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Chapter 21
Plant Structure and
Function
Specialized Tissues in Plants
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Seed Plant Structure (3 principal organs)
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Roots
Absorbs water and dissolved nutrients
 Anchors plants in ground
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Stems
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Support system, transport system, and defense
system
Leaves
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Main photosynthetic systems
Plant Tissue Systems
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Plants consist of three main tissue systems
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Dermal: “skin” that protects against water
loss and injury
Vascular: “bloodstream” that transports water
and nutrients throughout the plant
Ground: everything else
Dermal Tissue
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Consists of a single layer of epidermal
cells
Often covered with a thick waxy layer
(cuticle)
Guard cells: on underside of leaves
regulate water loss and gas exchange
Vascular Tissue
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Xylem
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Tracheid: long, narrow cells with walls that are
impermeable to water
Vessel element: arranged end to end; cell walls at
both ends lost when cell dies
Phloem
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Sieve elements: arranged like vessel elements and
ends have many small holes
Companion cells: cells that surround sieve element
and aid in movement of substances in and out of cell
Ground Tissue
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Composed of cells that lie between dermal and
vascular tissues
Parenchyma: thin cell walls and large central
vacuoles: in leaves they are packed with
chlorophyll
Collenchyma: strong, flexible cell walls that help
support larger plants
Sclerenchyma: extremely thick, rigid cell walls
Plant Growth and Meristematic Tissue
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Plant growth occurs at meristems that are
responsible for continuing growth throughout
the plant’s life
Meristematic tissue: undifferentiated (not yet
become specialized), only plant tissue that
produces new cells by mitosis
Apical meristem: produce increased length at
stems and roots
Differentiation: development into specialized
structures and functions
Roots

Types of Roots:
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Taproots: primary root grows long and thick
to reach water far below Earth’s surface
Fibrous roots: branch to such an extent that
no single root grows larger than the rest
Root Structure and Growth
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A mature root has an outside layer, the
epidermis, and a central cylinder of
vascular tissue
Root hairs: produce large surface area
through which water can enter the plant
Vascular cylinder: phloem and xylem
Root cap: protects root as it forces its way
through the soil (Fig 23.7)
Root Functions
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Uptake of Plant Nutrients
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Active Transport of Nutrients
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Use ATP to pump mineral ions from the soil into the
plant
Movement into the Vascular Cylinder
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Composition of soil determines plants present
Osmosis and active transport cause water and
minerals to move form epidermis to cortex
Root Pressure
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Pressure allows for upward movement of water
Stems
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Stem Structure and Function: they produce
leaves, branches, and flowers; hold leaves up to
the sunlight: and transport substances between
roots and leaves
Nodes: where leaves attach
Internodes: regions between the nodes
Buds: undeveloped tissue that can produce new
stems and leaves
Monocot and Dicot Systems
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Monocots
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Vascular bundle scattered throughout the cell
Phloem faces outside of cell and xylem faces
the center
Dicots
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Vascular bundles arranged in a cylinder
Pith: parenchyma cells inside ring
Cortex: parenchyma cells outside of ring
Primary Growth of Stems
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Refers to growth occurring at ends of a
plant
Produced by cell divisions in the apical
meristem. It takes place in all seed plants
Secondary Growth of Stems
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Method in which stems grow in width
In conifers and dicots, secondary growth
takes place in lateral meristematic tissues
called the vascular and cork cambium
Vascular cambium: produces vascular
tissues and increases thickness of stems
Cork cambium: produces outer coverings
of stems
Formation of Stems
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Formation of Vascular Cambium:
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Formation of Wood:
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Once secondary growth begins, vascular cambium
appears as thin layer between clusters of vascular
tissue
Heartwood: no longer conducts water (dark)
Sapwood: actively conducts water (light)
Formation of Bark:
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All of the tissues outside the vascular cambium
Leaves
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Leaf Structure: optimized for absorbing
light and carrying out photosynthesis
Blades: flattened sections that absorb
sunlight
Petiole: thin stalk that attaches blade to
stem
Leaf Function
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Photosynthesis: occurs in the mesophyll in
most plants (Fig 23-18)
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Stomata: porelike openings on underside of
leaf that allow CO2 and O2 to diffuse through
Guard cells: control opening and closing of
stomata
Transpiration: loss of H2o through leaves
Leaf Function (cont.)
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Gas Exchange: leaves take in CO2 and
release O2
Plants keep their stomata open just enough to
allow photosynthesis to take place but not so
much that they lose an excessive amount of
water
Transport in Plants
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Water Transport: the combination of root
pressure, capillary action, and
transpiration provides enough force to
move water through the xylem tissue
Capillary action: tendency of water to rise
in a thin tube; works by adhesion
(attraction between unlike molecules)
Transport in Plants (cont.)
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Transpiration: major force in water
transport to topmost branches and leaves
Controlling transpiration: controlled by a
series of feedback mechanisms
Transpiration and wilting: high
transpiration rates can lead to wilting
Nutrient Transport
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Functions of Phloem
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Carry out the seasonal movement of sugars within
the plant
Movement From Source to Sink
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Pressure-flow hypothesis:
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Source: where sugars are pumped into xylem
Sink: region that utilizes the sugars
When nutrients are pumped into or removed from the
phloem system, the change in concentration causes a
movement of fluid in the same direction