Plant organ system: Xylem tissue transports water and mineral ions from the roots to the stems and leaves. It is composed of hollow tubes strengthened by lignin adapted for the transport of water in the transpiration stream.

The idea

Xylem is a plant transport tissue that carries water and dissolved mineral ions upwards from the roots through the stem to the leaves. Xylem cells are dead at maturity and form hollow tubes, so there is an uninterrupted column of water with little resistance to flow. Their walls are strengthened with lignin, a rigid material that prevents the tubes collapsing under tension and helps support the plant. Water moves through xylem in the transpiration stream. Transpiration is the loss of water vapour from leaves, mainly through stomata. As water evaporates from leaf cells, water is pulled up the xylem to replace it, and this pull, together with cohesion between water molecules, maintains a continuous column. Mineral ions dissolved in the water are carried along with it.

What earns the marks

• Xylem transports water and dissolved mineral ions from the roots to the stems and leaves.
• Xylem tissue is made of hollow tubes so water can flow through with little resistance.
• Lignin strengthens xylem walls, preventing collapse and helping to support the plant.
• Water moves through xylem in the transpiration stream.
• Transpiration is the loss of water vapour from leaves, mostly through stomata.
• Evaporation from leaves creates a pull that draws water up the xylem.
• Cohesion between water molecules helps maintain a continuous column of water in the xylem.

Common mistakes and how to put them right

• Saying xylem transports food or sugars.

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

• Saying xylem cells are living.

  Put it right by explaining that mature xylem cells are dead, leaving hollow tubes for water transport.

• Confusing lignin with a waterproof layer.

  Put it right by describing lignin as a strengthening material that prevents the tubes collapsing.

• Saying transpiration is the movement of water up the plant.

  Put it right by defining transpiration as the loss of water vapour from leaves.

Exam tips

• State the direction of transport clearly: roots to stems to leaves.
• Link each structural feature of xylem to its function, for example hollow tubes to unobstructed flow.
• Use the term transpiration stream when describing the movement of water through xylem.
• Mention lignin as a strengthening material, not as a waterproofing material.
• Include cohesion between water molecules when explaining how the column is maintained.