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)
Evolution of photosynthesis era around 2 billion years ago. Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic. Oxygenic photosynthesis uses water as an electron donor, which is oxidized to molecular oxygen () in the photosynthetic reaction center. Timeline of Photosynthesis on Earth Several groups of animals have formed symbiotic relationships with photosynthetic algae. These are most common in corals, sponges and sea anemones. It is presumed that this is due to the particularly simple body plans and large surface areas of these animals compared to their volumes. In addition, a few marine mollusks "Elysia viridis" and "Elysia chlorotica" also maintain a