Without light, a plant can’t make its food by photosynthesis. Sunlight is so important to a plant that it will change the way it grows so that it points toward the light. This is called phototropism, from the Greek words for “light” and “turn.” You can see this with a houseplant: the leaves grow to point toward the window. If you turn it around it will eventually move to orient itself toward the window again.   How strong is this attraction to sunlight? Can a plant grow around obstacles to find the light? Time to find out!

What You Need:

  • Shoe box with a lid
  • A couple pieces of cardboard
  • Matte black paint (spray paint is easiest)
  • Tape
  • Small flowerpot or styrofoam cup
  • Potting soil
  • Bean seed

What You Do:

Setup for phototropism project

  • Have an adult help cut two pieces of cardboard that are as deep as the box and about 2/3 as wide.
  • Paint the inside of the box, the inside of the lid, and the two pieces of cardboard with black paint. This will help cut down on light reflection.
  • When the paint is dry, tape one of the cardboard pieces to the inside of the box so that it extends out into the middle of the box. (During the experiment the box will stand on its end — leave enough room for your flower pot or cup to stand below the piece of cardboard.) Tape the other piece of cardboard to the opposite side of the box a few inches above the first one.
  • Stand the box on its end and cut a small hole (about the size of a dime) in the top end.
  • Plant one or two bean seeds 3/4-inch deep in some damp potting soil in the flower pot or styrofoam cup. Place it in the bottom of the the box and put the lid on. (Make sure the lid fits tightly enough that no light can get in except through the hole in the top of the box.)
  • Keep your bean plant watered and check on it once a day to see how it is growing. Draw or take pictures of how it grows.

What Happened:

The bean plant grows towards the only source of light, the hole in the top of the box, even if it means growing around the cardboard obstacles you placed inside the box! The energy it needs to sprout and start growing was stored in the seed, but eventually that food will be all used up and the plant will need to make more through photosynthesis. The plant spends the energy from the seed trying to find light so it can survive.