Introduction: (Initial Observation)
As we study plants we notice that plants seem to “know” which way to grow. Germinating seedlings emerge above the soil and grow upward. On a window sill, plants turn toward light. Observations that downward growth of root (toward water) and upward growth of the shoot (toward light) are critical to a plant’s survival lead to questions such as:
- Why do plants grow upwards?
- Why do plants grow towards light?
- Is upward growing of a plant a response to light or gravity?
Find out about plant growth. Read books, magazines or ask professionals who might know about the effect of light and gravity on plant growth. Keep track of where you got your information from.
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.
What makes the plant to grow upward? light or gravity?
The purpose of this project is to design and perform an experiment to test whether gravity or light are the reason for plants to grow upward.
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.
Two variables that may affect the direction of plant growth and we will test them in this project are:
These are called independent or manipulated variables.
Direction of plant growth is the dependent or responding variable.
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. Hypothesis is an educated guess.
My hypothesis is that gravity makes plants grow upward, not light. This hypothesis is based on my daily observation of plants that get light from any direction other than above, but they still are growing upward.
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.”
Design an experiment to test the effect of gravity on the direction of plant growth.
- Use two young healthy and identical plants with straight stems for this experiment (you can also plant seeds and use seedlings for this experiment).
- Lay one of the plants on the side. This will be your experimental plant.
- The other plant will remain upright. This will be the control plant.
- Cover both plants wit empty boxes so they will receive no light.
- Check both plants after 24 hours. If the plant that you are testing are not very young, it may take more time for the plant to grow and change its direction upward again. In this case, keep the plant on the light, but cover it somehow that it only gets light from the sides, not from the above. In this way any upward growth will not be related to the light.
Originally, the pot was upright and the plant growth was vertical. After the pot was laid on its side, over time, the stem begins to grow upwards, instead of straight out of the pot (which would have been to the right here).
Note importantly that the plant does NOT bend. The new direction resulted from growth. Unlike bending/straightening an elbow, growth is not reversible.
For this experiment we also use two young plants. Use an electric lamp as a light source. Place the plants in two different shelves or two different levels and place the lamp between the two plants. In this way, one plant will get light from the above and the other will get light from below. Check the plant growth and the direction of stem every day for two weeks. This experiment must be performed in a dark or low light space so that the light bulb is the main source of light.
If the light is the cause of the plants growing upward, in this setup the stem and leaves of upper plant must bend down towards the light.
- Take pictures and make drawings of your experiments and use them for your display.
- Try to use your test plants and your experiment setup also as a part of your display.
- If you have enough time and you want to plant seeds for this experiment, get at least four small pots, some potting soil, and plant at least four seeds in each pot.
- Young plants and seedling respond much faster to light and gravity.
Need a graph?
If you need a graph, you must also record the plant height before and after the experiment for both plants. By having the initial height and final height for each plant you can calculate the change in plant height for both plants. Changes in plant height may be positive or negative. When the plant height increases 25 millimeters, this value is positive and you show it as +25. If the plant height decreases 12 mm, then you show it as -12.
With this information you can have a bar graph to visually display your experiment results.
Make a bar graph with 2 vertical bars, one bar for each plant. The height of each bar represents the changes in plant height for that specific plant. If the value is positive, the bar goes upward. If the value is negative, the bar goes downward.
Under or over each bar write its name so they can be distinguished. For example you can write “Light from above” or “Light from below”.
Materials and Equipment:
List of material can be extracted from the list of experiments.
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.
You may need to calculate the amount of plant growth or the change in plant height. Write your calculations in this section of your report.
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.
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.
List of References