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Holidays & Seasons        < Previous        Next >

 

Autumn Leaves: Where Did the Green Go?

 

Today's Snack: Make a salad by tearing up lots of bits of green lettuce and mixing with grape tomatoes cut in half, little bits of chopped orange pepper, and small chunks of cut-up yellow squash. Have a little ranch dressing on the side. Stir it around so that the colorful bits of veggies fall to the bottom of the bowl, and all you can see is the green lettuce. Now eat all the green lettuce, and voila! You can see the red, orange and yellow veggies! That's the same thing that happens with autumn leaves, going from all green to a wide variety of colors!

 

 

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Heyyyyyyy! One day, the leaves on the trees are glossy and green. Then, all of a sudden, in the fall, they all change colors radically. And next thing you know, they've completely fallen off!!!

 

But that's not bad news. That's GOOD news. Little kids believe that when it finally gets cold in the fall, "Jack Frost" goes around with a paintbrush and paints all those leaves to brilliant orange and yellow and red, in a matter of a few days.

 

But he had a lot of help: science!

 

And you can learn how the basic scientific process called "photosynthesis" - how plants use light to make their own sugar to use as food - is affected as the seasons change from summer to autumn to winter.

 

"Photo-" refers to light (as it does in photograph) and "synthesis" refers to combining or manufacturing something. So "photosynthesis" is the word used to explain how a plant's leaves make food for the plant using sunlight as the energy source to combine carbon dioxide from the air, water, and certain salts.

 

With a little razzle-dazzle called " watercolor chromatography," you can see how chlorophyll - the pigment inside leaves that gives them their green color - exists alongside other pigments. Chlorophyll is the other ingredient that is necessary for photosynthesis to take place.

 

Now, "pigment" doesn't have anything to do with pigs! A "pigment" is a chemical substance that creates color. Our skin has pigment; various paints and inks have pigment; well, green leaves have pigment, too.

 

The thing is, as long as it's warm weather outside and sunlight lasts for many, many hours, in the spring and summer, the chlorophyll is the only pigment that shows in the leaves on trees. All the leaves are green. It is the "dominant" (meaning strongest or most prominent) pigment while photosynthesis is going on and the leaves are making food for the plant.

 

But when the cool weather comes around in mid- to late autumn, and it stays dark longer in the morning, and gets dark earlier in the evening, then photosynthesis begins to shut down. That's because the sunlight that produces the energy that is necessary for the food-making process is no longer as available.

 

When photosynthesis starts to fade away in the fall, and there isn't as much sunlight to trigger photosynthesis in the leaf, then the OTHER pigments besides chlorophyll can finally show their colors. Chlorophyll is no longer so dominant. Another factor is that cold temperatures bring out the other colors, especially the reds, in the leaves, and that's why we see so many oranges and reds in the colder states the further North we go.

 

So here are two easy experiments to help you see how this works:

 

 

 

1.      Watercolor Chromatography

 

"Chroma-" means a measurement of the intensity of a color, and "-tography" means to make a record. So just as "photography" uses light and chemicals to create a photograph that is a record of something or someone, "chromatography" is a way to create a record of the colors that are within something.

 

It's the type of procedure that's used in science laboratories to separate things so that they can be studied. So this is a great introduction to an important science skill.

 

You will need:

 

  • coffee filters
  • water-based markers
  • small, clear glass jars (baby food jars or pimiento jars!), test tubes or other small, clear, glass containers

 

Cut the coffee filter into strips about 2 inches by 6 inches. Use a marker or markers to color in a thumb-sized dot 1 inch from the bottom of the paper. Black works particularly well, as does a combination of colors marked on top of each other. But you can try this with several colors in different intensities to see what happens with each.

 

Pour in just enough water to cover the bottom of the container, and gently insert the paper until its bottom edge just touches the water. Let the paper sit undisturbed for several minutes. The water will "wick" or suck its way up to the marker dot, and continue moving beyond it into the dry part of the filter paper. As it does that, it will be bringing with it a separation of the colors from the dot.

