the sieve tube element, allowing free movement of ATP, ions, water and metabolites between the two. They effectively function as a single physiological unit.

Active loading mechanism (brief preview — covered in detail in the next subtopic):

1. A proton pump (H⁺ ATPase) in the companion cell membrane pumps H⁺ OUT of the cell, using ATP. This creates a steep proton gradient — high outside, low inside.
2. H⁺/sucrose co-transporter proteins use the inward flow of H⁺ (down its gradient) to bring sucrose IN from the apoplast.
3. Sucrose passes from companion cell to sieve tube via plasmodesmata, raising the sucrose concentration in the sieve tube above that of the surrounding tissue.

Cambridge frequently asks for a direct comparison of xylem and phloem. Use comparative phrasing throughout.

Feature	Xylem vessel	Phloem sieve tube
Living state at maturity	Dead (no cytoplasm or organelles)	Living (but no nucleus, few organelles)
End walls	Broken down — continuous hollow tube	Perforated sieve plates — pores allow flow
Wall thickening	Heavily lignified	Thin cellulose, unlignified
Associated cells	None	Companion cells linked by plasmodesmata
Direction of transport	Upwards only (roots → leaves)	Source to sink (either direction)
Substances transported	Water + dissolved mineral ions	Sucrose, amino acids, hormones
Driving force	Transpiration pull (negative pressure / tension)	Pressure flow from source to sink (positive pressure)

This table is worth memorising — 5- and 6-mark 'compare xylem and phloem' questions appear on Paper 2 every cycle. Always use comparative phrasing ('xylem is X whereas phloem is Y') rather than two separate descriptions, which can cap your mark.

The position of vascular tissue varies between stems, roots and leaves — each arrangement reflects the mechanical and transport needs of that organ.

Dicotyledonous stem.

• Vascular tissue is organised into vascular bundles arranged in a ring around the periphery of the stem.
• Within each bundle: xylem is on the inside (towards the centre), phloem on the outside (towards the epidermis), with a layer of cambium between them.
• The peripheral arrangement gives the stem resistance to bending (like a hollow scaffolding pole — stronger than a solid rod of the same mass).
• The cambium can divide to produce secondary xylem and phloem, allowing the stem to thicken (secondary growth in woody plants).

Dicotyledonous root.

• Vascular tissue is central, forming a star-shaped stele (vascular cylinder).
• Xylem occupies the centre in a cross / star pattern (often described as having 'arms').
• Phloem lies in the bays between the arms of the xylem.
• The vascular cylinder is surrounded by a single layer of cells called the endodermis, whose walls contain a waterproof Casparian strip (suberin). This forces water from the apoplast pathway into the symplast, allowing the plant to control mineral uptake.
• The central arrangement gives the root resistance to pulling forces as the root anchors the plant in the soil.

Leaf.

• Vascular bundles run through the midrib and branching veins of the leaf.
• Within each vein, xylem is towards the upper surface, phloem towards the lower surface.
• The veins also provide mechanical support, holding the leaf flat to maximise light interception.
• Mesophyll cells around the veins are arranged for efficient gas exchange and photosynthesis.

You should be able to identify xylem and phloem from a transverse section under the microscope: xylem has very thick lignified walls (often stained red/pink with safranin), large empty lumens and a polygonal outline; phloem sieve tubes have thin walls, sieve plates visible in longitudinal section, and small companion cells alongside.

In the practical examination (Paper 3) and on Paper 2 photomicrographs, you are expected to identify xylem and phloem in stained transverse sections of root, stem or leaf.

Xylem features under the microscope:

• Thick, dark-stained walls (lignin takes up red dyes such as safranin or phloroglucinol → bright red / pink).
• Large empty lumens (cells are dead — no cytoplasm).
• Polygonal or rounded outlines in transverse section.
• Spiral or annular thickenings sometimes visible in longitudinal section.
• Located towards the inside of vascular bundles in stems, in the centre of roots.

Phloem features under the microscope:

• Thin walls that take up less stain (usually appear pale).
• Smaller cells than xylem.
• Sieve plates visible as cross-walls in longitudinal section (sometimes appear as 'dotted' end walls).
• Companion cells alongside sieve tubes — densely stained because of dense cytoplasm.
• Located towards the outside of vascular bundles in stems, in the bays between xylem arms in roots.

Calculating magnification or actual size: standard practice for AS and A2 — use the formula

$\text{magnification} = \frac{\text{image size}}{\text{actual size}}$

Always work in the same units (μm or mm), and remember 1 mm = 1000 μm.