Ch04 Lecture

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Ch04 Lecture

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  • Chapter 4Lecture Outline

    StemsCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

  • OutlineExternal Form of a Woody TwigOrigin and Development of StemsTissue Patterns in StemsHerbaceous Dicotyledonous StemsWoody Dicotyledonous StemsMonocotyledonous StemsSpecialized StemsWood and Its Uses

  • External Form of A Woody TwigCells produced by stem meristem become shoot system with branches and leaves.Woody twig consists of an axis with attached leaves.Node - Area of stem where leaves are attachedAlternate or spiralOpposite - Attached in pairsWhorled - In groups of three or moreInternode - Stem region between nodesLeaf has a flattened blade and is usually attached to the twig by petiole.

  • External Form of A Woody TwigAxil - Angle between petiole and stemAxillary Bud located in axil.Will become branches or flowers in flowering plantsBud scales protect buds.Terminal Bud at twig tipGrowth makes twig longer.Number of groups of bud scale scars tells age of twig.Stipules - Paired, often leaflike appendages at base of a leaf

  • External Form of A Woody TwigDeciduous trees and shrubs (lose all leaves annually) - After leaves fall, have dormant axillary buds with leaf scars belowBundle scars mark food and water conducting tissue within leaf scars.

  • Origin and Development of StemsApical meristem at stem tipContributes to increase in stem lengthDormant before growing season beginsProtected by bud scales and by leaf primordiaLeaf primordia - Tiny embryonic leaves that develop into mature leaves

  • Origin and Development of StemsApical meristem cells form 3 primary meristems.Protoderm - Gives rise to epidermisProcambium - Produces primary xylem and phloemGround Meristem - Produces pith and cortex, both composed of parenchyma cells

  • Origin and Development of StemsLeaf primordia and bud primordia develop into mature leaves and buds.Traces branch off from cylinder of xylem and phloem, and enter leaf or bud.Trace = strand of xylem and phloem

    Each trace leaves a gap filled with parenchyma in the cylinder of vascular tissue, forming leaf gap or bud gap.

  • Origin and Development of StemsNarrow band of cells between the primary xylem and primary phloem may become vascular cambium.Cells produced by the vascular cambium become components of secondary xylem toward center and secondary phloem toward surface.

  • Origin and Development of StemsIn many plants cork cambium (phellogen) produces cork cells with suberin and phelloderm cells.Function to reduce water loss and to protect stem against injuryLenticels - Parenchyma cells in cork for exchange of gases

  • Tissue Patterns in StemsStelesStele - Central cylinder of primary xylem, primary phloem, and pith (if present)Protostele - Solid core, phloem surrounds xylemIn primitive seed plants, whisk ferns, club mosses and fernsSiphonosteles - Tubular with pith in centerCommon in fernsEusteles - Discrete vascular bundlesIn flowering plants and conifers

  • Tissue Patterns in StemsCotyledons - Seed leaves attached to embryonic stemsStore food needed by young seedlingDicotyledons (Dicots) - Flowering plants that develop from seeds having two cotyledonsMonocotyledons (Monocots) - Flowering plants that develop from seeds with a single cotyledon

  • Tissue Patterns in StemsHerbaceous Dicotyledonous StemsAnnuals - Plants that die after going from seed to maturity within one growing seasonUsually green, herbaceous plantsMost monocots are annuals, but many dicots are also annuals.Tissues largely primary

  • Tissue Patterns in StemsHerbaceous Dicotyledonous StemsHerbaceous dicots have discrete vascular bundles arranged in a cylinder.Vascular cambium arises between primary xylem and primary phloem.Adds secondary xylem and secondary phloem

  • Tissue Patterns in StemsWoody Dicotyledonous StemsWood = Secondary xylemDifferences in wood:Vascular cambium and cork cambium active all year:Ungrained, uniform wood producedSome tropical treesIf wood produced seasonally:In spring: Relatively large vessel elements of secondary xylem produced - Spring wood.After spring wood: Fewer, smaller vessel elements in proportion to tracheids and fibers - Summer wood.In conifers, vessels and fibers absentTracheids in spring larger than later in season

