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Why do plants grow towards light?

Why do plants grow towards light?

Introduction: (Initial Observation)

Have you ever seen a flower faced to the wall?
A quick observation reveals that plants (leaves and flowers) are faced to the direction with more light. But do plants really grow towards the light?
What if the plant is on the edge of a table and the only available light source is on the floor?
Trees grow upward. Is it because they are growing towards light or they are growing against gravity?
In this project you will investigate the direction of plant growth.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.  

Project advisor

Information Gathering:

Find out about plants growth. Read books, magazines or ask professionals who might know in order to learn about the factors that affect the direction of plant growth .

Involuntary response of an organism, or part of an organism, involving orientation toward or away from something is called tropism.

If you do more Internet search using the keywords tropism and plant, you will find that depending on the type of stimuli that causes tropism, we have different types of tropism. For example plant movement cause by light is called phototropism and plant movements caused by gravity is called geotropism. Also search to see what are thigmotropism and hydrotropism?

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to find out why plants grow towards light.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

The manipulated variable in our experiment is the direction of light. The dependent variable is the direction of growth.

In other words we want to see how does the direction of light affect the direction of growth.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

Sample Hypothesis:

My hypothesis is that somehow that side of the plant’s stem that is toward light, stops growing and the other side continues to grow. That makes the plant to bend towards light and continue to grow that way.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1: Light Affects the Growth of Plants

The tropical rain forests are very lush and dense with foliage. All of the plants which make up this dense foliage need adequate amounts of light to grow. This activity mirrors what happens in the tropical rain forest as plants compete for light in order to grow.

Materials: Shoe box, a sprouting potato, small flower pot or plastic cup (large enough to hold the potato), potting soil, water, 2 pieces of poster board or large index cards to fit inside the shoe box (see diagram below), tape (clear or masking), scissors or blade, space near a window or other source of light

potato sprout.gif (7703 bytes)

Procedure: Obtain a shoe box (with lid) and cut a hole (caution students to be careful, or do it for them), about 6 cm across in one end of the box.

Inside, place 2 cardboard or index card baffles (see diagram below). Place a sprouting potato in a small cup or pot with a little potting soil and water and place it behind the baffle, farthest from the light source. Place the lid on the box and place near source of light.

Lastly, have one sprouting potato out of a box and in the light for comparison.

This activity can be done by students individually, in pairs or in groups.


  • Sketch your plant and the plant which was grown outside of a box. Describe the differences.
  • Which plant looks healthier? Support your opinion.
  • In crowded living conditions, such as the plants experience in the tropical rain forests, do you think that some plants have adapted to survival with different amounts of light? Or do you think that all plants thrive on the same amount of light? Support your opinion.
  • Try to be very patient and leave your experiment set up long enough to see if the potato plant can grow all of the way out of the hole in the end of the shoe box. If you do this, sketch and describe what occurs. Offer your best explanation of what happens when the stem reaches the outside of the box.

How does a plant respond to light?

Experiment 2:

To find the answer to this questions we design a simple experiment to see how does the stem segment (called the “hypocotyl”) of a young seedling responds to the light. In this experiment we place a young seedling in a dark chamber with only one small window on the side to see how does the plant react to the light entered from the small window. We have decided to use black film canister as a dark chamber and punch a hole on it’s side as a window. You can use any thing else such as a small box or plastic cup for this purpose. If what you have is clear, cover it with aluminum foil or black paper so inside your chamber will be dark and the only light source will be the small hole that you make. Here are some details:

Procedure (what to do):

First you’ll need to grow some seedlings for the experiments.

If you’ve never done this before, it is very easy to do and seedlings can be grown in a greenhouse or in relatively warm and well-lit areas, such as near a window.

  • Place a layer of paper toweling at the bottom of a container (or a seed tray or flowerpot) .
  • Add about 2 inches of potting soil and moisten the soil.
  • Sprinkle a few radish seeds on the top of the soil.
  • Add a thin layer of soil on the top.
  • Water again lightly to make sure that you don’t uncover the seeds.
  • Place Saran wrap on top of the container until the seedlings emerge (this may take a day or two).
  • Keep a pan of water under the container and water if needed from the top.

Preparing the Chambers

  • To make a “window’ in your chamber, punch a hole in the side of the film canister or any other dark box that you may have.
  • If you are using clear material for chamber (like a plastic cup), cover it with aluminum foil to make it dark inside.

Setting up the Experiment

  • Cover your plant with the dark chamber. Make sure the window of the chamber is open for some light to enter.
  • Leave the plants in this position for a few hours, then carefully open the chamber and look at the seedlings.
  • Water your plant if necessary, close the chamber and observe again after one day, two days, three days etc.
  • While you are waiting to observe the results, you can make predictions about what you expect to happen.

Making Predictions

Instead of just waiting to see what the plants will do, you can make predictions about the possible outcomes.

In the chambers with the “window” on the side

Do plants move as a response to gravity? (Geotropism)
(is recommended for higher grade students only)

Now we want to see the effect of gravity on plant movement (geotropism or Gravitropism).

Experiment 3:

In this experiment we want to see the effect of gravity on movement of plant.

Select one of your young seedlings with straight stem. Turn it on the side and cover it with a dark chamber with no windows. You want to see if the stem goes up or down as a result of gravity. We cover the plant by a dark chamber to make sure that any movement of plant is not caused by light.

Check the plant after a few hours, after a day and after 2 days. Report the results.

Materials and Equipment:

List of material can be extracted from the experiment section.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


No calculation is required for this experiment.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


Visit your local library and find books in Plant Biology. Review the books for chapters related to tropism. Include the books you review in your bibliography.