Lessons 2 and 3…
Having tried to establish that plants are, indeed, living organisms, it’s time to launch into experimental investigation time.
I display a range of plant material – garlic, mint, ginger, chilli, basil, rosemary… what have they got in common?
All nice in the kitchen for adding flavour to food. OK, fine, but why do these plants have this ability? They’re not making these tasty compounds for us, but for themselves. Why?
We circle back to the idea of how plants defend themselves, not just against predators, but also against infection.
I then ask them to design an investigation to test the anti-bacterial properties of various plant extracts. I used to provide this method…
…but I now much prefer, where possible, to let them design the experiment for themselves. Learn by doing. Learn by making mistakes. So I demonstrate the technique – extracting the compounds, soaking the discs, being aseptic – but let them decide on the approach. Most of them will forget a control disc, many won’t do repeats, they won’t all think of all the necessary control variables – but my feedback will highlight these basic omissions and they’ll do better next time….
This is motivating and interesting and different and they clearly enjoy it….
… and it’s a practical that really works. Garlic – particularly if you put it through a garlic crusher – inhibits bacteria to a spectacular extent, as they’ll discover in the next lesson, creating vast halos of inhibition around the paper discs. Homework is to finish their experimental write up and then design an advert for a garlic flavoured toothpaste.
Collecting the results doesn’t take much time, so it’s time to move things on.
So I ask them if they remember their babies, the little brassica seeds that they planted a week ago. We take a look. These Rapid Cycling Brassicas are not misnamed – one week is quite enough to see dramatic results. Oh look, I say disingenuously, one lot of plants went under the light bank, but this lot got left at the back of the lab. Tut tut, careless me, hem hem….
The difference is dramatic and the impact immediate. There are – I kid you not – audible coos and gasps when I do my big reveal.
I get them to make careful observational drawings of the difference between this “inadvertent” experiment. This is always a good exercise in forcing them to look closely and carefully at something. I also get them to list the controls and to think about the measurable differences between the two treatments.
They produce beautiful drawings and we discuss the differences. Plants under light bank have….
- larger leaves
- more leaves
- thicker stems
- purple stems
- much taller stems
- hairier leaves
We’ve controlled temperature and air and number of seeds per module and type of soil and volume of soil and availability of water and type and volume of fertiliser and species of plant.
Yep, the only difference is the light. Simple conclusion: light = growth. More light = more growth.
I’ve yet to come across any other demonstration or practical that so quickly and effectively gets this vital learning outcome across. I would choose this over and above all my other photosynthetic teaching resources.
But where to go next….? Find out next week!