Plant tissues: xylem and phloem

The idea

Xylem and phloem are the two transport tissues in flowering plants. Xylem carries water and dissolved mineral ions from the roots to the stem and leaves; it is made of dead, hollow cells joined end to end, with lignified walls that also support the plant. Phloem carries dissolved sugars, mainly sucrose, from the leaves to other parts of the plant for use or storage; it is made of living cells, including sieve tubes and companion cells. In the stem these tissues often form vascular bundles, with xylem towards the inside and phloem towards the outside. A useful method is to compare a stained transverse stem section under a microscope: xylem appears as large open vessels, phloem as smaller cells near the edge.

What earns the marks

• Xylem transports water and dissolved mineral ions from the roots upwards to the stem and leaves.
• Xylem tissue contains dead, hollow cells with lignified walls, which help to support the plant as well as transport water.
• Phloem transports dissolved sugars, mainly sucrose, from the leaves to other parts of the plant for use or storage.
• Phloem tissue contains living cells, including sieve tubes and companion cells, and is arranged in vascular bundles with xylem.
• Transpiration and translocation are the processes associated with xylem and phloem respectively.

Common mistakes and how to put them right

• Saying that xylem carries food or sugars.

  Put it right by stating that xylem carries water and mineral ions, while phloem carries dissolved sugars.

• Describing xylem as living cells.

  Put it right by stating that xylem cells are dead and hollow, whereas phloem cells are living.

• Reversing the direction of transport in phloem.

  Put it right by stating that phloem carries sugars from the leaves to other parts of the plant, not from the roots to the leaves.

Exam tips

• Use the terms xylem and phloem accurately and link each to its transported substance and direction.
• When describing adaptations, connect hollow dead cells and lignin to water transport and support, and sieve tubes and companion cells to sugar transport.
• If asked to compare, give one clear difference and one clear similarity between the two tissues rather than listing unrelated facts.