Xylem

In the syllabus and the exam

Xylem is studied first within plant tissue organisation and then again in more depth under transport in plants, so questions on it can appear across two related chapters. Candidates must know that xylem vessels are dead at maturity, lack cytoplasm, and have walls thickened with lignin, and that these features suit its transport role: the hollow, lignified tube offers minimal resistance to the upward flow of water and mineral ions while the rigid walls resist collapsing under tension. A frequently tested comparison sets xylem against phloem in a table, requiring candidates to state that xylem transport is one-directional, from root to leaf, is unaffected by whether the plant is photosynthesising, and moves water together with dissolved minerals rather than manufactured food. A common error is describing xylem as a living tissue or claiming that it transports sugar; sugar transport is the role of phloem, and confusing the two, or their direction of flow, is one of the most frequent sources of lost marks in this section. Some candidates also forget that xylem forms a continuous pathway from root hair cells through the stem to the leaf veins, which is essential for explaining how the transpiration stream works as a single connected system. Candidates should also be able to state that xylem vessels are arranged towards the inner side of the vascular bundle in a typical dicotyledonous stem, with phloem positioned towards the outer side, since this arrangement is a common feature tested in labelled cross-section diagrams of a stem or root. This is also why a cut flower placed in coloured dye shows the dye travelling up through visible strands within the stem, a simple classroom demonstration candidates can cite as evidence for the position and one-directional nature of xylem transport.

Example

Comparing the two transport tissues, a candidate writes: 'Xylem consists of dead, hollow, lignified vessels that transport water and dissolved mineral salts upward from the root to the leaves in one direction only.'

Related terms

PhloemTranspiration streamRoot hair cell

Source:SRC-DSKP-EN