Structure of transport tissues

Biology AS · Transport in plants. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Plants have two transport tissues. Xylem carries water and mineral ions up from the roots. Phloem carries assimilates such as sucrose. You must know where each tissue lies in a root, a stem and a leaf, and how its cells suit their job.

Xylem vessel elements are dead, empty cells with lignified walls and no end walls, so they form long open tubes. Phloem sieve tube elements are living but have no nucleus and very little cytoplasm, and their end walls are sieve plates with pores. Each one is served by a companion cell.

A plan diagram shows only the outlines of the tissues. No single cells are drawn.

Rules to remember

Root: xylem in the centre in a star or cross shape, phloem between the arms, endodermis around the outside.
Stem: vascular bundles in a ring, xylem on the inner side and phloem on the outer side.
Leaf: in each vascular bundle, xylem towards the upper surface and phloem towards the lower surface.
Lignin: strengthens xylem walls against collapse under tension and makes them waterproof.
Pits: gaps in the lignin that let water move sideways between vessels.
Companion cell: nucleus, dense cytoplasm, many mitochondria, joined to the sieve tube element by plasmodesmata.

Common mistake

Describing sieve tube elements as dead. They are living cells without a nucleus; only xylem vessel elements are dead.

Worked example

A transverse section of a plant organ is stained so that lignin is red. The red cells form a cross in the centre of the section, with small unstained cells between the arms of the cross. Which organ is it, and what are the unstained cells?

Lignified, red cells are xylem vessels.
Xylem in the centre in a cross shape is the pattern of a root, not a stem or a leaf.
In a root, the tissue between the arms of the xylem is phloem.

Answer: It is a root, and the unstained cells between the arms are phloem.

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