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Transcript of Shoot Primary Growth - Christian Brothers jmoore25/BotanyPPT/Shoot Primary Growth.pdf ·...

  • 10/6/2011


    Shoot Primary Growth

    Ch 25

    Outline Origin & growth of primary stem tissues Primary structure of stem Relation between vascular tissues of the stem & leaf Morphology and structure of the leaf Grass leaves Development of the leaf Leaf abscission Transition between the Vascular Systems of the Root

    & Shoot Development of the flower Stem and Leaf modifications

    Primary Shoot functions

    Support Conduction

    Food W t Water

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    Origin and Growth of the Primary Tissues of the Stem

    Apical meristem adds cells to the primary plant body Also, repetitively produces leaf primordia and

    bud primordia Results in repeated units called phytomeres

    Leaf primordia Develop into leaves

    Bud primordia Develop into lateral shoots

    Protects apical meristem

    R t d itRepeated units

    Shoot apical meristem Tunica-corpus organization

    Tunica consists of the outermost layer or layers of cells that divide anticlinally Perpendicular to the surface of meristem

    Corpus consists of a body of cells that lie beneath tunica layers. Initials of corpus occur beneath the tunica and

    add cells to the corpus by dividing periclinally Parallel with the apical surface

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    Number of tunica layers varies from species to species.

    Angiosperms have apices consisting of three superimposed layers:three superimposed layers: L1, L2, L3 Outermost = L1 Innermost = L3

    Central Zone

    Peripheral zone

    Two layered tunica

    Initial layer of

    Coleus blumei

    Initial layer of corpus is L3.

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    Central zone Mitotically quiescent

    Peripheral zoneMit ti ll ti Mitotically very active

    Young shoots tightly packed in buds; protected by bud scales.

    Bud scales: highly modified leaves initiated late in the previous growing season.

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    Buds open to reveal oldest rudimentary leaves.

    Internodal elongation has separated the nodes from one another.

    Terminal bud is a mixed bud,containing both leaves and flowers.

    Lateral buds produce only

    p yleaves.

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    Primary stem tissues go through periods of growth similar to the root Stem cannot be divided along its axis into regions of

    cell division, elongation, and maturation like roots. Shoot gives rise to leaf primordia in such rapidShoot gives rise to leaf primordia in such rapid

    succession that nodes & internodes can not be distinguished at first.

    Thus, increase in length of stem occurs by intrenodal elongation: This may occur simultaneously over several


    Apical meristem of shoot gives rise to same primary meristems found in root: Protoderm, procambium, and ground

    meristemmeristem. These meristems develop into the mature

    tissues of the primary plant body: Epidermis, primary vascular tissues, and

    ground tissue.

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    Primary structure of the stem

    In seed plants other than monocots, the vascular system of the internode appears as more or less continuous cylinder within the ground tissuethe ground tissue.

    In others, the primary vascular tissues develop as a cylinder of discrete strands, or bundles, separated from one another by ground tissueground tissue.

    In stems of most monocots and some herbaceous eudicots, the arrangement is more complex. Vascular bundles appear scattered throughout the ground tissuescattered throughout the ground tissue.

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    Tilia americana Vascular cylinder is composed of vascular

    bundles that are separated from one another by inconspicuous regions of ground y p g gparenchyma call interfascicular regions.

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    Sambucus canadensis The interfascicular regions pith rays are

    relatively wide, hence the procambial strands and primary vascular bundles form a system p y yof discrete strands around the pith. Very similar to Tilia

    Young stem, showing protoderm, ground meristem, and three discrete procambialstrands.

    Mature sieve element

    Primary tissues farther along in development

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    Stem near completion of primary growth.

    Cambia not formed yet!!!

    Will become woody.

    Sambucus has wide interfascicular regionsgreat portion of vascular cambium develops from interfascicular parenchyma

    Herbaceous stems

    Undergo little or no secondary growth. Medicago sativa (alfalfa) exhibits some

    secondary growth.G th i il t S b Growth similar to Sambucus.

