How do you do the plant responses required practical?

What does the plant responses practical investigate?

It tests how light or gravity affects the way young seedlings grow. Shoots grow towards light, which is called phototropism. Roots grow down with gravity and shoots grow up against it, which is called gravitropism (or geotropism). A plant hormone called auxin controls both.

This practical is only in separate GCSE Biology. If you take Combined Science (AQA Trilogy), you do not need it. This guide follows the light version, using cress or mustard seeds, which germinate within a few days. The gravity version is described with the method.

What equipment do you need?

For the light version you need seeds on damp cotton wool in three dishes, and a way to control where the light comes from: a dark cupboard and a box with a slit cut in one side.

• Cress or mustard seeds
• Three Petri dishes
• Cotton wool or filter paper
• Water, and a measuring cylinder or pipette to add the same volume to each dish
• A box with a slit cut in one side, and a lamp or a bright window
• A dark cupboard or a closed box
• A ruler marked in mm, and some thread for curved shoots
• Labels

What is the method?

Germinate seeds in three dishes, measure them, then leave each dish in different light for a few days and measure again. For the gravity version, fix germinated seeds to damp cotton wool pointing in different directions, stand the dish on its edge in the dark, and check a few days later: the roots bend down and the shoots bend up, whichever way each seed started.

• Put the same amount of cotton wool in three Petri dishes, dampen each with the same volume of water, and spread 10 seeds on each.
• Leave the dishes somewhere warm for a few days until the seeds germinate. Add the same volume of water each day so none of them dry out.
• Measure the height of each seedling and record it.
• Put one dish in a dark cupboard, one in the box so light reaches it through the slit, and one where the light is even all round.
• Leave them for the same number of days, still watering them equally.
• Measure each seedling again and note which way it is growing. Draw a few seedlings from each dish carefully, to show their shape.

What are the variables?

In the light version you change where the light comes from and measure how the seedlings grow. Everything else that affects growth stays the same.

How do you work out and present the results?

Work out the mean length of the seedlings in each dish, or the mean increase in length since your first measurement. Leave out seeds that never germinated, because they tell you nothing about light.

Worked example: five seedlings in the one-sided light measure 24, 27, 25, 30 and 29 mm. The mean is 135 ÷ 5 = 27 mm, and the range runs from 24 to 30 mm.

The conditions are categories, so draw a bar chart of mean length for each one, and use your drawings to show the direction of growth. Seedlings lit from one side should bend towards the light, and seedlings in even light grow fairly straight. Seedlings in the dark often grow tall and spindly, and turn pale yellow: with no light for photosynthesis, they are living on the food stored in the seed.

To explain the results with auxin: in a shoot lit from one side, auxin collects on the shaded side. The cells there grow longer than the cells on the lit side, so the shoot bends towards the light. With gravity, auxin collects on the lower side of both roots and shoots. In a shoot it speeds up growth, so the shoot bends up. In a root it slows growth, so the root bends down.

How do you make it accurate and safe?

Measure every seedling from the same point, such as where the shoot leaves the seed, to its tip. A ruler can't follow a curve, so lay a piece of thread along a bent shoot, then straighten the thread and measure it. More seeds in each dish give a more trustworthy mean, and equal watering stops one dish drying out faster than another.

This is a low-risk practical. Wash your hands after handling the seeds, and don't eat them. If mould grows on the cotton wool, don't open the dish: tell your teacher, because some people react to mould spores. Take care around hot lamps.

What do exam questions ask about it?

Many questions ask you to explain the results with auxin: which side it collects on, and what it does to the cells there. Others ask why the gravity version is done in the dark, or why every dish gets the same volume of water. An anomaly might be a seedling that dried out, or one shaded by its neighbours. You might calculate a mean length or a growth rate in mm per day, and a 6-mark question may ask you to plan the investigation.

An exam-style question we wrote: "Seedlings were grown in a box lit from one side. Explain why their shoots bent towards the light." A good answer: auxin collected on the shaded side of each shoot, so the cells on that side grew longer and the shoot bent towards the light.

Common mistakes

• Saying auxin collects on the lit side. It collects on the shaded side.
• Saying auxin always speeds up growth. In roots it slows growth, which is why roots bend down.
• Doing the gravity experiment in the light. Then you cannot tell whether light or gravity caused the bending.
• Measuring a curved shoot with a straight ruler. Use thread, then measure the thread.