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Unlock This CourseIn this required practical you investigate the effect of light or gravity on the growth of newly germinated seedlings. You already know that shoots and roots respond to these stimuli. Now you collect your own evidence.
You record your results in two ways. First, as length measurements. Second, as accurate, labelled biological drawings that show the effects. Both are needed, because a number tells you how much growth happened and a drawing shows you which way the seedling grew.
Key terms:
Use seedlings that have only just germinated, such as cress or mustard seedlings. They grow quickly, so you can see changes within a few days. Choose one stimulus to test. The two set-ups below are the usual choices.
Put seedlings in a box with a hole or slit cut in one side, so light reaches them from one direction only (one-sided light). Put a second set of seedlings in the same conditions but with light from above or all round, as a comparison.
Fix seedlings to damp paper or cotton wool in a dish, for example with tape, so they cannot slip. Turn the dish on its side, so the roots and shoots lie sideways. Keep the dish in the dark, so light cannot affect the result. A second dish stays upright as a comparison.
Choosing only one stimulus matters. If you test gravity, light must not come from one side, or you will not know which stimulus caused the bend.
A bent shoot is hard to measure with a straight ruler. Lay a piece of thread along the shoot, then straighten the thread and measure it.
The thing you change: the direction of the light, or the position of the seedlings (upright or on their side).
The thing you measure: the length of the shoots or roots, and the direction of growth.
Keep the same: type and age of seedling, temperature, water, compost and time.
Put your results in a table with a title and units in the column headings. Work out the growth of each seedling, then the mean.
Five seedlings in one-sided light were all 12 mm long at the start. After 4 days they measured 30, 28, 33, 29 and 30 mm. Total length: 30 + 28 + 33 + 29 + 30 = 150 mm. Mean length = 150 / 5 = 30 mm. Mean growth = 30 - 12 = 18 mm.
Drawings show effects that numbers cannot, such as a shoot bending or a root turning. A good drawing:
Drawing the seedling before and after lets you compare them. For example, you can show that a shoot grew upright at the start and curved towards the light by the end.
Examiners often ask you to suggest improvements. Common problems are:
A further investigation could change one thing, such as the colour of the light or the position of the slit.
Students draw a shoot with no title or labels, or shade it in. They also forget to keep the dish in the dark during a gravity test, so light affects the result as well. Many forget units, and some change two things at once, which makes the results unfair.
A student investigated the effect of one-sided light on cress seedlings. They put seedlings in a box with a slit in one side and measured the shoot length at the start and after 4 days. They also made a drawing of the seedlings at the end.
(a) Name the independent variable. [1 mark]
(b) Give two variables the student should control. [2 marks]
(c) Give one improvement to make the mean growth more reliable. [1 mark]
(d) Describe two features of a good biological drawing. [2 marks]
(a) The direction of the light, or light from one side only. (1)
(b) Any two from: type or age of seedlings, temperature, amount of water, type of compost, time left. (1) (1)
(c) Use more seedlings, or repeat the investigation, then calculate a mean. (1)
(d) Any two from: sharp pencil lines, no shading, large and in proportion, title, straight label lines, labels on parts. (1) (1)
When asked for a control variable, name something specific such as "same temperature". Never write just "keep it fair".