    Very discrete

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    Ranunculus (buttercup): herbaceousness Vascular bundle resembles many monocots VBs retain no procambium after primary

    vascular tissues maturevascular tissues mature Hence, bundles never develop a vascular

    cambium Lose potential for future growth

    Bundles are closed; All procambial cells mature precluding secondaryprecluding secondary growth.

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    Zea mays Exemplifies stems of

    monocots. Vascular bundlesVascular bundles

    appear scattered throughout the ground tissue in transverse section.

    VBs are closed

    Mature vascular bundle surrounded by a sheath of thick-walled sclerenchyma cells.

    Relation between Vascular Tissues and Stem and Leaf

    Extensions from the vascular system in the stem toward the leaves Leaf traces

    The wide interfascicular regions or gaps of ground tissue in the vascular cylinder located above the level where leaf traces diverge toward the leaveswhere leaf traces diverge toward the leaves Leaf trace gaps

    Leaf trace extends from its connection with a bundle in the stem (stem bundle) to the level which it enters the leaf.

    Buds commonly develop in the axils of leaves, and their vascular system is connected with the main stem by branch traces

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    Leaf arrangement Phyllotaxy

    Arrangement of leaves on the stem Most common helical or spiral.

    Quercus & Morus Other plants have a single leaf at each node

    i.e, grasses phyllotaxy is distichous When leaves are formed in pairs at each

    node Phyllotaxy = opposite (Acer & Lonicera)

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    If each pair is at a right angle to previous pair: Decussate phyllotaxy

    Lamiaceae (Coleus) Lamiaceae (Coleus) Three of more leaves at each node

    Whorled phyllotaxy Culvers root (Veronicastrum virginicum)





    Morphology & Structure of the Leaf

    Blade or lamina The expanded portion

    Petiole Stalklike portion

    Simple leavesSimple leaves Blades not divided into distinct parts

    May be deeply lobed Compound leaves

    Divided into leaflets each with own petiole Two types

    Pinnately & Palmately Pinnately leaflets arise from either side of an axis rachis

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    Distinguishing leaflets from leaves. 1. buds are found in the axils of leaves

    simple and compound but not in leaflets.

    2. leaves extend from the stem in various planes, whereas leaflets of a given leaf all lie in the same plane.


    Mesophyll Ground tissue of the leaf Palisade parenchyma

    Spongy parenchyma Spongy parenchyma

    Where photosynthesis takes place Chloroplasts


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    VBs found throughout mesophyll Known as veins in leaves

    VenationNetted or reticulate Netted or reticulate

    Veins arranged in branching pattern with successively smaller veins branching from larger ones.


    Parallel Veins that extend along the long axis of the leaf Monocots

    Sun vs. Shade leaves

    Light can have substantial effects on leaf development. Sun leaves

    Grown under high light intensities Grown under high light intensities Shade leaves

    Develop under low light intensities

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    Quercus rubra sun vs shade leaves

    Sinuses very shallow Petioles rarely reddish

    Sun leaves thicker Internal surface area of the mesophyll is

    higher in sun leavesSh d l t d t d t hi h li ht Shade leaves not adapted to high light intensities Lower maximum photosynthetic rates

    Sun Leaf

    Shade Leaf

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    Leaf Abscission

    Separation of leaf from stem Occurs near base of petiole Creates an abscission zone

    After separation occurs (i.e., leaf falls) the protective layer is recognized as a leaf scar on the stem.

    Overview Shoot apical meristem produces leaf primordia, bud

    primordia, and stem primary tissues. Three basic types of organization exist in the primary

    structure of stems. Leaves and stems are closely elated physically and

    developmentally. Leaf structure variation is related to habitat. Leaves exhibit determinate growth, stems exhibit

    indeterminate growth. Separation of a leaf from a branch by abscission is a complex

    process. A flower is a determinate stem tip bearing modified leaves. Stems may serve food-storage or water-storage functions.