Phototropism experiments

Plants may give the appearance that they hardly move at all. However, in reality, plants are constantly moving; it’s just that these movements are usually too small for us to notice. All plant movements are known as “tropisms,” with phototropism describing how plants move towards light. Plants display two forms of phototropism; positive and negative. Positive phototropism (moving towards light) is often witnessed in the stems and negative phototropism (moving away from light) is often seen in plant roots. There are many simple experiments you can conduct on phototropism from the comfort of your own home.

What you’ll need

Most phototropism experiments can be carried out with little cost and with materials that are relatively easy to source. First, you are going to want a relatively fast growing plant to reduce experimentation time. Lima beans and corn are fast growing plants that you can pick up from most supermarkets and garden centres. You will also need plant pots (empty milk cartons or drinks containers work fine), potting soil, a cardboard box and of course a source of light. With those materials to hand you’ll be ready to conduct the following experiments.

Light worshippers

For this experiment you’ll need a medium to large plant pot, lima bean or corn seeds, a light bulb and a dark box. Plant your seeds in the pot and keep them stored in the dark box for a few days until seedlings start to appear. Next, cut a hole in the top of the box in the centre so that it’s just large enough to fit your light bulb through. With the bulb hovering over the centre of your plant pot, place the box in a dark room, turn your light on, and leave for around 15 hours. When you come back to check on your seedlings you should see them all bending towards (or "worshipping") the light. Now, open the box, let natural light hit the plants, and after a few hours they’ll return to a vertical position.

One direction

For this experiment all you’ll need is a plant pot, lima bean or corn seeds and a dark box. As with the above experiment, you’ll need to plant your seeds and store the pot in the dark box until seedlings start to appear. When they do appear, cut a hole in the top of one side of the box and place your plant pot on the other side of the box. Make sure you place your box in an area where lots of light will get through the hole you have made. After a day or two, you should start seeing the seedlings growing in the direction of the hole you have made, towards the light.

If you want to extend this experiment, gather several boxes and place different coloured transparent plastic filters over the holes on each box. Now you can also see how different colours of light affect plant growth by comparing the plants from each box.

The science

It’s always worth knowing the science behind these experiments, especially if you’re doing the experiments as part of a school project or to teach your kids. All plants contain a hormone called auxin. Auxin regulates phototropism by modifying the cells in plants, encouraging them to grow towards light (which is essential for photosynthesis and giving plants energy). Notice that in the light worshippers experiment plants return to a vertical position. This is because of the other tropisms that affect plant movement. In this case, gravitotropism (sometimes called “geotropism”) has become dominant. As the plants are bathed in light from all directions, gravitotropism takes over, returning the plants to their vertical position.

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About the Author

Stuart Withers is a psychology graduate and digital savvy freelance writer. He is a master of writing web-based content, has worked in-house as a writer for massive media companies like Northcliffe Digital, and loves nothing more than bashing keys frantically on a keyboard.