Horse chestnut leaves. The horse chestnut tree (Aesculus hippocastanum) is a deciduous tree (its leaves are lost in autumn). The leaf contains the green pigment chlorophyll that is used to trap the energy in sunlight to form sugars from water and carbon dioxide (a process called photosynthesis). A network of veins carry water and nutrients in and out of the leaf. Photographed in autumn, as the leaves have just begun to lose their green colour.