Levetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles3. The Plant Body © The McGraw−Hill Companies, 2008313The Plant Body CHAPTER OUTLINE Plant Tissues 32Meristems 32 Dermal Tissue 32 Ground Tissue 32 Vascular Tissue 33 Plant Organs 35Stems 35 Roots 37 A CLOSER LOOK 3.1 StudyingAncient Tree Rings 38Leaves 39 A CLOSER LOOK 3.2 Plants That Trap Animals 42 Vegetables: Edible Plant Organs 44Carrots 44 A CLOSER LOOK 3.3 SupermarketBotany 45Lettuce 45 Radishes 45 Asparagus 46 Chapter Summary 47 Review Questions 47 Further Reading 48 KEY CONCEPTS 1. Tissues are groups of cells that perform a common function and have a common origin and structure. 2. Flowering plants are made up of three basic tissue types: dermal, ground, and vascular.3. These tissues make up the vegetative organs of higher plants: roots, stems, and leaves. CHAPTER Nepenthes is a group of carnivorous vines from the Asian tropics that forms pitchers, modified leaves that can trap and digest a variety of animals.Levetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles3. The Plant Body © The McGraw−Hill Companies, 200832UNIT IIIntroduction to Plant Life: Botanical Principles The earliest life forms were unicellular, and that single cell was capable of carrying out all the neces-sary functions of life. When multicellular organisms evolved, certain cells became specialized in structure and function, leading to a division of labor. Groups of specialized cells performing specific functions are usually referred to as tissues . In flowering plants, various tissues compose the familiar organs: roots, stems, and leaves. PLANT TISSUES MeristemsAll flowering plants are multicellular, with the cells all origi-nating from regions of active cell division. These regions are known as meristems. Plant growth is localized in meristems. The cells originating from meristems give rise to the various tissue types that make up a plant, such as the cells of the epider-mis that form the protective layer in a plant. The three basic tis-sue types in higher plants are dermal, ground, and vascular.Apical meristems are located at the tips of all roots and stems and contribute to the increase in length of the plant. Tissues that develop from these apical meristems are part of the primary growth of the plant and give rise to the leaves and nonwoody stems and roots. Some plants have additional meristematic tissues that contribute to increases in diameter. These are the vascular cambium and cork cam-bium. Tissues developing from them are considered part of the plant’s secondary growth ( fig. 3.1 ). Dermal Tissue Dermal tissues are the outermost layers in a plant. In young plants and nonwoody plant parts, the outermost surface is the epidermis ( fig. 3.2a ). It is usually a single layer of flattened cells. Epidermal cells in leaves and stems secrete cutin, a wax-like substance that makes up the cuticle on the external surface. The cuticle prevents evaporative water loss from the plant by acting as a waterproof barrier. In many leaves, the cuticle is so thick that the leaf has a shiny surface; this is especially true in succulents such as the jade plant and tropi-cal plants such as philodendron. In some plants, hairs (trichomes) may be present on the epidermis ( fig. 3.2b ). Although usually microscopic, they may be abundant enough to give a fuzzy appearance and texture to leaves or stems. Trichomes may also be glandular, often imparting an aroma when they are brushed, as you can experience by rubbing a geranium or tomato leaf. Scattered through the leaf epidermis are pores known as stomata (sing., stoma). Gases such as carbon dioxide, oxy-gen, and water vapor are exchanged through these stomata. A pair of sausage-shaped cells, guard cells, occur on either side of the pore and regulate the opening and closing of each stoma ( fig. 3.2a ). The guard cells are the only epidermal cells with chloroplasts. Stomata and guard cells can also be found in the epidermis of some stems. In plant parts that become woody, the epidermis cracks and is replaced by a new surface layer, the periderm, whichis continuously produced by the cork cambium as the tree increases in girth. The periderm, which consists of cork cells, the cork cambium, and sometimes other cells, makes up the outer bark seen on mature trees ( fig. 3.2c ). In fact, the cork in wine bottles is the periderm from Quercus suber, the cork oak tree native to the western Mediterranean. Cork is principally made up of dead cells whose walls contain suberin, anotherwaterproofing fatty substance. It prevents water loss and pro-tects underlying tissues (see Chapter 18). Ground Tissue Ground tissues make up the bulk of nonwoody plant organs and perform a variety of functions. The three categories of ground tissue are parenchyma, collenchyma, and sclerenchyma. The most versatile of these is parenchyma. Although often described as a thin-walled 14-sided polygon, parenchyma cells can be almost any shape or Figure 3.1 Plant meristematic tissues in a diagram of a shoot tip and root tip. Apical meristems contribute to increases in the length of the plant, or primary growth. Vascular cambium and cork cambium are present in plants that have secondary growth, an increase in the girth of the plant body. Cork cambiumShootApical meristemCork cambiumRootApicalmeristemRoot hairsVascularcambiumVascularcambiumLevetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles3. The Plant Body © The McGraw−Hill Companies, 2008CHAPTER 3The Plant Body 33provide mechanical support and protection. The extremely thick secondary walls of sclereids account for the hardness in walnut shells and the grit of pear fruit. Vascular Tissue Vascular tissues are the conducting tissues in plants. You can readily see the vascular tissues in a leaf; they are the veins.The vascular tissues form a continuum throughout the plant, allowing the unrestricted movement of materials. There are two types: xylem, which conducts water and minerals from the roots upward, and phloem, which transports organic materials synthesized by the plant. Both xylem and phloem are complex tissues composed of several cell types. Tracheids and vessel elements are the water-conducting cells in the xylem. Both cell types have secondary walls, and at maturity,