Practical skill 15: Conditions needed for germination

Aim

To find out whether water, oxygen and a suitable temperature are needed for seeds to germinate.

Background

A seed contains an embryo plant and a food store. Germination is when a seed begins to grow into a new plant. The seed needs three conditions to germinate: water, oxygen and a suitable temperature.

The seed absorbs water and swells, and its enzymes become active and break down the stored food so that it can be used for growth. Oxygen is needed for aerobic respiration, which releases energy for growth. Enzymes work best at a suitable temperature: too cold and reactions are too slow, and too hot and enzymes are denatured. Light is not needed for most seeds to germinate.

To show that a condition is needed, take it away from one set of seeds and keep everything else the same. The tube with all three conditions is the control. It shows that the seeds can germinate, so that failure in the other tubes can be linked to the missing condition. Light is kept the same for every tube by keeping them all in the dark, because the inside of a refrigerator is dark.

Hypothesis

Seeds will germinate only when they have water, oxygen and a suitable temperature, because water activates enzymes, oxygen is needed for respiration to release energy for growth, and enzymes work too slowly in the cold.

Variables

Equipment

• 4 boiling tubes (150 mm × 25 mm) and a test-tube rack
• 40 mustard (or cress) seeds of the same type
• Cotton wool
• Water and a dropping pipette
• 250 cm³ beaker, heat source and heatproof mat to boil water (with a lid or cover to cool it)
• Vegetable oil
• Marker pen or glass-marking pencil for labelling the tubes
• Dark cupboard at room temperature (about 20 °C)
• Refrigerator (about 4 °C), dark inside
• Thermometer, −10 °C to +110 °C
• Hand lens (at least ×6) for checking small roots

Risk assessment

Method

1. Label four boiling tubes A, B, C and D with the marker pen.
2. Put a pad of cotton wool in the bottom of tubes A, B and D.
3. Use a dropping pipette to add water to the cotton wool in tubes A and D until it is damp but not dripping. Leave the cotton wool in tube B dry.
4. Place 10 seeds on the cotton wool in each of tubes A, B and D.
5. Boil some water, then let it cool completely in a covered beaker. Boiling removes the dissolved oxygen.
6. Place 10 seeds in the bottom of tube C, with no cotton wool. Fill the tube with the boiled, cooled water so that the seeds are covered.
7. Pour a thin layer of vegetable oil on top of the water in tube C. This stops oxygen from the air dissolving in the water again.
8. Put tubes A, B and C in a dark cupboard at room temperature (about 20 °C). Check the temperature with a thermometer.
9. Put tube D in a refrigerator, which is dark inside, at about 4 °C.
10. Check the tubes each day. Add a few drops of water if the cotton wool in tube A is drying out, and keep the cotton wool in tube B dry.
11. After 5 days, take out each tube and count the seeds that have germinated (a root or shoot is visible). Use a hand lens to check any small roots.
12. Record the number germinated in each tube and calculate the percentage of seeds that germinated. Wash your hands.

Results

Fill this table in as you go. Print the PDF for a copy to write on.

Drawing the graph

Bar chart: tube (A to D) on the x-axis, label 'tube', and seeds germinated / % on the y-axis from 0 to 100 (2 cm per 20%). Draw bars of equal width that do not touch, because the x-axis shows separate categories.