The root cap is a thimblelike mass of parenchyma cells (for more about these cells, see this chapter) that covers and protects the growing root tip as it penetrates the soil. The root cap is pushed forward as the root tip grows longer. The cells on the periphery of the root cap are thrown off as the root cap is pushed forward; new cells are added by the meristem. The root cap also protects the meristem, aids the root as it penetrates the soil, and plays an important role in controlling the response of the root to gravity (called gravitropism, or the response of a plant growth to gravity). From their origin until they are sloughed off, root cap cells live four to nine days, depending on the length of the root cap and the plant species.