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What Are Leaves For?
Leaves are very important for trees - they provide food for the whole tree (or plant)! How do they do it? Leaves use a very special process called photosynthesis (say: foto-SIN-thuh-sis) to convert energy from sunlight into sugars and starches that a tree uses as food. Leaves have an important chemical inside of them called chlorophyll (say: KLOR-uh-fil), which is what makes them green, and is also what allows them to do photosynthesis. You might remember learning about photosynthesis in the newsletter about flowers a few months ago. If you missed that issue, you can read it here.
Leaves also help keep you cool on hot days by making shade. The leaves fill in the spaces between the branches to make a canopy, sort of like an umbrella, over the tree. Leaves also help make trees good homes for animals, like birds, squirrels, and bugs by providing them shelter, a place to hide, and even food!
Why Do Leaves Change Colors?
Different chemicals found in leaves are what cause them to change colors in the fall. As we already learned, chlorophyll is the chemical that gives leaves their green color while they make food through photosynthesis during the spring and summer months. When it starts to get cooler outside and the sun does not shine for as long each day, trees know it is time to start storing up food for winter. To do that, the chlorophyll in the leaves starts to break down and the food that the leaves have been making is stored inside the tree instead of in the leaves. Now that the chlorophyll is gone, instead of being green, the leaves become all the pretty colors of fall, like orange, yellow, red, or even purple! These colors have actually been in the leaves all summer long, we just couldn't see them because the green from the chlorophyll was blocking them. Chlorophyll and the other chemicals that cause the colors in leaves are called pigments. Pigments are also used to dye thread and fiber that clothes are made out of. The other leaf pigments besides green come from chemicals called carotenoids (say: kuh-ROT-in-oidz) and anthocyanins (say: an-thuh-SYE-an-inz). Carotenoids make leaves yellow, orange, and brown and are always in leaves, just like chlorophyll. Not all leaves have anthocyanins, which cause colors like red and purple to appear. Anthocyanin is formed when sugar gets trapped in a leaf after the chlorophyll is gone. Then, when the leaf is exposed to sunlight, the anthocyanin turns leaves red and purple! The leaves in the picture have a little of each of the pigments in them.
Why Do Leaves Fall Off Trees?
As the weather gets cooler and the days get shorter in the fall, trees start to prepare for winter. Trees use sunlight to make a special layer or seal between each leaf and the branch it is connected to. Then the leaves fall easily to the ground, leaving the branches of the tree protected from the cold that will come in the winter and also helping the tree store up food! Since leaves have water inside their cells, they can't survive freezing temperatures, because the water would freeze and the leaves would die When leaves fall to the ground, they eventually break down and provide nutrients for the soil, helping prepare for more plants to grow in the spring and also create a layer that helps the ground absorb water. Another reason that trees lose their leaves is because many of them get torn or damaged during the spring and summer by weather (such as hail stones), insects that eat leaves, or diseases that trees can get. Since they lose their leaves in the fall, they will be able to grow brand new ones in the spring!
Deciduous Trees vs. Evergreen Trees
You may have noticed that some trees don't lose their leaves in the fall at all and that their "leaves" look rather odd compared to the leaves you see on most trees. Trees that lose their leaves in the fall or winter are called deciduous (say: de-SID-joo-us). Trees that do not lose their leaves are called evergreen and their leaves are usually called needles. Just like their name says, they stay green all year long, because their needles can survive freezing cold temperatures and do not fall off in the fall! The cells inside the needles are different than the cells inside leaves from deciduous trees - they do not have water inside them that will freeze and they also have a smooth waxy coating on the outside that helps them stay warm during the winter.
Photosynthesis - a process that happens in the leaves of plants where sunlight, water, and carbon dioxide (from the air) are converted into food and oxygen.
Chlorophyll - a chemical that is in leaves throughout the year and that helps them make food through photosynthesis. It is also what makes leaves green.
Carotenoid - a chemical in leaves that makes them yellow and orange. Just like chlorophyll, it is in leaves throughout the year, but it is not as strong as chlorophyll.
Anthocyanin - a chemical that comes from sugar that gets trapped in a leaf after the chlorophyll is gone. Anthocyanin is only in some leaves, and only in the parts that have a lot of water. It makes leaves red and purple when the leaves are exposed to sunlight.
Use this worksheet with the Take A Nature Walk science activity to help your kids practice keeping a science journal. They can create a page for as many trees as they want and keep them in a three-ring binder to create a fall tree file book.
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