Topic 14'1 The Structure

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Topic 14.1The Structure Growth of Flowering
Plants

• Biology 1001
• November 14, 2005

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III. PLANT HORMONES

• Hormones are chemical signals that coordinate the
  parts of an organism
• A hormone is a molecule produced in one part of
  the body and transported to another where it bind
  to a receptor and triggers a response in the
  target cell or tissue
• In plants, hormones control growth development
  by affecting the division, elongation,
  differentiation of cells
• Plant hormones are small molecules required in
  very small amounts and they often have multiple
  effects on the plant
• Some hormones respond to environmental stimuli
• Hormones produce their effect by altering gene
  expression, affecting the activity of enzymes, or
  changing the properties of membranes

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Note multiple effects and interactions of plant
hormones
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AUXIN

• The term auxin is used for any chemical substance
  that promotes the elongation of coleoptiles
• Auxins have multiple functions in flowering
  plants
• The auxin that occurs in plants is indoleacetic
  acid
• IAA moves in one direction only in a plant from
  shoot tip to base called polar transport
• Rate is 10mm/h faster than diffusion
• Involves transport proteins concentrated at the
  basal end of each cell
• Apical meristems are the major sites of auxin
  synthesis
• A primary role of auxins, at low concentrations,
  is to stimulate cell elongation in young
  developing shoots

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Cytokinins, Auxin the Control of Apical
Dominance

• Cytokinins stimulate cytokinesis (cell division)
  and influence the pathway of cell differentiation
• Cytokinins are produced in actively growing
  tissues
• Cytokinins achieve their effects in the presence
  of auxin

• Apical dominance is the ability of the terminal
  bud to suppress growth of the axillary buds
• Cytokinins auxin interact in apical dominance
• Auxin transported down the shoot from the from
  terminal bud inhibits growth of axillary buds
• Cytokinin entering the shoot from the roots
  counters the effect of auxin by signalling
  axillary buds to grow
• Apical dominance determines the shape of the plant

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IV. Secondary growth of roots stems

• Secondary growth is the increase in girth of the
  stems roots of woody plants due to the lateral
  meristems
• The vascular cambium adds secondary xylem (wood)
  and secondary phloem (part of bark)
• The cork cambium replaces the epidermis with a
  thick protective covering called periderm (cork
  cambium and cork cells)
• Secondary growth occurs in the older parts of the
  plant at the same time that primary growth
  continues to elongate the roots and stem

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Secondary Growth of a StemFigure 35.18!!
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What Happens During Secondary Growth?

• The vascular cambium is a cylindrical meristem
  one cell thick located between the primary xylem
  the primary phloem
• Forms from parenchyma cells that regain their
  ability to divide
• Vascular cambium cells called fusiform initials
  produce secondary phloem to the exterior
  secondary xylem to the interior
• Ray initials produce xylem phloem rays, radial
  files of parenchyma cells
• The diameter of the vascular cambium increases
  with secondary growth
• As the girth of the stem increases, the tissues
  exterior to the vascular cambium (secondary
  phloem etc.) rupture and the cork cambium
  develops from parenchyma cells in the cortex
• The cork cambium produces cork cells which
  replace the epidermis
• Cork cells deposit a waxy substance, suberin, in
  their walls and them die
• The cork cambium and cork cells together form the
  protective periderm
• All tissues exterior to the vascular cambium are
  collectively called bark

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The Vascular Cambium and Secondary Growth

• The cells of the vascular cambium are called
  initials
• Initials divide either transversely to produce
  new cambium initials or radially to form xylem or
  phloem cells
• In each year of growth the vascular cambium lays
  down a new layer each of secondary xylem and
  phloem
• The secondary xylem layers accumulate and form
  wood
• The secondary xylem cells produced in the spring
  are larger than the ones produced in the autumn
  this causes the annual growth rings of a tree
  trunk

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Features of an Older Woody Plant

• Secondary phloem does not accumulate because it
  continually gets sloughed off as the girth of the
  tree increases there is just the most recent
  layer
• As old layers of periderm slough off a new cork
  cambium forms to the inside and produces a new
  layer of periderm
• In roots the older woody parts function to anchor
  the plant and transport water and solutes to the
  shoot
• The secondary xylem of the trunk becomes
  differentiated into heartwood and sapwood the
  younger sapwood functions to transport the xylem
  sap while the heartwood provides support

Anatomy of a tree trunk Figure 35.20!!