Vascular Tissues (Xylem and Phloem)

Xylem

Xylem is a complex tissue made mainly of vessels and tracheids, which are dead cells at maturity with no cytoplasm, joined end to end to form continuous tubes. Their walls are thickened with lignin, which makes them strong and waterproof and also provides mechanical support to the plant.

Xylem transports water and dissolved mineral salts from the roots to the rest of the plant in one direction only, upward.

Phloem

Phloem is a complex tissue made mainly of sieve tubes and companion cells. Sieve tube elements are living cells joined end to end with perforated end walls called sieve plates, but they lack a nucleus, so each is supported by an adjacent companion cell, which has a nucleus and provides the energy needed for transport.

Phloem transports manufactured food, mainly sucrose and amino acids, and can move substances in more than one direction depending on where they are needed.

Comparing xylem and phloem

How it is examined

Common questions ask you to identify xylem or phloem from a labelled diagram or micrograph, to link a structural feature (such as lignified walls or the presence of a companion cell) to its function, or to explain why xylem cells being dead does not stop them from transporting water.

Worked exam-style question

Question. Diagram P shows a cross-section of a plant stem with two vascular tissues labelled S and T. Tissue S consists of dead, hollow cells with thick, lignified walls, while tissue T consists of living cells with thin walls and perforated end plates, each next to a smaller cell with a prominent nucleus.

(a) Name tissue S and tissue T. (b) State the direction in which tissue S transports substances.

(c) Name the smaller cell next to each cell in tissue T, and state its function. (d) Explain why tissue S, although made of dead cells, is still able to transport water efficiently.

Model answer. (a) S is xylem; T is phloem. (b) Xylem transports water and mineral salts in one direction only, upward, from the roots to the rest of the plant.

(c) The smaller cell is a companion cell; it provides the energy and control needed by the neighbouring sieve tube element, which has no nucleus of its own. (d) Xylem vessels lose their cytoplasm and become hollow, lignified tubes at maturity; this hollow structure offers little resistance to the flow of water, so the absence of living contents is actually an advantage for transport rather than a problem.

Practice question

Try this. A ring of bark, including the phloem, is removed from around the trunk of a tree (a process called ring-barking), but the xylem beneath is left undamaged. Predict what would happen to the leaves above the ring over the following weeks, and explain why.

Exam tip

Key terms

These Chapter 16 terms name the tissues examined in this topic:

• Xylem, the vascular tissue of dead, lignified cells that transports water and mineral salts upward from the roots.
• Phloem, the vascular tissue of living sieve tube elements and companion cells that transports manufactured food.
• Meristem, a region of actively dividing, undifferentiated cells responsible for plant growth.
• Active transport, the energy-requiring process companion cells use to help load sugars into the phloem.

Source:SRC-DSKP-EN

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