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 centerphloem 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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