Plant fibres, xylem and phloemEdexcel A-Level Biology A: Revision notes

Plant cell walls are built from cellulose. Each cellulose molecule is a long, unbranched chain of β-glucose. Many chains lie side by side and are cross-linked by hydrogen bonds into microfibrils, which are very strong when pulled (high tensile strength) and are embedded in a matrix of other polysaccharides.

Some cells, including sclerenchyma fibres and xylem vessels, undergo secondary thickening. After the cell stops growing, extra layers of cellulose are laid down on the inside of the primary wall. Within each layer the microfibrils are parallel, but successive layers lie at different angles, so the wall resists pulling forces from any direction. The wall is then lignified: lignin, a rigid, waterproof polymer, fills the spaces between the microfibrils, stopping them slipping past each other and preventing the wall from collapsing or absorbing water. The cell contents die, leaving an empty lumen.

Sclerenchyma fibres are long, narrow cells with tapered ends that overlap along the stem. Their thick, lignified secondary walls and dead, empty lumen mean that their only function is support. They have few pits and do not transport anything.

Because fibres from plants such as flax, hemp and nettle are long and strong in tension, humans exploit them to make rope, cloth (linen), paper and building materials. The high tensile strength of a fibre comes from its cellulose microfibrils, the many layers of secondary thickening and the lignin.

Xylem vessels transport water and mineral ions upwards and also provide support. Vessel elements are dead cells joined end to end, and the end walls break down, so each vessel is a continuous, hollow, wide tube with very little resistance to flow.

Their walls are thickened with lignin, often in rings, spirals or a continuous layer. This makes them strong enough to resist collapse under the tension created by transpiration, and the lignin is waterproof. Unlignified gaps called pits let water move sideways between vessels or into neighbouring living cells, so water can bypass an air bubble or blockage.

Phloem transports organic solutes, mainly sucrose, in the process of translocation. Phloem is made of sieve tube elements joined end to end, with perforated end walls called sieve plates. Sieve tube elements are living but have very little cytoplasm, no nucleus and few organelles, so the flow is not obstructed.

Each sieve tube element is linked by plasmodesmata to a companion cell. The companion cell has a nucleus, dense cytoplasm and many mitochondria, so it supplies the ATP needed to load sucrose into the sieve tube. Phloem walls are unlignified cellulose, so they provide no real support.

In a young dicotyledonous stem the vascular bundles are arranged in a ring. In each bundle the sclerenchyma fibres form a cap on the outer side, then comes the phloem, then a thin layer of cambium, and the xylem lies on the inner side, towards the pith.

• Sclerenchyma fibres: dead, lignified, narrow with tapered ends, support only.
• Xylem vessels: dead, lignified, wide with no end walls, support and transport of water and mineral ions.
• Phloem: living sieve tube elements with sieve plates and companion cells, unlignified, translocation of organic solutes.

Similarity: all are elongated cells that run lengthways. Sclerenchyma and xylem both have thick lignified walls and are dead when functioning.

Cut a very thin transverse section of a stem, stain it (for example with toluidine blue, which turns lignified walls blue-green and cellulose walls pink-purple, or phloroglucinol, which stains lignin red) and view it with a light microscope on low power, then high power.

Identify xylem vessels (large, empty, thick lignified walls), sclerenchyma fibres (small, empty cells with very thick walls, forming a cap near the outside), and phloem (small cells with thin, unlignified walls: sieve tubes with companion cells). Make a labelled low-power plan, drawing the tissues but not individual cells.

Clamp one end of a fibre of a measured length, hang a mass holder from the other end and add masses (for example 100 g at a time) until the fibre snaps. The force at breaking is mass × gravitational field strength. Tensile strength can be compared as the breaking force, or more fairly as breaking force per unit cross-sectional area.

Control the fibre length, diameter, species, age and moisture, and measure the diameter with a micrometer. Repeat with several fibres and calculate a mean. Wear eye protection and put sand or a box beneath the masses.

Example: a fibre breaks at 2.4 kg, so the force is 2.4 × 9.8 = 23.5 N.