Scanning electron micrograph of a leaf section of a garden Zinnia. The picture shows the structure of the leaf as a sandwich of cell layers. The upper and lower layers (pale yellow green) are the epidermis, the function of which is to regulate the entry of air into the leaf interior, and to provide a water-proof covering. Air is admitted into the leaf through pores - stomata - in the lower epidermis, not shown here. The mesophyll cells (grey green) within the leaf are the site of photosynthesis. They are of two types; to the upper leaf surface, they are elongated palisade cells, that contain many chloroplasts arranged against their long walls in order to enhance gas exchange. Below them are smaller, spongy parenchyma cells, here seen as random shapes. They too are photosynthetic, but less efficient than the palisade cells, due to shading and their more spherical shape. Mag x220 at 10x8