A plant epidermis serves much the same function as those on an animal; the single layer of cells contains the inner parts of the plant, protecting them from outside damage. Directly adjacent to the epidermis is the cortex, a thicker row of undifferentiated cells that help protect the inside of the plant, differentiate to help heal and contain damage, and also may contain chloroplasts to aid in photosynthesis. Cortex cells help fix the shape and the structure of the plant, and also provide nutrients into the interior structures of the plant through diffusion. The outer cortex cells become thick through contact and outer movement, and are called collenchyma cells. Cortex cells are bounded on the outside by the epidermis, which is thin, and on the inside by the endodermis, which provides various chemical support for the plant including water management, starch storage, and protection from toxins.