26 Xylem and phloem

We are learning how to:

• identify the xylem and the phloem in a plant stem

You have already seen how water containing dissolved minerals is absorbed by the roots and passes to other parts of the plant. However, there is also a flow of other substances in the plant.

The leaves use energy from sunlight to make food in the form of glucose. The glucose is then distributed to the other parts of the plant either to provide the plant’s cells with energy, or to be stored as starch.

The plant therefore needs two vessels to transport dissolved substances:

• The xylem transports water and water-soluble minerals from the soil to different parts of the plant

• The phloem transports glucose, proteins and other organic chemicals within the plant

In the root of a plant the xylem and phloem together form a vascular cylinder.

Inside the vascular cylinder the xylem forms the centre, which is surrounded by the phloem.

A cross section of a plant stem shows that the vascular cylinder forms a series of vascular bundles. These are arranged symmetrically around the stem but the pattern is different in different plants. Each vascular bundle consists of xylem cells towards the centre of the stem and phloem cells towards the outside. These cells are separated by vascular cambium cells.

The flow through the xylem vessels is upwards from the roots towards other parts of the plant. The flow through the phloem may be in either direction as substances are carried between the different parts of the plant.

Activity B 3.26.1

Arrangements of vascular bundles

Here is what you need:

• poster-sized sheet of paper

• different coloured card × 2

• scissors

• glue

Here is what you should do:

1. Cut 20 discs of two different colours and sizes from coloured card. 10 will represent the xylem and 10 the phloem.

2. Arrange the discs to show how the xylem and phloem are arranged in the root of a plant and in the stem of a plant.

3. When you are satisfied with your arrangement, glue your discs in place so your poster can form part of a classroom display.

Check your understanding

1. Fig B 3.26.5 shows a section across a plant stem.

Key terms xylem vessels carrying solution of minerals from the soil to different parts of the plant phloem vessels carrying solution of nutrients and other chemicals around the plant a) Name the parts labelled A, B and C. b) What is carried in: i) A ii) B? c) In which direction is the flow through: i) A ii) B? vascular bundle structure containing xylem and phloem vessels vascular cambium cells between the xylem and phloem in a vascular bundle