 

The black color masks the reds, purples and yellows that are in a black marker color but not visible. It's the same way that xanthophyll (pronounced "ZAN throw fill," a yellow or orange pigment) and carotene (pronounced "CARE oh teen," a red, orange, yellow or brown pigment) in leaves are overshadowed by chlorophyll. Note that there's a lot of carotene in carrots and other fruits and vegetables that are orange, and the reason birch leaves turn bright yellow is that they are rich in carotene.

 

The formation of the reddish, blue or purple pigment -- anthocyanin (pronounced "AN the SIGH uh nin") -makes the most spectacular, red autumn leaves. That's why maple trees might be the most spectacular trees in your yard or the park in the fall, bursting bright red. The anthocyanin is produced when sugar builds up in leaves because of drought or other environmental stresses. When too much anthocyanin is present, leaves appear purple or red, rather than their normal color.

 

When autumn is in full force and photosynthesis begins to shut down, something mysterious happens inside each tree that we don't fully understand. It will make a membrane - a thin coating that is like a living barrier -- between the tree branch and the stem. This membrane gradually blocks the flow of nutrients to the leaf. It's as if the membrane puts the tree on a diet! But in turn, with less "demand" from the tree for food, it causes a decline in chlorophyll production in each leaf, and the green in the leaf begins to fade. With less chlorophyll, the other pigments that have always been in the leaf, but just not visible, are able to finally show.

 

 

2.      Leaf Color Experiment

 

Now let's get out into the real world and study this some more! It's best to do this in the autumn. You will need:

 

        Two or three green leaves

 

        Two or three leaves that have already changed color

 

        One small glass jar per leaf (you can lay out the coffee filters from the first experiment to dry, on paper towel, and rinse out and dry those jars for this experiment) and matching lid

 

        A blank address label or masking tape, and a marking pen

 

        Rubbing alcohol

 

        Plastic knife or spoon

 

        A big shallow pan

 

        Access to hot tap water

 

        Clock, stopwatch or kitchen timer

 

        Coffee filter and scissors, cut into one long strip per jar; label one end with the name of each leaf with ballpoint pen or marker pen

 

        Masking tape

 

 

Put a leaf next to each empty jar. Label each jar with the label or masking tape and marking pen with the name of the tree that the leaf came from (maple, river birch, oak, etc.). If you don't know, make up a nickname for that tree and look online at a tree identification website later. Also mark what color the leaf is - green, red, orange, yellow, purple, or whatever.

 

Now rip each leaf into little bitty pieces into the bottom of its jar. Make them as small as you can. Don't spill! Don't mix up the pieces! Do this one at a time.

 

Pour enough rubbing alcohol into each jar to cover the bits of leaf. Using the plastic knife or spoon, carefully grind and smash the bits of leaf inside the jar so that they're practically powder and dissolved in the rubbing alcohol. This is called "pulverizing," or grinding something into dust.

 

Put the lid on each jar loosely - don't screw it on very tight, because you want a little air to come and go.

 

Fill a large shallow pan with one inch of hot tap water. Carefully place the jars in the water. Leave the pan next to the sink on the counter or someplace where it's close to the water supply. Let the jars sit for 30 minutes or more. They should turn dark, or even black. The darker the color gets, the better.

 

Every five minutes, shake and jiggle each jar, and check the water temperature. If it's not very hot, you can quickly but carefully take the jars out, dump out the lukewarm water, put the jars back in the pan, and refill it with hot water.

 

After 30 minutes, take the lids off, and plunge one end of the labeled coffee filter strip into the alcohol solution in the jar. Fold over the tip that's sticking out of the jar, and tape it to the jar.

 

Now let 30 minutes to two hours pass, and watch the colors divide and creep up the filter.

 

When you think you've gotten as much color creep as you're going to get, take each filter out of the jar and carefully tape it on another piece of paper, separate from the other ones. Don't let colors drip on other filters!

 

Once dry, analyze - or study - the patterns you see.

 

Did the green leaves produce the same color patterns?

 

Did the red one produce a pattern that is more similar to the yellow one, or a green one?

 

Which leaf had the most hidden colors? Why do you think?

 

Which leaf had the least hidden colors? Why?

 

 

By Susan Darst Williams www.AfterSchoolTreats.com Holidays & Seasons 28 2008

 

 

 

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