A granum is a structural unit of a chloroplast in vascular plants, consisting of layers of thylakoids. Thylakoids are coin-shaped structures that contain chlorophyll and other pigments, and are the site of photosynthesis. Grana are connected by stromal lamellae, which are extensions that run from one granum, through the stroma, into a neighboring granum. The space between the inner membrane and the thylakoid membrane is filled with a central aqueous region known as the thylakoid lumen. The term "granum" comes from the Latin word for "grain".