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Horse-chestnut leaf. Light micrograph of a section through a leaf from a horse-chestnut, or conker, tree (Aesculus hippocastanum). This section was taken parallel to the lower epidermis of the leaf, and shows the cells of the spongy mesophyll. These cells are arranged like a sponge with a network of air spaces, to allow gas exchange for photosynthesis and respiration, and the movement of moist air for transpiration. The arm-like cells link together and contain chloroplasts for photosynthesis, and storage starch. Veins (darker tubular areas) bring water and minerals in through the xylem and take away food products in the phloem. Magnification: x70 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR KEITH WHEELER/SCIENCE PHOTO LIBRARY)