In September, we officially say farewell to summer as the autumnal equinox ushers in the first day of fall. Equinox (say: ee-KWUH-nox) comes from the Latin word aequus (equal) and nox (night). Around the equinox, which occurs once in spring and once in fall, there are nearly equal hours of day and night. So during the autumnal and vernal (spring) equinox, daytime and nighttime both last 12 hours! After the autumnal equinox, the hours of sunlight each day slips away due to the earth's tilt and orbit around the sun. The days get shorter, the temperature starts to drop and fall settles in, eventually giving way to winter.
In addition to losing sunlight, autumn, also called fall, reveals many other changes as well. The most obvious change occurs with leaves. Although leaves look different in the fall than other times of the year, those flaming reds and yellows are actually there all along—we just can't see them! Different chemicals in leaves cause them to change colors in the fall. Chlorophyll is the chemical responsible for leaves' green color as they make food through photosynthesis during the spring and summer months. But in autumn, when it gets cooler outside and the sun doesn't shine as long each day, trees begin 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 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! Even though the other colors were there all along, we couldn't see them because the green from the chlorophyll blocked them.
Chlorophyll and the other chemicals that cause the colors in leaves are called pigments and are also used to the dye thread and fiber that clothes are made from. Other colors in leaves 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. Anthocyanins take credit for colors like red and purple, but not all leaves have it. 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!
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 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!
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.
Equinox - the two times a year when the sun crosses the equator and all over the world, day and night are of equal length.
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.
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