In 1880 Charles Darwin and his son Francis published The Power of Movement in Plants. They lit canary grass seedlings from one side, and the young shoots bent towards the light. When they covered just the tip of a shoot with a little cap so no light reached it, most stayed upright, even though the part below was still lit. So they reasoned that some influence must travel from the tip to the lower part that curves. They had no idea what it was. In 1926 the Dutch botanist Frits Went collected that influence from oat shoot tips in small blocks of agar. We now call it auxin.
This lesson covers the plant responses required practical for AQA GCSE Biology Topic 5 (Homeostasis and response): investigate the effect of light or gravity on the growth of newly germinated seedlings. It's Biology only, so it isn't part of Combined Science.
What the practical asks you to do
You record results in two ways: as length measurements and as careful, labelled biological drawings that show the effects. AQA's teacher notes stress that this is about how germinated seedlings respond, not about the factors that affect germination, so the seeds can be germinated before the test begins.
The light method
AQA's method uses white mustard seeds, which are bigger and easier to handle than cress.
- Set up three Petri dishes, each with cotton wool soaked in an equal amount of water.
- Put ten mustard seeds in each dish and leave them somewhere warm, where they won't be moved, to germinate.
- Water each dish with equal amounts when needed.
- Remove seedlings so every dish has the same number, and measure each seedling's height in mm.
- Put one dish on a windowsill in full light, one in partial light, and one in a dark cupboard. For partial light, swap the dish between windowsill and cupboard on alternate days.
- Measure the height of each seedling in mm every day for at least five days. Rest the ruler on the cotton wool and hold the seedling gently against it.
- Calculate the mean height for each dish each day and plot mean height against day for all three.
Variables
- Independent variable: light intensity (or, in the gravity version, the direction of gravity).
- Dependent variable: seedling height in mm, plus what your drawings show.
- Control variables: the type of seed, the number of seedlings per dish, the volume of water, the temperature, and how long they grow.
What you should see, and why
Seedlings in the dark often grow taller at first, but they are thin and pale yellow because they don't turn green without light. So "tallest" doesn't mean "healthiest", and your drawings should show the colour and thickness as well as the height.
If light comes from one side, shoots bend towards it. This is positive phototropism. The explanation at GCSE is auxin: it collects on the shaded side of the shoot, cells there elongate faster, and the shoot curves towards the light.
The gravity version
AQA's handbook writes out only the light method. A common gravity version: lay the dish on its side, keep it in the dark so light can't cause bending, and draw the seedlings each day. Roots curve down (positive gravitropism, also called geotropism) and shoots curve up (negative gravitropism). In a shoot, auxin collects on the lower side and speeds up growth there, so the shoot turns upwards. In a root, auxin also collects on the lower side, but in roots it slows growth, so the upper side grows faster and the root curves down. A clinostat, which turns the seedlings slowly so gravity acts on every side in turn, is a useful comparison: its roots and shoots don't bend.
Common exam mistakes
- Putting auxin on the wrong side. In a shoot lit from one side, auxin is on the shaded side.
- Saying auxin always speeds up growth. In roots it slows growth.
- Calling the tallest seedlings the healthiest. Seedlings in the dark are tall, thin and pale.
- Giving "the water" as a control variable without saying what about it. Say "the same volume of water".
- Shading in a biological drawing. Use clear pencil lines and ruled label lines instead.
Frequently asked questions
What is the plant responses required practical in GCSE Biology?
It's AQA's required practical activity 8: investigate the effect of light or gravity on the growth of newly germinated seedlings, recording lengths and labelled drawings.
Why do seedlings grow taller in the dark?
Seedlings in the dark often grow tall and thin before they run out of stored food. They are pale because they don't turn green without light.
Does auxin make roots grow faster?
No. In roots auxin slows growth, which is why a root bends towards the side where auxin collects.
Is the plant responses practical in Combined Science?
No. AQA lists it as a GCSE Biology only practical.
Try it
Say each answer before you flip the card. Then put yourself in the Darwins' place: you've covered the tip, and the shoot stops bending towards the light. What would you have tested next?
Braineos is not affiliated with or endorsed by AQA.