The epidermis of leaves is a continuous layer of cells on all surfaces of the leaf, unbroken except for pores, the stomata ( stoma, singular), which facilitate the exchange of gases between the interior of the leaf and the atmosphere. The parenchyma cells of the epidermis fit together like paving stones and generally contain no chloroplasts except for those in the guard cells of the stomata. A cuticle composed of cutin and wax is deposited on the outer primary walls of the epidermal cells. It varies in thickness among different kinds of plants. Hairs or scales—called trichomes—are extensions of epidermal cells and are present on many leaves. Glands associated with trichomes often produce substances repugnant or toxic to herbivores. The physical presence of a tangle of trichomes on the surface of a leaf also deters many animals from eating or using the leaf.
Stomata consist of two kidney-shaped guard cells surrounding an opening, the stoma, and usually two to four subsidiary cells—ordinary parenchyma cells shaped to fit around the guard cells so no holes are left in the epidermal covering. (Note that “stoma” refers both to the small pore alone as well as to the entire apparatus of guard cells plus the pore.) The walls of the guard cells facing the stoma are thicker than the opposite walls and more elastic. When the guard cells fill with water (become turgid) the thinner walls elongate faster than those facing the pore, thus pulling the latter walls away from one another and opening the pore. Conversely, when the cells lose water and contract (become flaccid), the walls relax and the pore closes. The stomata regulate the passage of most of the water from the leaves and the movements of air in and out.
Depending upon where the plant lives and how its leaves are oriented, stomata may be present on both the upper and lower leaf surfaces, on one or the other exclusively, or be lacking from the leaves entirely, the latter case being characteristic of submerged aquatic plants.
The mesophyll tissue forms the bulk of most leaves and the chloroplasts in its cells are the principal sites of photosynthesis. The mesophyll is sandwiched between the epidermal layers. In leaves held horizontally on stems and in which there is a discernable top and bottom, the upper and lower mesophyll cells have different shapes whereas in leaves held vertically, the mesophyll is uniformly the same throughout.
If the mesophyll is differentiated, the upper layer is called the palisade mesophyll and consists of closely packed columnar cells with their long axis at right angles to the leaf surface. The lower tissue, called spongy mesophyll, is made of irregularly shaped cells, loosely arranged with much intercellular space. While both mesophyll types contain chloroplasts, the palisade has more than does the spongy mesophyll. The mesophyll, therefore, is a type of chlorenchyma—chloroplast-containing parenchyma. The spongy mesophyll with its air spaces is, additionally, an aerenchyma.
The wet surfaces of the mesophyll cells are the sites of water loss and gas exchange; the stomata are merely the gates through which the water and gases pass to the outside.
The mesophyll contains strengthening tissues, primarily around the veins, but also in scattered batches throughout the mesophyll. Sclereids are especially common and almost always collenchyma cells are used to strengthen veins. Fibers are common in the leaves of monocots.
Veins (vascular tissue)
Veins penetrate all parts of the leaf, forming a network that connects the leaf through the petiole to the vasculature of the stem and thereby to the root as well. Primary xylem cells occupy the upper part of the vein and phloem cells the lower. The vascular tissues are surrounded by a bundle sheath one or two layers thick, composed of fibers in the smaller veins and parenchyma in the larger.
Fibers and collenchyma are present in and around the veins and give strength to them and to the leaf as a whole. Bundle sheath extensions connect the bundle sheaths to either or both epidermises giving added stability to the blade. The large veins branch repeatedly becoming smaller each time they divide until they ultimately end with only one or two tracheids at the vein ending. Here the mesophyll cells are in direct contact with—or at most one or two cells away from—the raw materials carried in the xylem and used for photosynthesis. The phloem is equally convenient for export of photosynthetates. The bundle sheaths insulate the conducting cells and ensure the retention of materials in the pipeline.
The veins of tropical grasses and other plants with C4 photosynthesis are surrounded by two cylinders, the inner of thick-walled bundle sheath cells, the outer of thin-walled mesophyll cells. C4 plants are said to have a Kranz (from the German word for wreath) anatomy because of these. In addition, no distinct palisade or spongy mesophyll zones are present in the C4 leaves.