Saturday, November 6, 2010

Why Do Leaves Fall?

Most of the time people think when leaves start to fall it’s because of the wind and winter frost. Well, it’s not that! In fact, every single leaf on a deciduous tree has a thing called "abscission" cells, which acts just like scissors. How does this work?

Roughly around this time of year—Fall—in the Northern Hemisphere, changes in the weather trigger a hormone in the trees. The hormone sends a chemical message which tells the leaves that it’s about time to leave the tree. After the message is received, cells called “abscission” cells materialize where the leaf is attached to the stem. After a few days, each leaf receives a thin coat of bumpy lined cells. These cells push the leaf away from the stem. The tree then gives the leaf a push which leaves the leaf, in other words, hanging on a thread. The wind then carries off the leaf and essentially finishes of the “job”.

What’s the point of leaves falling off trees?

I really like the kitchen staff analogy to explain the role of leaves on a tree from Robert Krulwich.
“…leaves are basically the kitchen staff of a tree. During the spring, summer and early fall they make the food that helps the tree grow and thrive and reproduce. When the days get short and cold, food production slows down, giving the tree an option: It can keep the kitchen staff or it can let it go.
If trees kept their leaves permanently they wouldn't have to grow new ones, but leaves are not the brightest of bulbs (sorry!). Every so often, when the winter weather has a break and the days turn warm, Raven says leaves will start photosynthesizing. "They get some water up and they start operating and making food and then it freezes again."
When the cold snap's back on, the leaves will be caught with water in their veins, freeze and die. So instead of a food staff that's resting, the tree is stuck with a food staff that's dead. And when spring comes, the permanent help will be no help. The tree will die.”

List of Deciduous Trees

Deciduous trees are leaf-dropping trees. Here are some examples of deciduous trees:




For a more extensive list go here: List

Other source: