Dorothy Roush, age 12, of Chester, Pa., for her question:
WHY DOES A LEAF TURN YELLOW OR RED?
Leaves from many broad leafed trees, such as oaks and maples, turn from green to gold and yellow as summer ends and fall and winter approach. It’s all part of nature’s wonderful pattern.
The broad part of a leaf that holds the green food making cells is called the blade. It is attached to the petiole, which is the stemlike part that connects the leaf to the branch of a tree. As summmer ends, a disk of cells begins to turn into cork where the petiole joins the twig. These cork cells are called the abscission layer.
As the cork cells slowly grow across the petiole, the pipelines to and from the leaf are blocked. When the vein system of the leaf is finally completely cut off and without water, the leaf stops making foods. At the same time, chlorophyll begins to break down, and other beautiful colors that had been hidden in the leaf are revealed.
A bright yellow pigment called xanthophyll, which had been covered up by the green of chlorophyll, becomes visible. Another substance called carotene shows its orange yellow hues. The deep reds and purple hues of a compound called anthocyanin may also appear.
The secret of fall color is just that simple: a busy chemical “laboratory” of the leaf, with its flow of raw materials cut off, loses its green color aced reveals hidden yellow, red, orange and purple hues.
The autumn color of each kind of tree or shrub depends on which of the three compounds it has the most of, after the chlorophyll breaks down.
The life of a leaf ends when a breeze blows it from the twig. The petiole breaks off cleanly between the cork cells. The blade and its petiole flutter to the ground.
Long before a tree’s leaves fall, the tree has started the next year’s batch of leaves. A bud for the next year’s growth appears just above the point where the leaf stem joins the twig.
A typical leaf is born inside a leaf bud. The bud contains a tiny leaf, or leaves, folded up tightly.
In the world’s temperate regions, a woody plant that loses its leaves in winter has its next year’s buds ready by the end of the summer. The buds then stop growing for a time. They remain dormant or inactive during the cold months. Warm sunlight and plenty of water bring out the leaves in the spring.
Within a few weeks after the leaf is released from its bud, the blade is fully grown. During the warm sunny months, the leaf food factory is in full operation.
As summer approaches, the bright green of the young leaf gives way to a deeper, bluer green. It is at this stage that a disk of cells begins to turn into cork where the petiole joins the twigs.
Evergreens and trees with needle shaped leaves also grow new leaves. But their leaves fall off a few at at time throughout the year and are continuously being replaced by new leaves.