1.2 Controlled Experiments and Modelling a System
With a desire to introduce students to practical work as soon as possible and knowing that growing seeds is a surprising novelty, I designed this activity for the start of the year and the Practical Scheme of Work (PSOW).
Students had to work in small groups (pairs in my class) and design an experiment to test the effect of one factor on the germination and growth of cat grass seeds.
Here's a download of the instructions.
- Seed trays
- Cat grass seeds or similar
- Tap water
- Measuring cylinders
- Lights / light banks / growth chambers
- Solution of diluted liquid fertiliser
- general compost
- Balance (scales)
- Design and start an experiment that will test the impact of changing ONE (abiotic) factor on the germination and growth of your seeds.
- Make sure you understand the difference between germination and growth once the seeds have germinated.
- Make sure you record your plan. You should plan to collect enough data to be able to calculate a mean and standard deviation for each condition (5 different conditions).
- Design a table in which to record your results.
- You will need to decide how you are going to check if the plants need more water before your next practical class.
- The seeds will normally take between 2 and 7 days to germinate then a few days to grow.
- Count the number of seeds that have germinated after your teacher tells you to do this (perhaps one week later).
- Measure the height of the leafy structure.
- Measure the length of the roots.
- Record your data in a table and calculate the means and standard deviations for each condition.
- Plot a graph of your findings.
- Follow your teacher's instructions, if they would like to see a report on your findings or just check your graphs.
- Produce a system diagram(s) that represents this experiment.
Approaches to Teaching
I shouldn't have been, but I was surprised that not one of my new students knew what germination was. I was chatting away, explaining the task and could see the look in their eyes. Luckily they admitted this too so I gave them the background information I've drawn out above.
I emphasised that I would be looking for controlled experiments, that collected enough data to calculate means and standard deviations (minimum of 5 seeds per trial) and that I was looking for at least 5 conditions to give us the magic 5 by 5 for their experimental design. Apart from this, I was looking for the students to ask questions to clarify their strategies. "How can I control everything and only change one factor?" "How will we have enough data?" "Am I really only changing one factor?" I encouraged them to ask these questions.
One group only managed to compare presence or absence of sunlight and so I told them that this was ok but I'd be using it as a teaching tool when we looked at the data.
During the evaluation of the experiment, they should be asking questions like, "what went well?", "what could I do differently?".
By doing this you are encouraging students' metacognition.
Important outcomes of this introductory experiment (you will need about 2 hours for planning, execution and data collection), are a reminder of experimental design, data collection and presentation and using this experiment to make a system model of plant growth.