Xylem, phloem and sclerenchymaEdexcel International A Level Biology: Revision notes

In a young dicotyledonous stem the vascular bundles form a ring near the outside of the stem. Within each bundle:

• xylem lies towards the centre of the stem
• phloem lies on the outer side of the xylem
• a cap of sclerenchyma fibres lies outside the phloem, near the edge of the stem, where bending forces are greatest

A layer of cambium between xylem and phloem can divide to produce new vascular tissue. Parenchyma forms the pith in the centre and the cortex outside the bundles.

Xylem vessels transport water and mineral ions up the plant and give support.

• Mature vessels are dead and hollow, with no cytoplasm and a wide lumen, so water flows with little resistance.
• The end walls are broken down, forming a continuous tube.
• Walls are thickened with lignin, which is strong and waterproof. Lignin often forms rings or spirals, so the vessel can stretch as the stem grows, and it stops the vessel collapsing under the low pressure of the transpiration stream.
• Pits, where there is no lignin, allow water to move sideways between vessels and into surrounding living cells.

Sclerenchyma fibres have a supporting role only.

• They are long cells with tapered ends that overlap, so the strength spreads along the stem.
• They are dead at maturity, with a very narrow lumen.
• The walls are very thick, because of cellulose microfibrils and heavy lignification. They are rigid but can flex slightly.
• Fibres lie in caps on the outer side of the phloem and also within the xylem.

They resist both stretching and bending and are the main source of strength in stems used to make rope and fabric.

Phloem carries assimilates such as sucrose and amino acids by translocation, from sources (for example leaves) to sinks (for example roots and growing regions).

• Sieve tube elements are living but have very little cytoplasm, no nucleus and few organelles, so there is room for flow. Cellulose walls are thin and unlignified.
• The end walls are perforated to form sieve plates, so sap flows from cell to cell.
• Each sieve tube element has a companion cell with a nucleus, dense cytoplasm and many mitochondria. It supplies ATP for loading sucrose and keeps the sieve tube element alive. Plasmodesmata link the two cells.

Similarities: all three are elongated cells arranged in strands along the stem, and all three have walls that contain cellulose.

Differences:

• Xylem vessels are dead, hollow, lignified and without end walls, and they carry water and mineral ions upwards and support.
• Sclerenchyma fibres are dead, with narrow lumens and very thick lignified walls, and they give support only.
• Phloem sieve tube elements are living, with cellulose walls and sieve plates, and they carry assimilates in either direction.

In a stem the xylem is on the inside of a bundle, the phloem is outside it and the sclerenchyma cap is outside the phloem.

1. Cut a very thin section of root, stem or leaf with a sharp blade and keep it in water.
2. Stain it, for example with toluidine blue (lignified walls blue-green, cellulose pink or purple) or phloroglucinol (lignin red).
3. Mount in water and lower a cover slip at an angle to avoid bubbles.
4. Focus on low power and draw a plan diagram of tissues only. Then use high power to draw a few cells.
5. Calibrate the eyepiece graticule with a stage micrometer for each objective lens.

Worked example: if 100 µm on the stage micrometer covers 40 graticule divisions, one division = 2.5 µm. A vessel 14 divisions wide is 14 × 2.5 = 35 µm.

Magnification = image size ÷ actual size.