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Stems- Part 1 STEMS OUTLINE External Form of a Woody Twig Stem Origin and Development Stem Tissue Patterns Herbaceous Dicotyledonous Stems Woody Dicotyledonous Stems Monocotyledonous Stems Specialized Stems Wood and Its Uses STEMS Anytime you use something made of wood, you’re using something made from the stem of a plant. Stems are linear structures with attached leaves that provide support and transport of water and nutrients to the plant body. Most stems grow upward, helping to raise plant structures, such as leaves, off the ground. Some plants also use their stems for photosynthesis or for food and water storage. EXTERNAL FORM OF A WOODY STEM Woody twig consists of an axis with attached leaves. Alternately or Oppositely arranged. Leaves attached at a node. Stem region between nodes is an internode. Leaf has a flattened blade and is usually attached to the twig by a petiole. EXTERNAL FORM OF A WOODY STEM Axil - Angle between a petiole and the stem. Axillary Bud- located in axil. Terminal Bud often found at twig tip. Stipules - Paired appendages at the base of a leaf. Often remain throughout leaf life span. Deciduous trees and shrubs have dormant axillary buds with leaf scars left after leaves fall. Bundle scars mark food and water conducting tissue. ORIGIN AND DEVELOPMENT OF STEMS The apical meristem is located in a bud, called a terminal bud, at the tip of the stem. New growth in length occurs because of cell division in the apical meristem. Along with the apical meristem, terminal buds may contain leaf primordia, fully formed, tiny embryo leaves that are ready to expand and grow when the bud opens. ORIGIN AND DEVELOPMENT OF STEMS Buds are usually protected by hard, leaf-like structures called bud scales. When the terminal bud begins to grow, the bud scales fall off, leaving a ring of scars that go 360 degrees all around the stem. For deciduous trees growing in temperate climates, you can tell how many years a woody branch has been growing by counting the number of rings of bud scale scars. ORIGIN AND DEVELOPMENT OF STEMS Leaves grow from the stem at nodes. On branches from plants that drop their leaves, called deciduous plants, leaf scars show where the leaves used to be attached. If you look closely at leaf scars, you can see little circular marks within them that show where the vascular tissue from the stem ran out into the leaf. These vascular scars are called bundle scars because they mark the places where bundles of vascular tissue were located. ORIGIN AND DEVELOPMENT OF STEMS The spaces between nodes on the stem are called internodes. Lateral meristems are located in axillary buds that are tucked in the angles, or axils, between the leaves and the stem. When an axillary bud begins to grow, its lateral meristems function just like the apical meristem producing new growth through cell division. Axillary buds may grow into new branches, or they may produce flowers. ORIGIN AND DEVELOPMENT OF STEMS All plants grow by getting taller as the apical meristem produces new cells at the tip of the stem. The stems of some plants that grow for more than one year will grow thicker over time. So, two types of growth occur in stems. ORIGIN AND DEVELOPMENT OF STEMS Primary growth increases the length of the stem. Primary growth results from cell division in apical meristems and builds herbaceous stems, which are nonwoody stems. Secondary growth increases the width of the stem. Secondary growth results from cell division of the vascular cambium and builds woody stems, stems that contain secondary xylem tissue. PRIMARY GROWTH The apical meristem is dormant before the beginning of the growing season. When a bud begins to expand, apical meristem cells undergo mitosis and three primary meristem tissues develop--- Protoderm Procambium Ground meristem PRIMARY GROWTH Protoderm - Gives rise to epidermis. Procambium - Produces primary xylem and primary phloem cells. Ground Meristem - Produces tissues composed of parenchyma cells. Pith Cortex PRIMARY GROWTH Most flowering plants and conifers organize their vascular tissue into vascular bundles, groups of pipe-like tissues that run longitudinally through the roots, stems and leaves. In primary stems, vascular bundles have three components: A layer of thick-walled cells, sunch as sclerenchyma, that surround the bundle, protecting the bundle and supporting the stem. PRIMARY GROWTH A cluster of phloem cells, located on the side of the bundle closest to the outside of the stem. These cells transport food. A cluster of xylem cells, located on the side of the bundles closest to the inside of the stem. These cells transport water and minerals. PRIMARY GROWTH Flowering plants are divided into two groups, monocots and dicots, based on several differences, including differences in stem organization. PRIMARY GROWTH- Monocot Stem PRIMARY GROWTH-Monocot Stem Most monocots grow only by primary growth, remaining herbaceous throughout their lives. The vascular bundles form in a spiral arrangement around the stem. They can appear scattered. PRIMARY GROWTH-MONOCOT STEM Moving from outside to inside, monocot stems are made up of- 1. epidermis 2. ground tissue with vascular bundles that appear to be scattered randomly around the stem. PRIMARY GROWTH-Dicot Stem Many wildflowers and crop vegetables that are dicots grow only by primary growth and then grow by secondary growth. Dicot stems are surrounded by a sheath of epidermal tissue. The rest of the stem, with the exception of the vascular tissue, is made up of ground tissue. PRIMARY GROWTH-Dicot Stems In young dicot stems, ground tissue is organized into two layers of tissue: The cortex is made of several layers of parenchyma cells and is located between the epidermis and the vascular tissue. The pith is the group of parenchyma cells in the center of the stem. Dicots arrange the vascular bundles in a ring around the stem. PRIMARY GROWTH-Dicot Stems Moving from the outside to the inside, primary dicot stems are made up of….. Epidermis Ground tissue (cortex) Vascular bundles Ground tissue (pith) SECONDARY GROWTH Whether or not a plant does secondary growth depends largely upon how long the plant lives. Annual plants don’t do secondary growth. Perennial plants do secondary growth. Another factor that determines growth pattern is the type of plant. Monocots don’t do true secondary growth. Gymnosperms and dicots both do secondary growth in a similar way. SECONDARY GROWTH Two rings of meristematic tissue, called lateral cambia, produce the new cells that make up secondary growth. The vascular cambium makes new vascular tissue, called secondary xylem and secondary phloem. As the new cells are made, the vascular cambium pushes the secondary xylem toward the inside of the stem and secondary phloem toward the outside of the stem. SECONDARY GROWTH The cork cambium (phellogen) produces ground tissue (phelloderm) toward the inside of the stem and cork cells (phellem) toward the outside of the stem. Cork cells help protect woody stems because they are impregnated with a waxy substance called suberin, which makes the waterproof and resistant to fire damage, infection, and insect attack. These cork cells eventually replace the epidermal cells that break away as the stem thickens. Lenticels are spongy areas within the cork that allow gas exchange. SECONDARY GROWTH The bark of a tree consists of all the cells from the vascular cambium to the outside of the stem. This includes the secondary phloem, parenchyma, and cork cells. The outer bark, which consists of just the cork cambium and the cells it produces (cork and parenchyma), is called the periderm. SECONDARY GROWTH SECONDARY GROWTH The vascular cambium and cork cambium both develop from primary tissue in the stem as a stem enters into secondary growth. The vascular cambium forms from cells between the xylem and phloem within the vascular bundles and from parenchyma cells in the spaces between the bundles. These cells develop into meristematic cells that join up with each other to form a ring of cells just one cell thick within the stem. The cork cambium usually forms from a ring of cells within the cortex of the stem. These cells develop into meristematic cells that form the ring of cork cambium within the stem. .
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