Track A Plant’s Movement

Though you may not realize it, every day the plants around you are moving…all by themselves. Even though most plants have roots that bind them to the surface they grow on, plants are able to stretch, grow, and bend to adjust to changes in their environment.

Some plants, like Venus flytraps, can move quickly, but most move or change position so slowly that it’s difficult to tell that they are moving at all. However, by filming plants or recording their position over a long period of time, we can see just how much they move and get clues about why. Watch the Science Friday video “Unwinding the Cucumber Tendril Mystery” to learn more about how subtley plants move.

In this activity, you’ll monitor the position of a houseplant to find out whether or not it changes position in response to a change in sunlight.

1. Position your houseplant on a stable surface near a window, but don’t let it touch the window. If your houseplant already lives near a window, rotate the pot a quarter turn. You may already notice that your plant was “leaning” towards the window. That is totally fine (and cool!); just rotate the plant so that it is leaning away from the window.

2. Gently place a couple of toothpicks in the soil on the same side of each large stem (or bunch of stems) of your plant, being careful not to stab the plant. You will use these toothpicks to tell whether your plant is actually changing position in the pot.

3. Next, tape one end of a blank piece of paper up to the window in front of the plant so that you can record the plant’s position. 

4. Find a leaf tip or the stem tip closest to the window. Carefully mark the spot that you found with a bit of nail polish, marker, paint, or tape. You are now set to measure your plant’s position in two ways!

5. To measure your plant’s distance from the window, hold a centimeter ruler perpendicular to the window so that it touches both the window and the spot you marked on your plant. Measure how far the painted spot is from the window. Record your measurement in a data table you keep near your plant.

   *Note: most rulers have extra length at each end, but don’t worry! Because these measurements are being repeated with the same ruler and compared to one another, you do not need to account for the extra length in your measurements.

6. Keeping the ruler perpendicular to the paper, mark on the paper where the bottom edge of the ruler touches. Write the date and time next to your mark so you’ll be able to tell it apart from future observations. If the leaf moves from side to side or up and down, the marks on the paper will help you trace its path.

7. Return hourly or daily to measure your plant’s movements. Between measurements, it is important to let as much light reach the plant as possible without moving the paper. In order to keep the paper in position but still allow sunlight to reach the plant, flip up the paper and apply a second piece of masking tape to hold it in this “up” position. When you are ready to make another observation, flip the paper back down.

8. After you’ve made a few observations, take your paper down to see how the marked leaf moved. Did it move closer to the window? Has it moved up, down, or sideways? Why do you think it moved this way?
9. When you’re done with your leaf observations, take a moment to observe the position of the plant in the pot by looking at where it is in relation to the toothpicks you placed earlier. Did the plant move in the pot? Describe the position of your plant relative to the toothpicks in the notes section of your data table.

As you probably noticed in your experiment, the leaves and stems of your plant were able to move, but the position of the plant in its pot stayed the same. That’s because plants can’t travel from place to place—their roots anchor them in the ground. Yet, a plant can respond to environmental changes by altering the texture of stem and leaf parts and growing leaves in specific directions. Many plants perform these movements, called tropisms.

One of the most common tropisms is what you observed in your own houseplant. It’s called phototropism, and it happens when plants move towards sunlight. Everywhere plants grow, sunlight provides the energy for photosynthesis, the chemical reactions necessary to convert water and carbon dioxide into oxygen, which animals breathe, and glucose, which the plant uses as food. Without sunlight, plants would not be able to produce the food they need to survive. Phototropisms allow plants to maximize sunlight exposure on their leaves by orienting them towards the sun. Plant phototropism can be so extreme that some plants, such as sunflowers, will actually change their orientation to track the position of the sun in the sky over a single day!

Want to try to observe a different plant tropism? Let’s try gravitropism, which is a plant’s growth response to gravity. Seed sprouts demonstrate gravitropism when they are deep underground. In other words, gravitropism is the reason why plants grow up! If you want to see a plant grow against gravity, carefully rest the pot of a young plant on a couple of soup cans or a box. Make sure the pot is tipped at a slight angle. Repeat the experiment above and see how your plant responds to the relative change in the direction of gravity!

Gravitropism (also called geotropism) in maritime pine (Pinus pinaster) filmed for the first 24 hours after the plant was tilted. From Herrera et al 2010, CC-BY-2.0)

• 4-LS1-1 Construct an argument that plants and animals have internal and external structures that function to support survival, growth, behavior, and reproduction.
• MS-LS2-1 Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.
• MS-LS1-5 Construct a scientific explanation based on evidence for how environmental and genetic factors influence the growth of organisms.