Growth of seedlings required practical

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This topic is for students studying the full biology course only

Investigate the effect of light or gravity on the growth of newly germinated seedlings.

Record results both as length measurements and as accurate, labelled biological drawings to show the effects.

Plants grow in directions that will best enable them to survive. Plant roots grow away from light and with gravity as this will most likely cause them to grow into soil, which provides the plant with minerals, water and anchorage. Plant shoots grow towards light and against gravity as this will cause them to to be exposed to the highest light intensities which is needed for photosynthesis and the production of sugars. However, newly germinated seedlings normally grow taller in darkness than in light because the lack of light suggests they’re underground and they need to grow through the soil to reach the light. To test this:

Place a layer of cotton wool in three Petri dishes so that it covers the base and reaches to the top of the dish
Put 10 cm3 of distilled water in each dish using a measuring cylinder
Use forceps to place 10 evenly spaced cress seedlings in each dish
Place one dish in a dark cupboard, the second dish in the corner of a room away from a window and the third dish on a window sill in bright sunlight - the three areas should be the same temperature
Leave the dishes for one week, watering each every day with the same volume of water to keep the cotton wool moist
At the end of the week measure the height of the 10 seedlings in each dish and calculate a mean average
Make accurate biological drawings of the seedlings, using the following criteria:

Experimental setup and assessment

- Use a sharp pencil
- Do not colour or shade the drawing
- Use continuous lines that don’t overlap
- Draw everything to the same scale
- Label everything you can identify

It is expected that the seedlings in the dark will grow the tallest, followed by the seedlings in the dim light and then the seedlings in bright light. The experiment could be made more reproducible if different numbers of lamps were used in dark rooms so that light intensity can be controlled more accurately.

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