I love the fall. It’s my favorite season. I love sweatshirts and campfires. I love having the windows open. I love breathing cold, crisp air again after practically drowning in humidity with every breath in the summer. I love the feeling of change. And the leaves are nice, too. Apparently, some of you think the leaves are really spectacular and you may even travel all over to see them. Unfortunately for me, I’m a little “color deficient”, and I miss a lot of the red color. I see the yellows and oranges, particularly on the maple trees, but most of the reds just sort of blend in. Oh, well.
Photo: US Forest Service
So, to add to the enjoyment of all you fully-color-competent leaf-peepers out there, I thought I might try to heighten your appreciation of all those autumnal fireworks by explaining the basics of what is going on in all of those gazillions of leaves to provide you with your show.
As most of you know, leaves exist mostly to absorb sunlight to produce energy for plants. They do this through a truly, truly amazing process called photosynthesis. Even the word “photosynthesis” is cool. Like most science terms, it is from the Greek—photo, meaning “light” and synthesis, meaning “putting together”—so photosynthesis is “putting together using light”. Visible light is actually a specific range of wavelengths of electromagnetic energy. I’ll do an article on electromagnetism, soon, because it is one of those scientific subjects that comes up over and over again. Anyway, “visible light” is a form of energy. We’ll leave it there for now. Plants have developed the ability to absorb that energy and use it to power the production of energy-containing molecules (sugars, mostly) and other structural molecules, like cellulose, they need to survive and grow.
Plants can take carbon dioxide from the air and water from (usually) the soil and, using the energy from sunlight, covert them into sugars and oxygen. The plants then use the sugars for their own energy needs (plant cells use energy just the same as animal cells do) and release oxygen into the atmosphere as a waste product. Each leaf is, in effect, a little solar cell combined with a self-contained factory for producing sugars. Pretty amazing stuff. I’m going to have to do an article on photosynthesis, soon, too.
“Visible light” is called visible light because, well, it’s visible to us. We have developed sensory organs that can detect electromagnetic waves in this particular wavelength range. The “white light” that we generally see is actually a mixture of all the wavelengths in the visible spectrum. You’ve undoubtedly seen what happens when you pass white light through a prism (or a crystal, or a lot of raindrops)—the white light splits into a band of colors. What we see as red are electromagnetic waves at the longer-wavelength, lower frequency end of the visible spectrum. Violet is at the other end of the spectrum, with the shortest wavelengths and highest frequency. All the yellows, greens and blues lie in between. All of those various wavelengths of light carry energy that photosynthetic organisms can use to synthesize sugars.
In order to be able to use the energy in light, plants have to be able to absorb the light. Now this part gets a little tricky, so try to stay with me. When we see a thing—anything—what we actually see is the light that the object we are seeing is reflecting and that our eyes are detecting. So when we see a leaf that appears green to us, it’s because it’s REFLECTING green light, which we can see. That leaf is ABSORBING all the other wavelengths of light—all the reds, blues, yellows and violets. If it wasn’t absorbing the other colors, it would reflect them all, and it would just look white. If it absorbed all the colors, it wouldn’t reflect anything, and it would appear black. Look up “photosynthetic absorption spectrum” on the internet to help understand this.
You’ve probably heard of chlorophyll, which you’ve understood to be the green pigment in plants. So, here’s a question for you. Does chlorophyll absorb green light or reflect it? Think on it a minute. I’ll wait…Okay, good. It REFLECTS green light, which is why things look green. There are actually a couple of different varieties of chlorophyll. One of them absorbs light in the red end of the spectrum and the other absorbs light more up toward the blue and violet end. Both of them, however, REFLECT all the wavelengths around the green range. Have you ever looked at a grow-light–a light bulb specifically designed to foster plant growth–and noticed the color of the light? You can also see this if you look at a high-tech greenhouse. The light will look a little blue-ish. That is because plants absorb light in the red range and in the blue range best. Of the two colors (red and blue), blue has the most energy. I’ll explain why that is so when I do that article on electromagnetism. For now, just trust me when I say blue light is more energetic than red light. Can you guess what would happen if you used green light in your greenhouse? Correct. Your plants would all die, because the leaves can’t absorb green light—they reflect it—so they wouldn’t be able to get any energy from it and they wouldn’t be able to conduct photosynthesis.
What does all this have to do with the leaves turning all red, yellow and orange in the fall? Okay. There’s a little bit more to the story. In previous articles, I’ve mentioned how efficient nature is, in several different ways. White light contains all the colors of the visible spectrum, and chlorophyll absorbs in the red and blue. That means that all the other wavelengths of light are not absorbed by chlorophyll. That is inefficient, so nature doesn’t like that. Therefore, many plants also produce a few other photosynthetic pigments, called accessory pigments. They generally absorb more in the green-ish wavelengths, in between the reds and blues. These additional pigments allow plants to harvest energy across more of the wavelengths of visible light than chlorophyll could by itself. If the accessory pigments absorb in the green, what colors do you think they might reflect? Exactly! Why do you think the leaves look green then? It’s because there is SO much more chlorophyll than any other pigments in the leaves. The green masks the other colors. In the fall, chlorophyll products stops and the green pigments start to break down. As the green pigment goes away, it allows the other colors to be visible. That means the reason the leaves turn is not (mostly not) because the leaves start making red and yellow pigments—it’s because the red and yellow pigments that were always there are revealed when the green goes away. There is a little more production of the red and yellow pigments in some plants in the fall, perhaps to take advantage of the change in the quality of sunlight in the fall, compared to the higher, brighter sun of summer, but mostly it’s just the green breaking down.
Some plants have normally reddish or yellowish leaves. A lot of these, like Japanese maples, are used as ornamental plants, because they add color to the landscape and often grow in low light conditions. Usually, these plants are species that developed in particular environments where the quality of the light is different. For instance, in a dense forest, the canopy of leaves above filters out most of the blue light and passes the green. Plants that are going to thrive in this greenish light had better be good at using the energy in that light. Green pigments might not be as good as red, so the leaves might be redder or yellower on these understory plants.
Doesn’t knowing WHY the leaves turn make watching the process more fun? The leaves of fall are still as beautiful, but now they might, I hope, seem a little more magnificent, because they are truly wonders of nature.