Examine the reasons why leaves change color and fall from trees each year. You and your kids will see how easy (and fun) it is to explore the science behind the changing color of fall leaves using items you likely already have at home.
Most people list fall as their favorite season, and for good reason. The cooler temperatures bring relief from summer’s heat and humidity. Fall is the season for bonfires, warm sweaters, caramel apples, and pumpkin spice everything. But one of the most anticipated aspects of the season is the leaves changing color. It’s not uncommon for people to plan trips around locations where they can view the most spectacular fall foliage.
Have your children ever asked why leaves change color in the fall? It all has to do with the biological purpose of a leaf.
The Purpose of a Leaf
Each part of a plant is designed for a specific function.
A plant’s roots anchor the plant in the ground and absorb water from the soil. A flower houses the plant’s reproductive organs. The leaves of a plant function to make food. In an amazing process called photosynthesis, leaves use the energy of the sun to create sugar from water and carbon dioxide.
But photosynthesis takes more than just sunlight, water, and carbon dioxide. After all, if you put a jar containing water and carbon dioxide in a sunny windowsill, you won’t end up with any sugar. Photosynthesis takes place because the cells of the leaf contain the pigment chlorophyll, a bright-green pigment that gives leaves their green color.
During photosynthesis, chlorophyll creates sugar from water and carbon dioxide.
Where does the water and carbon dioxide come from? The water is absorbed through the plant’s roots. It is then drawn up the stem delivered to the leaves.
Where does the carbon dioxide come from? On the underside of leaves are tiny holes called stomata (from the Greek stoma, “mouth”). These little “mouths” can open and close to let in carbon dioxide. However, each time the stomata are open, water can escape from the leaf. In order to minimize water loss, leaves keep their stomata open during the day (when sunlight is available) and closed at night.
You can watch a video of stomata opening and closing as viewed using a microscope here:
The Right Conditions for Photosynthesis
Given what you know about the ingredients necessary for photosynthesis, what seasons do you think are optimal for it to occur? If you guessed spring and summer, you’re right.
During spring and summer, there is ample sunlight available during the long days to supply the energy needed for photosynthesis. Additionally, chlorophyll functions best within the temperature ranges found in spring and summer.
But as the summer draws to an end, the days start to shorten. Long before the temperatures drop, the amount of sunlight starts to decrease a little each day. Plants have circadian rhythms, and as they sense the waning hours of sunlight they begin making preparations for winter. Since the shorter days and colder temperatures mean that the conditions suitable for photosynthesis are ending, it is no longer worthwhile for the plant to continue expending energy and resources to keep the leaves alive. When we start to see leaves changing color in the fall, we are witnessing the trees preparing for winter.
The Role of Chlorophyll
What Causes Leaves to Change Color in the Fall? As fall approaches, trees seal off leaves from the rest of the plant in order to prevent water loss from stomata. Once the leaves are sealed off, the leaf stops producing chlorophyll. Additionally, any chlorophyll remaining in the leaves begins to break down. As the chlorophyll degrades in the leaves, the bright green color disappears. With the green pigment gone, the colors of the other pigments within the leaf can finally be seen.
In other words, those beautiful oranges and yellows that we associate with fall leaves are actually present in the leaves all along—they have just been masked by the green chlorophyll. With the chlorophyll gone, their true colors can be seen.
It’s easy to demonstrate that the orange and yellow pigments are present even in green leaves by doing some simple paper chromatography. You likely already have everything you’ll need to do this activity.
Hands-on Activity to Investigate Leaf Pigments
Paper chromatography is a procedure used by scientists to separate colored parts of a mixture. You can use this same technique to separate the different pigments within leaves.
- Leaves *
- Wooden spoon
- Glass or jar
- Rubbing alcohol (also called isopropyl alcohol)
- Aluminum foil or plastic wrap
- Hot Water
- White paper towel or coffee filter
- Fork or spoon
*If no green leaves are available where you are, you can use fresh spinach leaves from the grocery store.
- Remove any stems from leaves and use your scissors to cut leaves into small pieces and place in glass
- Add enough rubbing alcohol to cover your leaves in the glass (about 1 tablespoon). NOTE: rubbing alcohol is poisonous. Do not drink. Use caution around children and infants.
- Use a wooden spoon (or something similar) to grind and crush your leaves for at least 5 minutes.
- Cover your glass with aluminum foil or plastic wrap and place glass in a pot or dish. Add enough hot water to the pot or dish so that the bottom one inch of your glass is submerged in the water. Leave the glass in the hot water for at least 30 minutes.
- While the glass is sitting in the hot water, prepare your chromatography strips. Cut a long strip from either a white paper towel or white coffee filter approximately an inch wide and long enough to touch the bottom of your glass and extend over the top.
- Use a fork or spoon to remove the leaf pieces from your glass while leaving the liquid behind.
- Place your chromatography strip in the glass so that just the very bottom is touching the liquid. Try to avoid having the strip touch the side of the glass. Extend the strip up and over the edge of the glass. You can tape the top of the strip to the outside of the glass to hold it in place.
- Allow the alcohol to travel up the strip overnight.
- The next day, remove your strip from the glass and allow it to dry. It is as your strip dries that the colors will be revealed.
When I did this experiment, I was able to see green, yellow, and orange pigments in my samples taken from green leaves indicating that there are many different colored pigments present in green leaves.
You can take this further by performing this experiment on leaves from different types of trees. Do the leaves of different trees produce different pigments? If you can find a tree in which some of the leaves have started to change while others remain green, use chromatography to see how the types of pigments change as a leaf changes color.
This video does a great job reviewing why leaves turn color in the fall. It also explains why evergreens stay green all year long.
This is just one example of how science is working all around us in our everyday lives. Science doesn’t have to be confined to a textbook. As we make observations about the world around us, those observations can be the first step in exploring the science of our amazing world.
- Why Leaves Change Color
- Abscission: The Reason Why Leaves Fall
- Leaf Pigments
- Hands-on Activities to Study the Plant Vascular System
- Leaf Science Online Unit Study
Dr. Kristin Moon earned a doctorate in Molecular Genetics from the University of Florida. She left lab life behind to stay home and raise her two sons. Not only did she homeschool her own children from birth through high school graduation, she has taught hundreds of students in homeschool co-ops, science clubs, online academies, and live lab intensives. She blogs about the science of everyday life in simple-to-understand ways, and has developed several self-paced, online courses which can all be found on her website Kristin Moon Science. You can also find Kristin on Facebook, Instagram, and Pinterest—where she shares resources and ideas for helping others understand, teach, and love science.