Under the upper epidermis (the thin layer of cells that forms the top surface of the leaf) there are primarily two layers of cells. The top one is the pallisade parenchyma and consists of elongated cells, tightly arranged in a vertical manner. In this layer resides most of the chlorophyll, a protein that is responsible for capturing the solar energy and power the process of photosynthesis. The lower level is the spongy parenchyma, consisting of irregularly shaped cells, with a lot of air spaces between them, in order to allow the circulation of gases.