Horse chestnut tree (Aesculus hippocastanum). This deciduous tree can reach a height of around 30 metres. It flowers from May to June, and its seeds (conkers) are dropped in September. The leaves are normally green, but change colour and drop off the tree in autumn. Autumn leaves are seen on the ground around this tree, and the leaves on the right-hand side of the tree have turned yellow. This is mostly due to a reduction in the amount of chlorophyll, the pigment that gives leaves their normal green colour. This reduction is also seen in trees with chlorophyll deficiency (chlorosis), and reveals the other pigments in the leaves. Photographed in Hyde Park, London, UK.