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TAMU BIOL 101 - 08-wk-02-stems-sec grow-root-soil-lipids
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1Biology 101Biology 101Fall, 2008Fall, 2008Week 2 – Stems-Secondary growthRoots & SoilsLipids, Membranes, TransportWeek 2 – Stems-Secondary growthRoots & SoilsLipids, Membranes, TransportSTEMS transport  support  production of CH2O  storage of water / starchSTEM ANATOMYnodenodenodenodeinternodeinternodeinternodeinternodeaxillary budaxillary budnodenodeaxillary budaxillary budLeaf blade petioleterminal budstem2Woody twigterminal budaxillary budinternodenodeslenticelsterminal budscale scarleaf scarbundle scarVASCULAR BUNDLE: xylem and phloem  primary xylem & phloem differentiate from procambium xylem towards stem centerphloem towards peripherySTEM ANATOMY dicots: ring of vascular bundles monocots: vascular bundles randomly dispersed3XYLEM Tracheids tapering ends with pits characteristic of gymnosperm xylemXYLEM Vessels larger than tracheids annular or spiral thickenings on inner walls end walls disintegrate or have perforation plates with pitsPHLOEM Sieve-tube member Companion cells4Secondary growthThese meristems evolved in the Middle Devonian (some 370 million years ago) and provided support that enabled the growth of large trees.cork cambiumvascular cambium...arises from two lateral meristems:pithprimaryxylemxylem raysecondaryxylemprimaryphloemtissues (periderm) derivedfrom the cork cambiumvascularcambiumsecondaryphloemSecondary thickening5meristemsecondary meristemxylemphloemSECONDARY GROWTH lateral meristems cylinders in subapical regions vascular cambium cork cambium (phellogen)VASCULAR CAMBIUM Dicots / gymnosperms Highly vacuolate Two types of initials: Ray initials Fusiforminitials6VASCULAR CAMBIUMRay initials elongated cells  perpendicular to stem axis  form ray parenchyma  transport water and solutes radiallyFusiform initials elongated cells parallel to stem axis  produce secondary xylem and phloem transport water & solutes vertically periclinal divisions several divisions precede differentiationWOOD: SECONDARY XYLEM Spring wood  large, thin-walled cells Summer wood  smaller thick-walled cells Annual rings  boundaries of spring and summer wood  >1,000 years old7WOOD: SECONDARY XYLEM Softwoods gymnosperms  many tracheids no vessels  resin ducts  amber: fossilized resinWOOD: SECONDARY XYLEM Hardwoods angiosperms vessels  tracheids fibersSAPWOOD outer few centimeters of secondary xylem transports water and dissolved nutrients  light, weak HEARTWOOD center of trunk (pith)  no transport cells clogged with resins, gums, oils and tannins  dark, durable, aromatic8CHARACTERISTICS OF WOOD Cuts radial sections (quartersaw cuts)  tangential sections (planesaw cuts)  cross sections (transverse)  often split along raysCHARACTERISTICS OF WOOD Knots Grain Texture Density DurabilityRaysSecondary phloemphellem (cork)phellodermphellogen (cork cambium)Peridermouter derivative of vascular cambiumall tissues outside vascular cambiumBARK:9PERIDERM Lenticels raised areas loosely packed cells arise from phellogen (cork cambium) penetrate phellem (cork)  gas exchangereplaces broken epidermal and peridermaltissuescells alive, often parenchymatous, and may be photosyntheticcells suberized, dead, and lack intercellular spacesPhelloderm (secondary cortex)Phellem (cork)MODIFIED STEMS: ABOVE GROUND STEM TENDRILS e.g. cucurbits RUNNERS asexual reproduction (vegetative propagation: e.g. strawberries) Stolons – Stern says are non-horizontal runnersMODIFIED STEMS: ABOVE GROUND THORNS  e.g. mesquite SUCCULENT STEMS  e.g. cacti10MODIFIED STEMS: ABOVE GROUND CLADODES (cladophylls) e.g. Christmas cactusMODIFIED STEMS: Below Ground BULBS e.g. onion CORMS e.g. GladiolusMODIFIED STEMS: Below Ground TUBERS e.g. potato(at the tip of stolons) RHIZOMES e.g. irisand ginger11COMMERCIAL USES OF SECONDARY XYLEM (WOOD) Fuel U.S., 10%  developing countries, up to 90%  Paper pulverized wood (pulp) Charcoal burning chunks of wood in limited air Lumber U.S., 35 tree speciesCOMMERCIAL USES OF SECONDARY PHLOEM (INNER BARK) Rope baobab tree Dyes alder  Drugs quinine  Cinnamon bark of Cinnamomumzeylanicum Maple syrup from Acer saccharumsapwood!FUNCTIONS OF ROOTS• Absorption• Anchorage• Conduction•Storage12How Roots Develop• When a seed germinates, the embryo’s radiclegrows out and develops into the first root.– May develop into thick taproot with branch roots.• Dicotyledonous Plants– May develop adventitious roots that develop a fibrous root system.• Monocotyledonous PlantsTYPES OF ROOT SYSTEMS• Adventitious– do not arise from radicleFibrous - monocot Taproot - dicot• Fibrous– short lived radicle– roots arise from stem– similar-sized roots13Fibrous - monocot Taproot - dicot• Taproot– Enlargement of radicle (primary root ) – Secondary roots (branch / lateral)– dicots and conifers• Fibrous– short lived radicle– roots arise from stem– similar-sized roots Root Structure• Root Cap - Thimble-shaped mass of parenchyma cells covering each root tip.– Protects tissue from damage.– Function in gravity perception.• Region of Cell Division - Composed of apical meristem in the center of the root tip.– Most cell division occurs at the edge of the inverted cup-shaped zone.ROOT STRUCTUREROOT CAP • Columella cells – amyloplasts•peripheral cells:•Golgi produce mucigel14•Zone of maturation– cellular differentiation – root hairs•Zone of elongation– cells elongate primarily by water uptake in vacuoles•Zone of division– Division of densely cytoplasmic cellsStele CortexPericyclePithVascularEndodermisCasparian stripStorage parenchymaEpidermisApoplastic water transportSymplastic water transport15CORTEX• endodermis – innermost layer of the cortex. It is characterized by the: – Casparian strip (suberized radial & transverse walls) • storage parenchyma – typically several layersSTELE: All tissues inside cortex • vascular tissue – dicots• core of lobed xylemand phloemalternate. Example here is tetrarch. Epidermis:single layer - no stomataHypodermis:often suberizedCORTEXSTELEEndodermis – with Casparian stripPericycle – origin of lateral roots16GROWTH PATTERNSDeterminate: short embryonic period


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TAMU BIOL 101 - 08-wk-02-stems-sec grow-root-soil-lipids

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