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It’s fall! That means shorter days and cooler nights. And, in many parts of the United States, it means that trees are shedding their leaves after a final colorful salute to summer.
Fall is a great time of year for learning about the process that gives life to trees and most other plants. As you know, this process is called photosynthesis, which literally means “putting together with light.” As winter nears, less sunlight and less water — elements essential to the process of photosynthesis — will be available to trees. That means less food for deciduous (leaf-shedding) trees! Soon the trees’ photosynthesis “factories” will shut down, the trees will recapture nutrients from their leaves, the leaves will drop to the ground, and the tree will rest until spring when water and light return and awaken the process.
Did you know that a tree’s leaves are orange and yellow in addition to green — even in summer? Most leaves include the pigments of all three colors. It’s just that the pigment chlorophyll (the pigment that makes the leaves green) is a much stronger and more abundant pigment than the others. It covers the yellow (xanthophyll) and orange (carotene) pigments that are natural to a tree’s leaves. But, come fall, a tree’s leaves produce less chlorophyll as their photosynthesis "factories" shut down. Now the other colors can show through in brilliant explosions of color!
Another pigment (anthocyanin), which produces reds and purples, isn’t present all year long in most green leaves. It only shows up as the nights get cooler, and many scientists think it functions like a "sunscreen." It may be surprising to learn that a tree's leaves cannot absorb an unlimited amount of sunlight. Some leaves will get the equivalent of a "sunburn" if they get too much sun. When this happens, the leaves get stressed out and may eventually die. When trees are breaking down and reabsorbing important nutrients from their leaves in the fall, their photosynthetic tissues are especially unstable and vulnerable to too much light and other stresses. Yet trees need the energy from photosynthesis to drive the processes that allow them to recapture as many of those nutrients as possible. Just as this reabsorption process begins, leaves start producing large amounts of anthocyanins near the leaf surface. By blocking some of sunlight's harmful UV rays, scientists hypothesize that anthocyanin pigments help to protect the leaves’ dwindling ability to generate energy during this period.
Why do leaves fall off the tree? The roots and branches of most trees are tough enough to endure freezing temperatures, but leaves are more susceptible to damage when the water inside them solidifies and expands. This kind of damage to living tissues can expose a tree’s internal plumbing system to the open air, increasing the chances of infection and speeding up water loss. To ensure the tree’s survival, any part of the plant that is unable to live through the winter must be sealed off and shed before freezing temperatures set in. As the days get shorter and the amount of available sunlight decreases, the vessels that carry water to and from the leaves gradually close off. A “separation layer” of cells forms at the base of each leaf stem. When the separation layer is completely formed, the connection to the branch is weakened and the leaf soon falls off the tree. (Interestingly, evergreen trees do not lose their needles in the winter because they are covered with a heavy wax coating and contain fluids inside that resist freezing.)
Now that you better understand why trees change color and lose their leaves in the fall, you will likely never look at those colorful displays in the same way again. Here's to a wonderful season of learning!
ANSWER THE QUESTIONIn some cases, only about half of a tree’s leaves turn red and the other half remain green. Why might this occur?
Using the information provided in the essay, construct an argument to explain why this might occur. Cite evidence from the essay and provide reasoning in support of your argument.
Your answer must include:
the claim you are making to explain the observation. (4 pts)
the evidence from the essay that supports your claim. (4 pt)
the reasoning that links the evidence to your claim. (4 pt)
Claim: Only about half of a tree’s leaves turn red while the other half remain green due to variations in the presence and production of the pigment anthocyanin.
Evidence: The essay states that the pigment anthocyanin produces reds and purples in leaves, and it only shows up as the nights get cooler. It is also mentioned that a tree's leaves start producing large amounts of anthocyanins near the leaf surface during the reabsorption process.
Reasoning: The presence and production of anthocyanin pigment in leaves are influenced by external factors such as temperature and light exposure. As the nights get cooler, the production of anthocyanins increases, resulting in some leaves turning red. However, not all leaves may experience the same levels of cooling or exposure to trigger the production of anthocyanins. Therefore, only about half of the tree's leaves may turn red, while the other half, experiencing different conditions, remain green. This variation in the presence and production of anthocyanin pigment can explain why only about half of the leaves change color.