Why do trees lose their leaves? In certain parts of our world, known as temperate zones, trees must have some sort of protection to survive the freezing temperatures and cold, drying winds of winter. Stems and buds have coatings and structures that allow them to withstand harsh conditions. Broad flat leaves are quite delicate and could freeze in the winter, so the trees either lose their leaves all at once, or they produce small, narrow, flattened leaves with extra protection to survive the winter. Leaf abscission refers to the loss of leaves in deciduous trees. It's preceded by events that protect sites where leaves were once attached. Where the petiole joins the stem, an abscission layer forms, consisting of weak, colorless, thin-walled cells that allow the leaf to separate easily when a gust of wind comes along. Meanwhile, a protective layer of densely colored cells has formed between the abscission layer and the stem. These protective cells are filled with waxy material that forms a thick seal over the place where the leaf will detach. New growth comes from the axillary bud that's already formed in the angle between the leaf and the stem. Think of losing a leaf as scraping your skin, only to find a protective "scab" already in place. Now let's watch leaf abscission. Notice that the leaf changes color during abscission. Why do leaves change color before they fall? During the autumn months, when temperatures begin to decrease and daylight diminishes, the abscission layer begins to form, slowly cutting off the transport of water and sugars between leaf and stem, and signaling the end of sugar production in the leaves. At the same time, chlorophyll breaks down and the intense green color of the leaf begins to fade. With the fading of the chlorophyll, we begin to see the yellow and orange hues of the carotenoids, pigments that were present all along in the chloroplasts. Simultaneously, if the abscission layer forms before all the chlorophyll has broken down and while the days are still sunny and warm, sugars may be trapped in the vacuole of each palisade cell. If sugars are trapped and prevented from moving away from the blade to the stem, their relative concentration within each cell increases and triggers the production of deep purple or scarlet red anthocyanin pigments. The combination of yellow, orange, red, and purple within dying leaves provides spectacular fall color in some parts of the world. Sometimes we see colorful patterns in a single leaf, as in this ornamental pear.