FALL

Why Leaves Change Color in Fall

Why do leaves turn color in autumn?  That's a very exciting question -- with an equally exciting answer!

Inside a leaf there are millions of little packages of color in green, yellow and orange.

• The green packages are called chlorophyll
• The yellow packages are called xanthophyll
• The orange packages are called carotene
  
  
  
  
  

  Autumn Leaves

  In the summer, the green chlorophyll packages are very, very busy.  The green packages in the leaves catch sunlight and, using it for energy, they change water from the ground and a special gas from the air ("carbon dioxide") into sugar ("glucose").  This sugar is the food for the tree. All summer long the green chlorophyll produces food for the trees.  Because the green chlorophyll is so busy, the green color covers up all of the other colors in the packages. The green packages of chlorophyll in the leaves need the water from the ground to do their job.  The water is soaked up by the trees roots, travels up the trunk and enters the leaves through tiny tubes in the leaf's stem.   For more information on where the water comes from, visit the water cycle. As fall approaches, the weather grows colder.  The tree realizes that winter is near and begins to get ready. A thin layer of cells grows over the water tubes in the leaves and closes them up in preparation for the winter.  No more water can get into the leaf! Without the water, the green chlorophyll starts to disappear and the other colors in the leaf -- the yellow xanthophyll packages and the orange carotene packages -- can finally be seen. The leaves don't really "turn" a certain color -- they just lose their green. Well...  That  explains yellow and orange.  But why are some leaves red or purple? Remember when we talked about the cells building a wall to cover over the tubes in the leaf's stem so the water couldn't get in anymore? The sap in the trees uses the same tubes to carry the sugar around to feed the rest of the tree.  When the wall covers the tubes, sometimes sugar gets trapped inside the leaf.  This sugar may cause the sap to turn red or purple. And this makes the leaves look red or purple! The brown leaves are easy to explain.  When the leaves stop having water flow into them and food made inside them, they start to die.  The green chlorophyll dies first, but the yellow and orange packages die too.  When all of the packages are gone, the leaf is dead and brown. The dead leaves are usually dry and crunchy. What about the trees that don't change color? There are two types of trees -- deciduous trees and evergreen trees.  Deciduous trees are the ones we've been talking about.  They change their leaf colors and lose their leaves in the autumn to prepare for the winter. Evergreen trees stay green all year long.  Their leaves are tough, green "needles".  The needles don't freeze in winter and they don't lose water as quickly as other kinds of leaves.  By holding onto the water inside them, they stay alive and green -- even in the winter!