Leaf of the Red oak Quercus rubra, in autumn colour. The yearly colour change of the leaves of deciduous trees occurs as a biochemical process. A change is signalled by the accumulation of lipid droplets (fat) in the chloroplasts of leaf cells which contain the green chlorophyll pigment. As photosynthesis in the chloroplasts slows down due to shorter daylight hours and lipid production increases, leaf colour changes occur. Until finally the leaf loses its green colour. Enzymes in the abscission zone (point where the leaf breaks from the stem) weaken the cell walls and cause the leaf to fall. This leaf specimen was found in Scotland.