Complex permanent tissues

Tissues made of many kinds of cell

A complex tissue has several kinds of cell, and they work as one unit. The two complex permanent tissues are xylem and phloem. Together they make the vascular tissue, the pipework of the plant.

Xylem

Xylem carries water and minerals from the roots to the stem and the leaves. It also gives the plant mechanical strength. It has four kinds of cell.

• Tracheids are long cells with tapering ends. The wall is thick and lignified. The cell is dead and has no cytoplasm. Water moves from one tracheid to the next through pits. Tracheids are the main water carriers in ferns and in gymnosperms.
• Vessels are long tubes. Each is built from many short cells called vessel members, joined end to end. The cross walls between them have openings called perforations, so the vessel is one open pipe. The members are dead. Vessels are a mark of the flowering plants, the angiosperms.
• Xylem fibres have very thick walls and almost no space inside. They are dead and give strength.
• Xylem parenchyma is made of living cells with thin walls. It stores food, such as starch and fat, and other things such as tannins. Its rays carry water sideways, across the stem.

Three of the four kinds of cell are dead. Only the parenchyma is living. A dead pipe is a good pipe: an empty tube carries water more freely than a tube full of cytoplasm.

Protoxylem and metaxylem

The first xylem to form in a young organ is the primary xylem. It is formed in two steps.

• Protoxylem is the xylem that forms first. Its cells are narrower.
• Metaxylem is the xylem that forms later. Its cells are wider.

Their place tells the organ.

• In a stem, protoxylem is towards the centre and metaxylem is towards the outside. This is endarch xylem.
• In a root, protoxylem is towards the outside and metaxylem is towards the centre. This is exarch xylem.

You should find that the two points swap places when you change from a stem to a root.

Phloem

Phloem carries food, mostly sugar made in the leaves, to every part of the plant. In an angiosperm it has four kinds of cell.

• Sieve tube elements are long tubes, joined end to end. Their end walls have many small holes, like the rose of a watering can. These walls are called sieve plates. A mature sieve tube element has no nucleus, but it is living and has a thin layer of cytoplasm at its edge.
• Companion cells are small, living cells that lie beside each sieve tube element. A sieve tube element has no nucleus, so its companion cell controls it. The companion cell has the nucleus for both. It also helps to keep up the pressure that moves the food.
• Phloem parenchyma is made of living cells that are long and pointed at the ends. It stores food, and resins, latex and mucilage. Most monocots do not have it.
• Phloem fibres are made of sclerenchyma cells. They are dead at maturity. They are found only in the secondary phloem. People use them: jute, flax and hemp are phloem fibres, also called bast fibres.

In gymnosperms the phloem is different. It has sieve cells and albuminous cells, in place of sieve tube elements and companion cells.

Protophloem forms first and metaphloem forms later, as in the xylem.

Xylem and phloem side by side

Try it in Python

The program lists the cells of xylem and phloem and picks out the ones that are living.

Practise with new numbers

Common mistakes

• Giving a vessel a nucleus. The vessel members are dead. They have no cytoplasm.
• Saying that xylem is all dead. Xylem parenchyma is living.
• Saying that phloem is all living. The phloem fibres are dead.
• Saying that a sieve tube element has a nucleus. It has none. Its companion cell has the nucleus for both.
• Mixing up endarch and exarch. In a stem the protoxylem is towards the centre. In a root it is towards the outside.

Remember these

• Complex tissue: more than one kind of cell. Xylem and phloem.
• Xylem: tracheids, vessels, xylem fibres, xylem parenchyma. It carries water and minerals. Only the parenchyma is living.
• Phloem: sieve tube elements, companion cells, phloem parenchyma, phloem fibres. It carries food. Only the fibres are dead.
• Protoxylem forms first. Stem: endarch. Root: exarch.
• Vessels are a mark of angiosperms. Gymnosperms have sieve cells in place of sieve tubes.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the four kinds of cell in xylem.

2. Which cell of the xylem is living?

3. What is a sieve plate?

4. Why does a sieve tube element need a companion cell?

5. What does endarch mean?