Ultrastructural observations were made on the mesophyll cells in leaves at different stages of development. In the ground meristem of the leaf primordium, the plastids and mitochondria, both are approximately the same in dimention, locate around the large nucleus (Fig. 1). The plastids contain several lamellae and small starch grains (Fig. 4). The plastids in the intercalary meristem have more developed lamellae and larger starch grains than those in the leaf primordium. The protuberance formation of the arm-palisade cell progresses in parallel with the cell enlargement, and has completed just before emerging from sheath (Fig. 18). At the stage of cell enlargement, the number of lamellae, the size of starch grains and the rudimentary grana increase in the plastids (Fig. 6, 10). The dividing plastids are frequently observed (Fig. 6). The plasma membrane of the vacuolated cells is observed to possess numerous invaginations (Fig. 7). These invaginations continue to increase in size and project into the large central vacuole (Fig. 8, 9). These structures are bounded by two membranes, an outer tonoplast and inner invaginated plasma membranes. Some invaginations may separate from the peripheral cytoplasm and subsequently attach to the plastids (Fig. 10, 11, 12). The inner membranes of the invagination attached to the plastid, are connected with the plastid envelope. It is assumed that the invaginations may possibly be related to the formation of plastid envelope. In the leaf portion emerged out from sheath, all of the plastids take up a peripheral position in the cell and increase in size (Fig. 14). Another prominent features of the plastids are the increase in both the number of grana and thylakoids per granum (Fig. 16). The microbodies of 0.5-1.0 μm in diameter, are found in the mesophyll cells of expanded leaves (Fig. 17), but they are not found at the stage of cell division and cell enlargement.