  • Tissue Patterns in StemsWoody Dicotyledonous StemsOne years growth of xylem = annual ring Vascular cambium produces more secondary xylem than phloem.Bulk of tree trunk consists of annual rings of wood.Indicates age of a treeIndicates climate during trees lifetimeVascular Rays consist of parenchyma cells that function in lateral conduction of nutrients and water.Xylem Ray - Part of ray within xylemPhloem Ray - Part of ray through phloem

  • Tissue Patterns in StemsWoody Dicotyledonous Stems

  • Tissue Patterns in StemsWoody Dicotyledonous Stems

  • Tissue Patterns in StemsWoody Dicotyledonous StemsTyloses - Protrusions of adjacent parenchyma cells into conducting cells of xylemPrevent conduction of waterResins, gums, and tannins accumulate, and darken wood, forming heartwood.Heartwood - Older, darker wood in center Sapwood - Lighter, still-functioning xylem closest to cambium

  • Tissue Patterns in Stems Woody Dicotyledonous StemsSoftwood - Wood of conifersNo fibers or vessel elementsHardwood - Wood of dicot treesResin canals - Tubelike canals scattered throughout xylem and other tissuesLined with specialized cells that secrete resinCommon in conifersIn some tropical flowering plants Frankincense and myrrh

  • Tissue Patterns in Stems Woody Dicotyledonous StemsBark - Tissues outside the vascular cambium, including secondary phloemMature bark may consist of alternating layers of crushed phloem and cork.Laticifers - Ducts found mostly in phloem that have latex-secreting cellsRubber, chicle (chewing gum), morphine

  • Tissue Patterns in Stems Monocotyledonous StemsMonocots stems have neither a vascular cambium nor a cork cambium.Produce no secondary vascular tissues or corkPrimary xylem and phloem in discrete vascular bundles scattered throughout the stemVascular bundles oriented with xylem closer to center of stem and phloem closer to surface.Parenchyma (ground tissue) surrounds vascular bundles.

  • Tissue Patterns in Stems Monocotyledonous StemsIn a typical monocot vascular bundle:Two large vessels with several small vesselsFirst formed xylem cells stretch and collapse.Leave irregularly shaped air spacePhloem consists of sieve tubes and companion cells.Vascular bundle surrounded by sheath of sclerenchyma cells.

  • Specialized StemsRhizomes - Horizontal stems that grow below-ground and have long to short internodesIrises, some grasses, fernsRunners - Horizontal stems that grow above ground and have long internodes.StrawberryStolons - Produced beneath the surface of the ground and tend to grow in different directions.Potato

  • Specialized StemsTubers - Swollen, fleshy, underground stemStore foodPotatoes - Eyes of potato are nodesBulbs - Large buds surrounded by numerous fleshy leaves, with a small stem at lower endStore foodOnions, lilies, hyacinths, tulips

  • Specialized StemsCorms - Resemble bulbs, but composed almost entirely of stem tissue, with papery leavesStore foodCrocus and gladiolusCladophylls - Flattened, leaf-life stemsGreenbriars, some orchids, prickly pear cactus

  • Wood and Its UsesIn a living tree, 50% of the wood weight comes from water content.Dry part of wood composed of about 60-75% cellulose and about 15-25% lignin.Density - Weight per unit volumeDurability - Ability to withstand decayTannins and oils repel decay organisms.

  • Wood and Its UsesTypes of SawingRadially cut (quartersawed) boards show annual rings in side view.Tangentially cut (plain-sawed or slab cut) boards are cut perpendicular to rays.Show annual rings as irregular bands of light and dark streaks

  • Wood and Its UsesKnots - Bases of lost branches covered by new annual rings produced by the cambiumWood ProductsAbout half of U.S. and Canadian wood production is used as lumber, primarily for construction.Sawdust and waste used for particle board and pulp.Veneer - Thin sheet of desirable wood glued to cheaper lumberPulp is second most widespread use of wood.Paper, synthetic fibers, plastics, linoleumIn developing countries, about half of cut timber is used for fuel.Less than 10% in US and Canada.

  • ReviewExternal Form of a Woody TwigOrigin and Development of StemsTissue Patterns in StemsHerbaceous Dicotyledonous StemsWoody Dicotyledonous StemsMonocotyledonous StemsSpecialized StemsWood and Its Uses