27 Movement of substances through the xylem and phloem

We are learning how to:

• describe the movement of substances along the phloem and the xylem of a plant

Both the xylem and the phloem appear to be long thin tubes, like narrow drinking straws, but their structures are very different.

Structure of the phloem

Phloem tissue consists of two main types of cells; sieve elements and companion cells, together with other types of cells that make up the structure.

Sieve elements are long narrow cells that join together to form a sieve tube. At the ends of each sieve element there is a sieve plate. Sieve plates are porous and therefore allow substances to flow between cells along the sieve tube.

Sieve elements have no nuclei but they do have thick rigid cell walls made of cellulose. These are needed to withstand the hydrostatic pressures that bring about the flow through the phloem.

Sieve elements would not be able to function without companion cells. The companion cells make the movement of materials through the sieve plates possible.

Substances can move in both directions through the sieve tubes. Movement is the result of hydrostatic pressure from the xylem.

Structure of the xylem

The xylem consists of stacks of dead cells, which are joined together to form tubules.

water and minerals no end walls between cells thick walls stiffened with lignin

Since the xylem doesn’t contain living cells it is not possible for the solution of mineral salts to pass up the plant by osmosis. Instead, the solution of minerals rises up through the xylem by capillarity. This occurs when liquids are in tubes that have very small diameters.

Activity B 3.27.1

Examining the tubes that carry water up a stem

Here is what you need:

• solution of red ink

• stick of celery

• sharp knife

Here is what you should do:

1. Cut off the bottom part of a fresh celery stalk and stand it in the red ink solution at the start of a lesson.

2. Near the end of the lesson take the stalk out of the ink solution and cut across the middle with a sharp knife.

3. Examine the cut part of the celery. Look for the cut xylem tubes which will look like red dots where the ink has been drawn up through the stem.

Check your understanding

1. Decide whether each of the following statements describes the xylem, the phloem, or both the xylem and the phloem.

a) Carries substances dissolved in solution.

b) Substances travel in two directions.

c) Substances can only travel by capillarity.

d) Contains sieve elements.

e) Consists of stacks of dead cells.

Interesting fact

Cut flower stems can absorb solutions that will alter the natural colour of the flowers.

Florists use this trick to make unusual floral displays.

Key terms

sieve tube a tube composed of sieve elements in the phloem capillarity movement of a liquid up a narrow tube