The developmental anatomy of apically stable periclinal chloroplast chimeras was studied in a number of monocotyledonous genera. Their ontogeny is basically similar to that of dicotyledons. In both there are three independent apical layers (L‐I, L‐II, and L‐III) whose derivatives can be traced in stem, leaf and flower. The bulk of the stem tissue is derived from L‐III with only the epidermis and one or two hypodermal cell layers from L‐I and L‐II. All three layers participate in formation of the leaf with great flexibility in the amount of tissue from each. There is more instability in growth of most monocotyledonous leaves than in dicotyledonous leaves. As a result there is relatively more tissue derived from L‐I and L‐II and less from L‐III. The same is true in floral development so that a significant number of gametes are of L‐I origin. As in dicotyledons, there is variation in direction, timing, and frequency of cell division. However, an overriding genetic control results in normal size, shape, and structure. The evidence from genetic and cytochimeras in both monocotyledons and dicotyledons has provided direct experimental proof of the functional reality of the apical layers described by Hanstein and Schmidt.