JOHN HESELTINE / SCIENCE PHOTO LIBRARY. JOHN HESELTINE / SCIENCE PHOTO LIBRARY.
Autumnal colours showing on a collection of maple leaves Acer sp. The yearly leaf fall among deciduous trees results from a combination of leaf senescence (death) & absicission (fall). A change is signalled by the accumulation of lipid droplets (fat) in the chloroplasts (photosynthetic bodies within the cell, which contain the green pigments, chlorophyll). As photosynthesis slows down due to shorter daylight hours & lipid production increases, colour changes occur. The abscission zone (point where leaf breaks from stem) is stimulated by the production of ethylene. Ethylene is involved in the secretion of enzymes which weaken the cell wall in the abscission zone.
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