Transcript secondary phloem

Topic 14.1
The Structure & Growth of Flowering Plants
Biology 1001
November 14, 2005
III. PLANT HORMONES
 Hormones are chemical signals that coordinate the
parts of an organism
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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
~Note multiple effects and interactions of plant hormones~
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
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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
Cytokinins, Auxin & the Control of Apical
Dominance
 Cytokinins stimulate cytokinesis (cell division) and
influence the pathway of cell differentiation
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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
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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
IV. Secondary growth of roots & stems
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Secondary growth is the increase
in girth of the stems & roots of
woody plants due to the lateral
meristems
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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
Secondary Growth of a Stem
Figure 35.18!!
What Happens During Secondary Growth?
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The vascular cambium is a cylindrical meristem one cell thick
located between the primary xylem & the primary phloem
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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
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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
The Vascular Cambium and Secondary Growth
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The cells of the vascular cambium
are called initials
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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
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Figure 35.19!!
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
Figure 35.18b!!
Features of an Older Woody Plant
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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!!