As days grow shorter and temperatures cooler, deciduous trees prepare for winter dormancy. Lacking sufficient light and water, photosynthesis shuts down, and trees must live off food stored during the growing season. In spring, leaves lay the groundwork for their demise. A special layer of cells forms at the base of each leaf, called the abscission or separation layer. Its work is to transport water to the leaf and take food, created by photo synthesis, back to the tree. In autumn, the cells of this layer begin to swell and the bottom of this layer forms a corklike substance that eventually cuts off all transfer between leaf and tree. Meanwhile, the top of the layer begins to disintegrate, making it easy for the leaf to detach. As photosynthesis ceases, the leaves lose their chlorophyll, which gives them their green color. Without chlorophyll, other colors emerge. Yellow and orange, for example, are normally present in the leaves but are overshadowed by the chlorophyll.
Maple leaf red occurs because glucose remains when photosynthesis shuts down. Drab oak-leaf brown represents wastes left in the leaves.
A Tree From Ages past
The long-needled Wollemi pine is a survivor from the age of the dinosaurs. While fossil records made the 200 million-year-old species known to us, it was believed to be extinct. Then, in 1994, an Australian parks officer found a single tree in the Blue Mountains in Wollemi National Park. Subsequently, a hundred adult trees were counted there. Conservation efforts funded in part from the sale of saplings go to save and strengthen the species.
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