Leaf Ultrastructure of Peperomia obtusifolia, P. camptotricha, and P. scandens

Three parenchyma tissues are arranged in separate periclinal layers in the leaves of Peperomia obtusifolia, P. camptotricha, and P. scandens. A transparent, multiple epidermis is situated on the adaxial side of the leaf and has large, tightly packed cells containing mostly starch-filled chloroplasts. A median (palisade) mesophyll, situated between the multiple epidermis and an abaxial spongy mesophyll, has small, moderately packed cells. Chloroplasts occupy a large volume of the median mesophyll cells and contain little starch but extensive thylakoid membranes with numerous, highly stacked grana. A spongy mesophyll is situated on the abaxial side of the leaf and has cells much larger than those in the median mesophyll. The spongy mesophyll chloroplasts contain mostly starch, and the thylakoid membranes have fewer, less stacked grana than those in the median mesophyll. The cellular ultrastructure of each tissue type is similar among these species, indicating a specialized function for each tissue. However, differences in leaf anatomy among species may be correlated with previously reported differences in their diurnal patterns of gas exchange and organic acid fluctuation.