The protoderm gives rise to the dermal tissue system (the epidermis which produces guard cells). The epidermis is a single layer of cells which protects the plant. The ground meristem produces the ground tissue system. Ground tissue makes up the majority of the plant body, and is responsible for photosynthesis and storage of starch. The procambium produces the vascular tissue system. Vascular tissue contains phloem (sugars) and xylem (water) transport systems between roots and shoots. The root apical meristem is protected by a group of cells called the root cap. Cells produced by the meristem constantly replenish the cap, which regularly loses cells. Root cap cells also sense gravity and determine the direction of growth. They also secrete mucigel, a slimy substance which helps lubricate the root tip as it moves through the soil. Zone of cellular division: contains the apical meristem, where cells are actively dividing, and the protoderm, ground meristem, and procambium.