Introduction: (Initial Observation)
Think about all different plants that you have seen so far. What is their direction of growth?
For many years people have been wondering why do plants grow upward? Some believed that plants are moving upward because light is coming from that direction. Others believe that plants grow against the direction of the earth gravity. What do you think?
In this project you will perform an experiment to see the effect of light on the direction of plant growth and find out for yourself
Find out about plants and the factors that affect their growth. Read books, magazines or ask professionals who might know in order to learn about the effect of light on plant growth. Keep track of where you got your information from.
Plants live in a very slow world, in which time is measured in months and seasons, not hours and seconds.
Plants need light in order to grow. The process by which green plants use carbon dioxide, water and sunlight to make their own food is called photosynthesis. It comes from the words: photo, which means light, and synthesis, which means putting together. One by-product of photosynthesis is oxygen.
Did you know that plants can move?
They usually do so by slowly growing in one direction or another, towards light, the soil, water or away from other plants. These growth movements are called “Tropisms” and are all based on the same principles of “asymmetric growth” (a scientific term which means that plants grow unevenly on one side of the stem as compared to the other).
Scientists know what causes plants to move (factors such as light, temperature, chemicals & gravity) and roughly how this is achieved, but the details of the steps between perception and response are not fully understood.
Plants’ movements caused by light is called phototropism.
To help you understand the tropism do this simple experiment.
- Clasp your hands together in front of you, keeping your elbows together and your wrists nice and relaxed.
- Keeping your right hand still, push upwards with your left. Remember to keep your wrists relaxed.
Q: What happened?
Did your hands tilt to the right?
- Now do the opposite, keep your left hand still and push up with your right.
Did your hands tilt to the left?
You have just done what plants do every time they want to grow somewhere!
They stop growing on the side of the stem where they want to turn and keep on growing on the other side. This causes them to tilt in the direction they want to grow, just like your hands!
The purpose of this project is to see if plants really grow towards light. So we design an experiment that can positively prove or disprove our hypothesis.
If we decide to repeat our experiments with different plants or different color lights in order to compare the results, then different plants and different color lights are called variables. Primary students usually do not need to do those experiments and define variables.
Based on your gathered information, make an educated guess about the effect of light on plants direction of growth. Some possible hypothesis are as follows:
- Plants grow towards light.
- Plants bend towards light, but grow against gravity.
- Plants grow only against gravity and light has no affect on the direction of growth.
Remember that only after you complete your experiments, you can analyze the results and prove or disprove your hypothesis.
To test if plants really do grow towards light (phototropism) we suggest following experiment designs that you can pick from.
You need: a shoe box, a pot, some soil, bean plant, scissors, tape and glue.
Cut a window in one end of the box (where the shoe heel would go) and make two cardboard shelves that stick out from the sides. Plant a bean in a pot and put the lid on the box. The bean plant will grow around the shelves and make its way out the top window towards light. Don’t forget to water the plant. Click here for a picture.
You need: tomato plants with small leaves, so we suggest to start with tomato plant seeds.
You will need to use some of your tomato plants for this experiment. You can plant extra seeds to allow you to conduct additional experiments.
What to do?
Place the tomato plants in a sunny window. Over a period of several days, observe how the leaves of the plant move towards the light source.
Once the leaves are pointing towards the light source, rotate the plant and observe the changes.
Record the results of placing the plants in a sunny window.
Record the changes when the plant is rotated.
Additional optional experiments:
Put some plants in a dark room and observe the growth patterns.
Put plants under different colored lights and observe the results.
This experiment is a neat way to illustrate the ability of plants to detect and grow towards light. Even if there is a small amount of indirect light, the plant will detect it and grow towards it. The growth response of plants to light is called phototropism. Plant growth towards the light (the shoot) is called positive phototropism. Plant growth away from the light (the roots) is called negative phototropism.
To understand the concept of phototropism and what it means to plants.
One bean seedling in a pot, shoe box or cardboard box with dividers, scissors, tape
- Build a maze out of the box. Try to make the holes staggered. There should be one hole to the outside to let in sunlight.
- Place seedling in pot in bottom corner of box (away from holes). Put the lid on the box so that the only source of light comes from the hole in the top.
- Depending on the size of the maze, after 1-2 weeks the seedling should make it to the outside.
- Observations and discussion.
Material you need:
small pot filled with damp potting soil
blocks and spools for your maze
sprouting potato (To sprout a potato, leave it out at room temperature. It might take as long as six weeks for it to sprout, depending on where your potato is and the temperature in the room.)
Plant the potato in the pot filled with damp potting soil with most of the sprouts facing up.
Make a small, round, quarter-sized hole in the short side of the shoe box.
Put the potato plant in the shoe box opposite the hole.
Now put the blocks and spools in front of the potato to make an obstacle course.
Then put the box on a sunny windowsill with the hole facing the light.
Check your box after three or four days to observe the growth. After a while, you’ll see that the sprouts find their way to the light by going around the blocks and the spools. Because the plant needs light to produce food, it will always grow towards the light.
Once you’ve built a potato plant maze, try experimenting with other plants. Think of a question that you’d like answered. Like, “Do all plants grow the same way? How about a dried lima bean?” (Don’t try this with the kind of lima beans that come frozen or in a can.) Make a prediction and then test it. Write down your results and also email them to your project advisor.
Materials and Equipment:
Shoe box, pot, carton box, bean plant or seed, soil, fertilizer, scissors, tape.
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 project
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.
Phototropism is the natural inclination of plants to grow towards the light source. Plants will actually bend toward the window and thus need to be rotated or have supplemental artificial light placed so that the plant receives even light. Otherwise, the plant will have excessive growth on one side and little growth on the opposing side.
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.
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.