Terrariums

This project requires students to design one or more experiments, bringing together observation of a natural process and data-collection and interpretation.  Depending on the variables studied, this could involve averages and surface area, as well as data collection, presentation and interpretation.

What does this project offer your club?

It provides ideas for a STEM club for up to a term, and would be a great project for a STEM club wanting to integrate Maths and Science with observation over a prolonged period.

Possible approach

Before setting up the Science experiment, students could be given this brief:


Then explain how they should make their terrariums using the empty plastic bottles and petri dishes.

If you are not a science specialist ...

The BBC programme How to Grow a Planet: Life from Light has useful background material on photosynthesis.  About 17 minutes into the programme, the presenter enters a sealed environment (similar to a Terrarium) to demonstrate how plants sustain life.

Photosynthesis is the process by which plants harness the power of the sun to create glucose, which they then use in their cells to release energy when they respire (just as humans do).
The process uses the raw materials of water and carbon dioxide, and the energy from the Sun’s rays turn them into glucose and oxygen.
This is summarised in the equation below:

Word equation
Symbol equation (unbalanced)
Symbol equation (balanced)
This process takes place in the green chloroplasts of the plant, which contain chlorophyll. The chlorophyll acts to trap the Sun’s rays, which provide the energy which the process requires.

Note: in a balanced equation, there are the same number of atoms of each element on both sides of the equation, so that it is clear where each constituent of the molecules formed has come from.  

Light absorption

Filters
White light is made up of the three primary colours Red, Blue and Green. Light from the sun contains all the colours of the spectrum (as seen in a rainbow).  A filter acts to absorb some colours but not others, for example a green filter will absorb all the colours except green.  The plastic of the terrarium acts as a filter.
Colours
We see an object as the colours of light that it reflects: a blue T-shirt, for example, reflects blue light but absorbs the other colours.

Possible Conclusions

You may wish pupils to make predictions before they start the experiment.  Although a green bottle which only allows green light through to the plants should stop the ‘useful’ light reaching them, since a plant mainly reflects green light, in practice this may not have a great effect on the plant growth. 
This may be many explanations for this:

• Are the bottles perfect filters?
• Is light the only thing that is having an effect?  Can plants germinate without light?

External Research

It has been shown (seen below) that the colour of light falling on a plant does have effect on how it grows.

Light colour and plant growth
Blue light helps leaves to grow.  Cool white fluorescent lights emit light high in the blue wavelength, so they are excellent at promoting leaf development.  The blue light is also useful for starting seedling growth.  Combining red and blue light encourages flowering.

Research by NASA suggests that plants can achieve optimal growth when exposed to LED (light emitting diode) lights composed of red (about 15 percent) and blue (about 85 percent) spectrum. 

If you are not a maths specialist ...

Data needs to be collected for a purpose, and that should be discussed prior to collecting the data to ensure that the right data is collected in an efficient way.  So before you start the data collection, decide which questions you want to focus on, for instance:

• What is the best method of determining which plants have grown the best?
• What maths do you need to use to help you?
• How and when will you record results?

The first question offers a lot of freedom to the pupils, which is why this can work as a very open project with different groups doing very different work.  Some of the suggested ways of measuring the best growth include colour, height, taste (do make sure that they grow edible plants if they want to taste the plants) and surface area.

The first question leads into the second.
For example, if a student wants to use height as a measure of ‘best growth’ are they going to use the tallest plant in each of the containers, or the shortest, or the average height, ...?  Will they exclude particularly tall or small shoots and look to gauge what they estimate the average height to be?  Things such as time and ease of method used could be discussed here.

The third question gets the pupils to think on the practicalities of running the experiment. Students need to draft a form to record their data, and trial it to ensure that it is easy to use and does actually help them to record the data they want. You may need to decide that observations will be done during a certain period of the day - say, the lunchbreak - rather whenever the students think about doing it.

When you have your data, it can be used in a number of ways for further work.  This might include graphs to display the data in some way, especially if you want to show the progress of the growth and make comparisons between different groups' terrariums.

If they want to use an average for comparisons, they should consider whether it makes more sense to use the mean (where you add up all the values and divide by the number of values), the median (the middle/central value) or the mode (the value which occurs most frequently)?
The mean has the advantage of making use of all the data, but the disadvantage that it is skewed by extreme values.  The median has the advantage of being unaffected by extreme values, but doesn't make use of all the data.  The mode has the advantage of being easy to find and the only average you can find for non-numerical data (like species type), but may not tell you anything very interesting.

If they quote an average, it is also good practice to quote the range of the values (this is the difference between the most extreme values) for numerical data or the number of different items observed in the case of non-numerical data.

Key questions

What do we want to find out?

How can we go about finding that out?

How are we going to record and display our data?  What does it tell us?

Other links

The background tab takes you to the resources used in the second STEM teacher inspiration day in March 2012, where you will find a whole section on terrariums, including ideas for incorporating maths and technology, as well as science.

More ideas links to ideas on how to design an experiment in plant science.

More advanced projects links to ideas for experiments to investigate the effects of different coloured light on plant phototropism.

Read: science is an article on phototropism.

Scientific data analyst links to an interview with a mathematician and statistician who tries to find patterns in large data sets in life science technology.