Fall Leaves

USDA Forest Service

We almost always think of trees as being green, but there is one time of year when their leaves turn a myriad orange, red, yellow and brown: the beautiful and chilly days of fall. Those living in the Eastern or Northern United States come to anticipate the change in color every year starting in September or October. What causes the leaves to change color, and why?

Just like many animals that hibernate for the winter, trees experience a unique change during the winter months. During summer, plants use the process of photosynthesis to transform carbon dioxide found in the air into organic compounds like sugars using energy from the sun. During the winter, however, there is less light to go around, and their ability to create food from the photosynthesis process is limited. What does that have to do with a leaf’s color? The substance that allows trees to turn carbon dioxide into food (chlorophyll) is also the cause for the leaf’s green sheen. As the photosynthesis process wanes in the colder months due to the lack of sun, so does its greenish hue, allowing other elements present in the leaf to show through. Believe it or not, the yellows and oranges that appear in fall have actually been there all year in the form of nutrients like carotene (also found in carrots). The intense green color of the chlorophyll had simply overshadowed them.

But what about the reds and browns? And what causes the leaves to fall away after they change color? The bright reds and purples in each leaf come from a strong antioxidant that many trees create on their own because of their protective qualities. The antioxidant helps protect the trees from the sun, lower their freezing levels, and protect them from frost. As winter comes, so does the need for the antioxidant (similar to the way a dog gets more fur during winter to stay warm).

As for the leaves falling, that is another story. At the base of each leaf, there is a layer of cells that carry food and water from the leaf to the tree during the summer months to keep it fed. In the fall, that layer actually starts to harden, preventing the passage of nutrients. Because of this, the nutrients and waste that previously passed from the leaf into the tree become trapped in the leaf with no fresh water to clean it. Not only does this cause the leaf to turn brown, eventually it causes the cells within it to harden so much that the leaf tears and blows away. Thus the pile of leaves you enjoyed jumping in as a child.

Because each tree, and each leaf, contains a unique amount of nutrients depending on how well-nourished it was over the spring and summer, the way each leaf breaks down during the winter months is also quite different. The result is the unique and complex facet of colors we see in neighborhoods and forest each fall.

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Egyptian Pyramids

Today, we have high-tech cranes and other machines to help us create massive skyscrapers and other modern works of architecture. Still, some of the most breath-taking architecture in the world, such as the ancient pyramids of Egypt, were created before those high-tech machines even existed. So how did those ancient civilizations create them?

Believe it or not, though they are one of the most studied and admired relics in history, there is no evidence to tell historians exactly how the Ancient Egyptians built the pyramids. Thus, they have been left to create their own theories as to how Egyptians created such amazing and awe-inspiring works of art.

According to one theory, the Egyptians placed logs under the large stone blocks in order to roll or transport them to the pyramid building location. Large groups of men would work to push or pull them into place (although historians also disagree on whether these men were slaves or skilled artisans). Still more, once the men moved the blocks to the pyramid location, they needed to lift them to ever-increasing heights to reach the top levels of the pyramid as it grew. Without modern cranes, many scientists have been baffled as to how they were able to do it. Some believe they used a ramp system that would allow them to roll the blocks upward around or through the pyramids; others believe they must have used a combination of pulleys and lifts. Still, most agree that once they did, they used a mixture of gravel and limestone to help fill any crevices and hold the mound together.

With such a primitive yet impressive building process, it’s obvious that the pyramids must have taken a great deal of time to build. With an estimated 2 million blocks weighing an average of 2.5 million tons each, the Great Pyramid of Giza, for instance, is estimated to have taken some 20 years to build. At 481-feet tall, it held the record of tallest building for 3,800 years – not bad for a building created almost entirely by hand.

Even though scientists don’t know exactly how the Egyptians did it, they do know that the method the Egyptians used to build pyramids changed over time. In the early days, the pyramids were made completely of stone, with limestone used to create the main body and higher quality limestone being used for the smooth outer casing. Later on, the pyramids were made mostly of mud brick with a limestone casing. Though they were likely much easier to build, they didn’t stand up nearly as well over time, leaving archaeologists with even fewer clues about their creation.

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From The Great Round World and What's Going On In It magazine