Red oak (Quercus rubra) leaf developing its autumn colour. The yearly colour change & leaf fall among such deciduous trees results from a combination of leaf senescence (death) & abscission (fall). A change is signalled by the accumulation of lipid droplets (fat) in the chloroplasts (photosynthetic bodies in the cell, containing the green pigments, chlorophyll). As photosynthesis slows due to shorter daylight hours & lipid production increases, colour changes occur. The production of ethylene stimulates the secretion of enzymes in the abscission zone (the point where the leaf breaks from the stem) which weakens the cell walls to allow the leaf to fall.