Leaves: Hanging in There
 

	Fun Experiment to Try at Home!
		Purpose To see that in order for leaves to fall, the chemical produced by the decrease in daylight is needed.   Materials - twig with at least four green leaves attached - vase   Procedure 1.  Stand the twig in an empty vase. (Don't add water!) 2. Set the vase somewhere where you can see it — but where it won't be disturbed — for four or more weeks. 3. Check in on the twig and leaves as often as possible, but be careful not to touch them. You'll see that even though the leaves turn brown and die, they won't fall off the twig.   Sum It Up! Why do these dead leaves stay on the stem when the leaves outside are falling? Because they died before the abscission layer dissolved. Remember: It's the chemical produced by the decrease in daylight that causes leaves to fall, not the death of the leaves themselves.

In autumn, trees near city streetlights may keep their leaves longer than their country cousins.
 
To understand this surprising fact, you need to first know a bit about how leaves are designed. Transport tubes, which carry sap into and out of the leaf, run through the stalk. A layer of thin-walled cells in the stalk, the abscission layer, holds the stalk to the tree stem.

In the fall, the abscission layer disappears and only the transport tubes remain to hold the leaf to the stem. The tubes aren't strong enough to hold the weight of the leaf combined with the force of the wind, so they break and the leaves fall.

What causes the abscission layer to disappear in the fall? The decrease in daylight — it triggers the layer's cells to produce chemicals that dissolve it. After September 22, the fall equinox, there are more hours of darkness than daylight. (In fact, night gets a little longer each day until December 22, the winter solstice.)

For trees growing near city streetlights, the decrease in light during the fall isn't the same as it is for trees that get light only from the sun. That's why these city trees may retain their leaves longer than trees in the country.

For more information on leaves, see Janice VanCleave's Plants (New York: Wiley, 1996)