About this experiment
Plant four bean seedlings and look after each one differently. One gets everything it needs, the others miss out on light, water or warmth. Let three weeks pass in a few seconds and compare the plants. Pupils learn what plants need to grow well and how to plan a fair test, where only one thing is changed at a time.
Learning goals
- Name the things a plant needs to grow well: light, water, warmth and air
- Describe how a plant looks when it is short of light or water
- Explain why a seedling can grow for a while in the dark
- Plan a fair test by changing only one thing and keeping the rest the same
- Measure and compare plant heights over time
How to use it
- 1Before you start, predict which pot will grow best and which pot will do worst.
- 2Let the days pass. Stop at day 7 and day 14 and describe each plant: height, colour and leaves.
- 3Look at pot B in the dark box. Why is it tall but yellow? What will happen to it later?
- 4Read the fair test notes. Change pot C so that it tests too much water instead of no water.
- 5Make pot D the same as pot A. Why is it useful to have two pots kept the same?
- 6Use the chart to find on which day each plant was tallest.
The model
Assumptions
- This is a simple story model that shows what a class usually sees. The heights are typical of a bean seedling, not measurements of one plant.
- A little water each day keeps the soil moist. A flooded pot cannot drain, so the roots get no air and slowly rot. A pot with no water dries out in about five warm days.
- The seed holds food for about ten warm days. In the dark the seedling uses it to grow tall and thin while it looks for light, then weakens because it cannot make food.
- A cold fridge slows everything down: growth, water use and the use of the seed food.
- All pots have the same soil, air and seeds, and each pot is watered at the same time of day.
Try this in class
- Q1Which pot shows what plants need to grow well? How can you tell?
- Q2Why does the plant in the dark grow tall but not strong?
- Q3A pupil tests light by putting one pot in the sun with water and one pot in the dark with no water. Is this a fair test? Why not?
- Q4What would happen to a plant that is given too much water?
- Q5Why do farmers plant seeds in spring, not in winter?
Frequently asked questions
Do plants need soil?
Not always. Soil holds water and gives minerals and support, but plants can be grown in water with minerals added (hydroponics). The things a plant cannot do without are light, water, air (carbon dioxide and oxygen) and a suitable temperature.
Why is the plant in the dark yellow?
Plants only make the green pigment chlorophyll when they get light. Without it they cannot make food, so a seedling in the dark uses the food stored in its seed to stretch upwards, hoping to reach light. Scientists call this etiolation.
How can we do this in class?
Soak broad bean or cress seeds overnight and plant them in identical pots. Put one pot in a cupboard, leave one unwatered, keep one in the fridge and one on the windowsill, and measure the heights every few days. Cress shows results within a week.