Coming from a place where the leaves on the trees turned brilliant colors each fall, I guess having the calendar say that it is fall made me yearn for some color. Is there any better way to get children outside than to go check if any tree leaves are turning their autumn colors yet?

During your trip you or your children may have some questions about the whys and wherefores of leaves turning yellows, oranges, reds, and maroons.

Most trees have green leaves during the growing season. Look closely, however, and you will notice not all the leaves on a tree will be the same color. New leaves will be a lighter, brighter green. Older leaves will be dark green. Leaves that are about to fall off are yellow.

How do scientists study the colors in plants? One way is to perform a simple color separation or “chromatography.” The materials that reflect or absorb certain colors when subjected to light are called pigments. In the following test, we are looking for the pigments found in growing plant leaves.

The chromatography does require some time, at least an hour, so prepare your children in advance. It also requires rubbing alcohol. I tried to use water as a solvent instead and it just didn’t work.  Note: If you are working with small children around, make sure they won’t try to taste the mixture, because of the rubbing alcohol.  Read and follow the warnings on the label carefully.

You will need:

  • A spinach leaf per child (spinach is full of pigments)
  • Small clean containers, like a yogurt cups or jars  (paper leaks) for one per child
  • Tool to chop the spinach leaves (requires adult supervision)
  • Rubbing alcohol (adult should pour)
  • Stirring utensil that will fit in containers
  • Coffee filter cut into about ¾-inch-wide strips that will fit into the container from top to bottom, one per container

Chop up the spinach leaves into tiny pieces. Put the chopped bits into the bottom of the containers. They should cover the bottom in a layer, up to ¼-inch deep. An adult should pour in just enough rubbing alcohol to cover the leaf bits. Stir a few seconds. Slide the coffee filter strip into the container, so that the bottom touches the alcohol/leaf mixture. Rest it against the side. Now wait for about an hour.

The alcohol should slowly move up dry part of the coffee filter, carrying the pigments as it goes. The heavier pigments will travel more slowly, the lightest most quickly. Once the wet alcohol front nears the top of the strip of coffee filter, remove it. Check the colors. Lay it on a piece of paper towel or paper and let it dry a bit. Often the colors of the pigments will show more intensely once the alcohol has evaporated.

Your coffee filter might look like this:

 

The green bands at the bottom are chlorophyll pigments. The plant uses chlorophyll to convert the sun’s energy into food. If you look closely, there are two bands of green, with some yellow. Those are the two chlorophyll pigments, named chlorophyll a, and chlorophyll b.

Just above the green is an intense yellow band. The yellow is a group of pigments called xanthophylls. Xanthophylls make corn yellow. When chickens eat plants with xanthophylls, they can use it in their eggs to make the yolk yellow.

At the very top, is a thin orange band. Those pigments are carotenes, named for the first plant tissue they were isolated from, carrot roots.  Carotenes are also found in pumpkins and squash.

Now compare the colors in the growing spinach leaf to those of the autumn leaf. What colors are the same? What colors are missing in the leaves? What colors are in the leaves but not in the spinach?

 

First I notice very little green in the autumn leaves. The chlorophyll pigments break down in the fall leaves. I do see the oranges and yellows. The carotenes and xanthophylls have been there all along in the growing leaf, but masked by the deep green. When the green disappears, we can see them.

I see another color, however, that wasn’t present before, an intense red. The red pigments are called anthocyanins, and are known for making flowers, and vegetables like beets, red or purple. It turns out that certain trees make a lot of anthocyanins in their leaves in the fall. It wasn’t there earlier in the year.

Right now scientists don’t know for sure why certain types of trees produce the anthocyanins. One suggestion is that the anthocyanins act like sunscreen to help protect the leaves while the last bits of nutrients are being moved out and down to the roots for storage over the winter. Another suggestion is that red trees are less visible to insects. Harmful insects may lay their eggs on trees in the winter, and may choose trees that are yellow over those that are red.

In any case, aren’t the colors amazing?