One striking feature of leaf development is that the number of layers produced in a leaf remain very constant and increase only when the final stages of differentiation occur. The layers could be traced back to the Submarginal Meristem and this was interpreted to mean that the MM & SM acted like Initials and produced all of the Epidermis & Mesophyll. However, subsequent studies have shown that there are several separate meristematic regions in a developing leaf rather than one. These considerations go beyond the level of depth on this topic that we can cover in this course. Let us use the classical concept of  "Marginal & Submarginal Meristems" to represent the surface and internal meristematic layers & regard it as the only meristem present in developing leaves. Remember that this is a simplification!