Complex permanent tissues: xylem and phloem

Tissues made of several kinds of cells

Complex permanent tissues are made of more than one type of cell, working together as one unit. The two complex tissues carry fluids through the plant; they are the vascular tissue.

Xylem

Xylem carries water and minerals from the roots up to the stem and leaves. Its parts are:

• Tracheids and vessels: long, narrow tubes of dead cells with thick walls and no contents. Water flows in them; the vessels are wider.
• Xylem parenchyma: living cells that store food.
• Xylem fibres: dead, thick-walled cells that give support.

Most of the xylem is dead, and that helps: water flows with no cell contents in the way, and the hard walls give strength. Wood is mostly xylem.

Phloem

Phloem carries the food made in the leaves to all parts of the plant, up and down. Its parts are:

• Sieve tubes: living cells joined end to end. The walls between them, the sieve plates, are full of pores.
• Companion cells: small living cells with a nucleus, beside the sieve tubes, which they help.
• Phloem parenchyma: living cells that store food.
• Phloem fibres: dead cells that give support.

All the cells of phloem are alive, except the fibres.

How does water climb?

Water in a very thin tube is pulled up by capillary action. The thinner the tube, the higher the water rises:

where $T = 0.0728\ \text{N/m}$ is the surface tension of water, $\rho = 1000\ \text{kg/m}^3$ its density, $g = 9.8\ \text{m/s}^2$, and $r$ the radius of the tube.

You should find that even a tube of radius 2 µm lifts water only about 7 m. So in a tall tree, capillary action is not enough. The loss of water from the leaves, transpiration, pulls water up, and it is the main force.

Try it in Python

Practise with new numbers

Common mistakes

• Saying water moves in the phloem. Water and minerals move in the xylem; food moves in the phloem.
• Saying xylem carries things down. It carries them up only; phloem goes both ways.
• Thinking dead xylem is a problem. Empty dead tubes let water flow easily.
• Thinking capillary action lifts water to the top of a tree. Transpiration is the main pull.

Remember these

• Xylem: water and minerals, upward, mostly dead cells (vessels and tracheids).
• Phloem: food, up and down, living cells (sieve tubes and companion cells).
• Water rises higher in a thinner tube. Transpiration also pulls water up in a tall tree.

Check yourself

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

1. Which tissue carries food from the leaves to the roots?

2. Why is it not a problem that most xylem cells are dead?

3. What is the use of companion cells?

4. In which tube does water rise higher, one of radius 10 µm or one of radius 40 µm?