Plant cell parenchyma cells, that covers the external surfaces of leaves, stems and roots. Several cell types may be present in the epidermis. Notable among these are the stomatal guard cells that control the rate of gas exchange between the plant and the atmosphere, glandular and clothing hairs or trichomes, and the root hairs of primary roots. In the shoot epidermis of most plants, only the guard cells have chloroplasts. Chloroplasts contain the green pigment chlorophyll which is needed for photosynthesis. The epidermal cells of aerial organs arise from the superficial layer of cells known as the "tunica" (L1 and L2 layers)
Fractionation of carbon isotopes in oxygenic photosynthesis be less fractionated in C (ie, δC is relatively less negative) compared to C3 plants with low water-use efficiency. C4 plants have developed the C4 carbon fixation pathway to conserve water loss, thus are more prevalent in hot, sunny, and dry climates. These plants differ from C3 plants because the Calvin Cycle in C4 plants produces a four-carbon molecule, malate (rather than 3-phosphoglycerate), after fixation of CO. Malate, the four-carbon compound is the namesake of "C4" photosynthesis. This pathway allows C4 photosynthesis to efficiently shuttle CO to the RuBisCO enzyme and maintain high concentrations of CO within bundle sheath cells.