A cross section one micrometer thick of a California live oak (Quercus agrifolia) leaf. Plant biologists use images like these to 'lift the hood' of the leaf to identify and measure cells and tissues, labeled in this image. The waxy coating on the outside of the leaf is called the upper and lower cuticle (UC and LC, respectively); beneath the cuticle is the upper and lower epidermis, or leaf skin (UE and LE). Photosynthesis occurs in the central spongy and palisade mesophyll cells (SP and PA, respectively). Veins (V) for water and sugar transport are wrapped in a ring of cells known as the bundle sheath (BS) which extend to the epidermis to the epidermis (bundle sheath extensions; BSE). Cells measured in the cross-section can be mathematically modelled in 3-D as cylinders, capsules or spheres.