Ageing and decay. Sequence showing the decay of a leaf from a maple tree (Acer sp.). When a leaf falls from a tree, it is cut off from the supply of water that kept it alive. It will shrivel up and dry out, turning from a flexible structure to a brittle one. The leaf material will be broken down by the bacteria and fungi in the soil to form a leaf mould, returning nutrients to the soil. The image at right shows the skeletal structure of the leaf veins after the rest of the leaf has rotted away. These veins supplied water to the leaf and removed the nutrients formed by photosynthesis, the primary role of a leaf.