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Plant stem6/5/2023 ![]() The periderm substitutes for the epidermis in mature plants. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The bark protects the plant against physical damage and helps reduce water loss. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. In woody plants, cork cambium is the outermost lateral meristem. The cells of the secondary xylem contain lignin, which provides hardiness and strength. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside, and secondary phloem (sieve elements and companion cells) to the outside. The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium (see ). The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles. ![]() Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have disintegrated. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. A series of sieve-tube cells (also called sieve-tube elements) are arranged end to end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. Water moves through the perforation plates to travel up the plant. Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Vessel elements are xylem cells with thinner walls they are shorter than tracheids. Water moves from one tracheid to another through regions on the side walls known as pits, where secondary walls are absent. Tracheids are xylem cells with thick secondary cell walls that are lignified. The latter two types conduct water and are dead at maturity. Xylem tissue has three types of cells: xylem parenchyma, tracheids, and vessel elements. In (b) monocot stems, vascular bundles composed of xylem and phloem tissues are scattered throughout the ground tissue. Sclerenchyma fibers cap the vascular bundles. The xylem tissue is located toward the interior of the vascular bundle, and phloem is located toward the exterior. In (a) dicot stems, vascular bundles are arranged around the periphery of the ground tissue. The apex (tip) of the shoot contains the apical meristem within the apical bud. An axillary bud is usually found in the axil-the area between the base of a leaf and the stem-where it can give rise to a branch or a flower. The stalk that extends from the stem to the base of the leaf is the petiole. The stem region between two nodes is called an internode. Nodes are points of attachment for leaves, aerial roots, and flowers. Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes ( ). It also helps to transport the products of photosynthesis, namely sugars, from the leaves to the rest of the plant. The stem of the plant connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. A stem may be unbranched, like that of a palm tree, or it may be highly branched, like that of a magnolia tree. Their main function is to provide support to the plant, holding leaves, flowers and buds in some cases, stems also store food for the plant. ![]() Stems may be herbaceous (soft) or woody in nature. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground. They may range in length from a few millimeters to hundreds of meters, and also vary in diameter, depending on the plant type. ![]() Stems are a part of the shoot system of a plant. ![]() List and describe examples of modified stems.Distinguish between primary growth and secondary growth in stems.Compare and contrast the roles of dermal tissue, vascular tissue, and ground tissue.Describe the main function and basic structure of stems.By the end of this section, you will be able to: ![]()
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