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CHAPTER 34 STEMS AND TRANSPORT IN VASCULAR PLANTS Stems o Link a plant’s roots to its leaves and are usually located aboveground o Can be herbaceous, with soft nonwoody tissues o Can be woody, with extensive hard tissues of wood and bark Functions of stems o Three main functions Stems of most species support leaves and reproductive structures Stems provide internal transport Conduct water and minerals from roots to rest of plant Conduct sugar produced in photosynthesis to rest of plant Stems produce new living tissue Continue to grow throughout a plant’s life, producing buds that develop into stems with new leaves and/or reproductive structures EXTERNAL STEM STRUCTURE IN WODY TWIGS External anatomy of woody stems o All stems have buds Embryonic shoots o Buds that are located at the tip of a stem are called terminal buds o Terminal buds that are not actively growing are covered with bud scales Modified leaves that protect the terminal buds o Axillary buds ( lateral buds ) are located in the axils of a plant’s leaves Axils are the upper angle between a leaf and the stem to which it is attached o Stems are attached to leaves at the nodes o Areas between two successive nodes are called the internodes o Bud scale scars are formed when the terminal bud resumes growth and the bud scales fall off Number of sets of bud scale scars indicate its age o Leaf scars show where each leaf was attached to the stem Can be looked at to determine the arrangement of leaves o Bundle scars are formed from the vascular tissue that extends from the stem out into the leaf Located within the leaf scars o Woody twigs have lenticels Sites of loosely arranged cells that allow oxygen to diffuse into the interior of the woody stem
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Look like tiny specks on the bark of a twig STEM GROWTH AND STRUCTURE Plants have two types of growth o Primary growth increases the length of a plant Occurs at the apical meristem Located at the tips of roots and shoot and also within the buds of stems o Secondary growth increases the girth of a plant Occurs at the lateral meristems Located within stems and roots All plant have primary growth whereas some have both primary and secondary growth o Plants with only primary growth are herbaceous o Plants with both primary and secondary growth are woody Herbaceous eudicot and monocot stems differ in internal structure All stems have a protective covering, one or more types of ground tissue, and vascular tissues Vascular bundles of herbaceous eudicot stems are arranged in a circle in cross section Anatomy of herbaceous eudicot stems o Epidermis provides protection in herbaceous stems Outer covering o Epidermis is covered by the cuticle Waxy layer of cutin Reduces water loss from the stem surface o Stomata permit gas exchange o Inside the epidermis is the cortex Cylinder of ground tissue that may contain parenchyma, collenchyma, and sclerenchyma cells
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