Apical meristem or growing tip. The epidermal (L1) and subepidermal (L2) layers form the outer layers called the tunica. The inner L3 layer is called the corpus. Cells in the L1 and L2 layers divide in a sideways fashion, which keeps these layers distinct, whereas the L3 layer divides in a more random fashion.
A meristem is a tissue in plants made of dividing cells. They are in parts of the plant where growth can take place.
Plant cells which are differentiated generally cannot divide or produce cells of a different type. Therefore, cell division in the meristem is necessary to get new cells. This allows for the growth of tissues and new organs needed by the plant.