External Features, Origin, and Internal Structure
Taxonomists use an inordinate number of terms as a means to separate and name plants. The terminology applied to the way leaves are attached to the stem, for example, includes alternate—the arrangement shown in Figure —as well as opposite and whorled and is based on the number of leaves attached at each node: one (alternate), two (opposite), and three or more (whorled). If a single blade is attached to a petiole, as in Figure , the leaf is simple; if the blade is divided into two or more individual parts, the leaf is compound and may be pinnately or palmately so depending upon how the leaflets (the individual separate units of the blade) are attached to the extension of the petiole (the rachis). Other standard terms are used for venation, overall shape, shape of the tip, condition of the edge of the blade (toothed, smooth, lobed), hairy (what kind of hairs) or smooth (on both upper and lower surfaces or just on one) and more.
Leaves arise in the shoot apex of stems in cells immediately below the protoderm. Division and expansion of the cells in this area result in a leaf primordium in which meristematic regions soon become identifiable in the upper and lower regions of the tissue destined to become the blade. A strand of procambium from the shoot, the leaf trace, makes connection with differentiating vascular tissues of the primordium thus assuring the continuity of the conducting tissues throughout the plant. The area on the vascular cylinder of the stem where the leaf trace diverges into the leaf primordium is called a leaf gap, a confusing name; it is not a hole but an area filled with parenchyma cells. “Gap” refers to the absence of xylem and phloem cells at this point in the vascular cylinder.
The tissues of the evolving blade develop faster on the lower ( abaxial surface) than those on the upper ( adaxial surface) with the result that the primordium bends inward towards the shoot apex. The elongating primordia arch over and protect the apical meristem of the shoot. Cells divide and elongate in the primordium, differentiating downward from the tip and the intercellular spaces characteristic of the mature leaf soon appear among the young blade tissues. Cell divisions cease when the leaf is less than full size, and subsequent enlargement consists of elongation and expansion of cells and intercellular spaces. Leaves thus have determinate growth, whereas the apical meristem, with its cells that continue to divide indefinitely, has indeterminate growth.
The standard leaf has three tissue regions: the epidermis, the mesophyll, and the vascular bundles or veins (Figure ).
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.