The yellow pigment is known as xanthophyll , the orange is carotene . The pigment anthocyanin produces bright red, blue and purple colors. The abscission layer that causes the leaf to eventually break from the plant is made up of parenchyma cells. Soon enzymes develop and dissolve the substance called pectin that holds these cells together. Without the pectin, the parenchyma cells separate.
Tannic acid forms in the leaf.
The leaf is held to the main plant by a brittle network of veins that breaks with the slightest gust of wind. After the leaf falls, a thin layer of cork cells seals the scar where the leaf was attached. The trees and plants surrounded by last year's growth of leaves are still very much alive. The leaves that now cover the ground serve as mulch for the soil. Soon a new growth of leaves will appear.