Practical skill 14: Phototropism and gravitropism in seedlings

Aim

To investigate how the shoots and roots of young seedlings respond to light from one side and to gravity.

Background

Phototropism is a response in which parts of a plant grow towards or away from the direction of the light source. Gravitropism is a response in which parts of a plant grow towards or away from gravity.

Shoots grow towards light and away from gravity (upwards). Roots grow away from light and towards gravity (downwards). These responses help the plant: the shoot reaches light for photosynthesis and the root grows down into the soil to take in water and mineral ions.

To investigate one response you must change only one factor. To test phototropism, the light must come from one side only. To test gravitropism, the seedlings must be in the dark, so light cannot affect the direction of growth, and their position relative to gravity is changed. A dish that is left upright, or lit evenly from above, is the control because it shows what normal growth looks like.

Hypothesis

Shoots will bend towards light from one side and upwards when a dish is turned on its side, because shoots grow towards light and away from gravity; roots will bend away from the light and downwards.

Variables

Equipment

• 4 clear plastic Petri dishes with lids, each with the base lined with cotton wool and a filter paper on top
• About 30 cress seeds
• Modelling clay to hold the dishes upright
• 2 light-proof cardboard boxes: one with a window 2 cm wide cut in the left-hand side only, one with a window cut in the top
• 2 more light-proof boxes (or one large box) to keep dishes C and D in the dark
• LED lamp (it gives out little heat)
• Ruler graduated in mm
• Protractor
• Marker pen or glass-marking pencil for labelling and tracing
• Water in a beaker and a dropping pipette
• Clear sticky tape

Risk assessment

Method

1. Label four Petri dishes A, B, C and D. Use a dropping pipette to damp the cotton wool and filter paper in each dish with 5 cm³ of water. It should be damp, not dripping.
2. Place 6 cress seeds, evenly spaced, in a straight line across the middle of the filter paper in each dish. Put on the lids and tape them in place.
3. Stand each dish on its edge, held with modelling clay, so that the line of seeds is horizontal. Keep all four dishes in a dark cupboard at about 20 °C for 2 days, until the roots and shoots are about 1 cm long.
4. Dish A (phototropism): put it in the box with the window in the left-hand side only. Stand the lamp 30 cm from the window so that light reaches the seedlings from the left.
5. Dish B (control for light): put it in the box with the window in the top, with the lamp 30 cm above, so that light reaches the seedlings equally from above.
6. Dish C (gravitropism): turn the dish through 90° so that the line of seeds is now vertical and the shoots point sideways. Put it in a dark box.
7. Dish D (control for gravity): leave the dish upright, in a dark box.
8. Leave all four dishes in the same room at about 20 °C for 3 days. Do not move them during this time.
9. After 3 days, take each dish out and lay it flat on a bench. Choose 5 seedlings in each dish and trace the outline of each shoot on the lid with a marker pen.
10. Use a protractor to measure the angle through which each shoot has bent from its starting direction (vertical for dishes A, B and D, horizontal for dish C). Record the angles to the nearest 1°.
11. Look at the roots in each dish and record in which direction they have grown (for example down, or away from the light).
12. Calculate the mean angle of bend of the shoots for each dish. 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 of mean angle of bend of the shoots (y-axis, label 'mean angle of bend / °', scale 0 to 90 with 10° per 1 cm) against the dish (x-axis, label 'dish', bars for A, B, C and D). Draw bars of equal width that do not touch, because the x-axis shows separate categories.