Leaves, produced as appendages or outgrowths from the vegetative stem apical meristem, have a different tissue arrangement than the stems and roots. The component tissues of the leaf are the epidermis, the mesophyll, and the vascular bundles. The epidermis is formed from the surface cell layer of the stem apical meristem. The epidermis serves a protective function.
Vascular bundles, also called veins, are cylindrical traces of xylem and phloem that diverge from the stem's central stele. The leaf's midvein is the largest medianly positioned vascular bundle. Minor leaf veins develop laterally on either side of the midvein. Both the midvein and minor veins differentiate through the stalk of the leaf (the petiole) into its flat blade.
Transverse section of a dicot leaf. Redrawn from Van de Graaff et al., 1994.
compound a substance formed from two or more elements organelle a membrane-bound structure within a cell chloroplast the photo-synthetic organelle of plants and algae
The blade is composed mostly of mesophyll tissue that develops interior to the enveloping leaf epidermis and surrounds the vascular bundles. Most leaves have two types of mesophyll cells, the palisade and spongy mesophyll. Palisade mesophyll develops near the upper epidermis and is composed of columnar-shaped cells. The spongy mesophyll cells are more spherically shaped and are characterized by the presence of numerous intercellular spaces between adjacent cells. Plants produce most of their food during photosynthesis in cells of the mesophyll tissue.
Although considerable variation in anatomy occurs in the internal leaf tissues of dicots and monocots, the differences are mostly based on environmental rather than taxonomic criteria. There is a distinct difference, however, in the leaf anatomy of certain species, both dicot and monocot, based on whether the first product produced in photosynthesis is a three-carbon or a four-carbon (C4) compound. C4 plants, best represented by tropical grasses, develop a thick-walled cell layer around each leaf vascular bundle, called the bundle sheath cells. This concentric organization of surrounding bundle sheath cells is referred to as Kranz (German for "wreath") anatomy. Additionally, the bundle sheath cells contain more organelles such as mitochondria and chloroplasts. Dicot and monocot species in which the first photosynthetic compound produced is a three-carbon compound do not exhibit Kranz anatomy.
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