Xylem vessel structure and function

Xylem vessels are a type of plant tissue that transport water and dissolved mineral salts from the roots up to the stem and leaves, while also giving the plant mechanical support.

A xylem vessel is really a chain of dead cells joined end to end into one long tube. Questions on this structure usually test whether you can link each feature, dead, hollow, lignified, no cross-walls, to the transport of water, and whether you can explain that water is pulled up rather than pumped.

Parts and functions

How structure suits function

The xylem vessel is adapted to carry a continuous column of water upward with little resistance and to support the plant at the same time. Being dead and hollow clears the way for water, the broken end walls make one long tube, and the lignin both waterproofs and strengthens the wall.

Each adaptation links to one part of the vessel's job, and an examiner awards the mark for the link, not for the feature alone.

• Dead, hollow and empty, no cytoplasm or nucleus blocks the lumen, so water flows with little resistance.
• End walls broken down, adjacent cells form one continuous, unbroken tube from root to leaf.
• Lignified walls, waterproof, so water is not lost sideways, and strong enough to resist the tension of the water column without collapsing.
• Narrow lumen, helps water rise by capillary action and keeps the water column continuous.
• Pits in the wall, let water move sideways to neighbouring vessels and to living cells.
• Rings and spirals of lignin, support the plant while still letting a young stem bend and grow.

Related processes

Water is transported through xylem by the transpiration stream. Water evaporates from the mesophyll cells of the leaf and diffuses out through the stomata as transpiration.

This loss lowers the water potential of the leaf cells, so they draw water from the xylem. Because water molecules are cohesive, sticking to one another by hydrogen bonds, and adhesive, sticking to the vessel walls, the whole column is pulled up the xylem in one piece under tension, which is the cohesion-tension mechanism.

Water enters the plant through the root hair cells by osmosis and crosses the root to the xylem, where root pressure can also push it a short way up. As the water rises it carries dissolved mineral ions, such as nitrate and magnesium, to the stem and leaves.

Alongside transport, the lignified xylem gives the plant mechanical support, holding stems upright and, in trees, forming the wood that supports the whole plant.

Common labelling errors

Worked question

Question. The diagram shows part of a xylem vessel from a plant stem. (a) State two features of the vessel that adapt it for transporting water, and explain each.

(b) Name the process that provides the main force pulling water up the xylem. (c) Explain why water travels up the xylem as a continuous column rather than in separate drops.

(d) Suggest why a plant with damaged xylem wilts on a hot day.

Model answer. (a) The vessel is dead and hollow with the end walls broken down, forming a continuous tube with no contents to block the flow, so water flows freely; its walls are lignified, which is waterproof and strong, so the vessel does not collapse under the tension of the water column. (b) Transpiration (the transpiration pull).

(c) Water molecules are cohesive, sticking to one another by hydrogen bonds, and adhesive, sticking to the vessel walls, so the column stays unbroken and can be pulled up as one. (d) On a hot day transpiration is fast, so water must be replaced quickly; damaged xylem cannot carry enough water to the leaves, so the leaf cells lose turgor and the plant wilts.

Source:SRC-DSKP-EN

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