Plant Organs: Leaves

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Plant Organs Leaves

• Chapter 8

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LEARNING OBJECTIVE 1

• Describe the major tissues of the leaf
  (epidermis, mesophyll, xylem, and phloem)
• Relate the structure of the leaf to its function
  of photosynthesis

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Typical Leaf
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Blade
Veins
Petiole
Axillary bud
Stipules
Stem
Fig. 8-1, p. 152
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Animation Simple and Compound Leaves
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KEY TERMS

• BLADE
• Broad, flat part of a leaf
• PETIOLE
• Part of a leaf that attaches blade to stem

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Leaf Morphology
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Stepped Art
Fig. 8-2, p. 154
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KEY TERMS

• PHOTOSYNTHESIS
• The biological process that includes the capture
  of light energy and its transformation into
  chemical energy of organic molecules (such as
  glucose), which are manufactured from carbon
  dioxide and water

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Tissues in a Leaf Blade
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Animation Leaf Organization
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Epidermis

• The transparent epidermis allows light to
  penetrate into the mesophyll, where
  photosynthesis occurs

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KEY TERMS

• CUTICLE
• Waxy covering over epidermis of aerial parts
  (leaves and stems) of a plant
• Enables the plant to survive in the dry
  conditions of a terrestrial environment

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Trichomes
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KEY TERMS

• STOMA
• Small pores in epidermis of stem or leaf
• Permit gas exchange for photosynthesis and
  transpiration
• Flanked by guard cells
• GUARD CELL
• Two guard cells form a pore (stoma)

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Stomata

• Stomata typically open during the day, when
  photosynthesis takes place, and close at night

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KEY TERMS

• MESOPHYLL
• Photosynthetic ground tissue in the interior of a
  leaf
• Contains air spaces for rapid diffusion of carbon
  dioxide and water into, and oxygen out of,
  mesophyll cells

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Vascular Bundle

• Leaf veins have
• xylem to conduct water and essential minerals to
  the leaf
• phloem to conduct sugar produced by
  photosynthesis to rest of plant

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KEY TERMS

• BUNDLE SHEATH
• One or more layers of nonvascular cells
  (parenchyma or sclerenchyma) surrounding the
  vascular bundle in a leaf

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LEARNING OBJECTIVE 2

• Contrast leaf structure in eudicots and monocots

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Animation Monocot and Dicot Leaves
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Bundle Sheath Extensions
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Upper epidermis
Bundle sheath extension
Bundle sheath
Midvein
Bundle sheath extension
Lower epidermis
Fig. 8-5, p. 157
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Leaf Cross Sections
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Leaf Cross Sections
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Upper epidermis
Palisade mesophyll
Midvein
Lengthwise view of vein
Spongy mesophyll
Privet
Air space
Lower epidermis
Stoma
Xylem
Phloem
(a) Privet (Ligustrum vulgare), a eudicot, has a
mesophyll with distinct palisade and spongy
sections.
Fig. 8-6a, p. 158
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Bundle sheath cells
Midvein
Mesophyll
Parallel vein
Upper epidermis
Lower epidermis
Phloem
Xylem
Fig. 8-6b, p. 158
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Monocot and Eudicot Leaves

• Monocot leaves
• Usually narrow
• Wrap around the stem in a sheath
• Have parallel venation
• Eudicot leaves
• Usually have a broad, flattened blade
• Have netted venation

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Bulliform Cells

• Large, thin-walled cells on upper epidermises of
  leaves of certain monocots (grasses)
• Located on both sides of the midvein
• May help leaf roll or fold inward during drought

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Bulliform Cells
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(a) A folded leaf blade. The inconspicuous bullifo
rm cells occur in the upper epidermis on both
sides of the midvein.
Bulliform cells
Midvein
Fig. 8-7a, p. 159
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Bulliform cells
(b) An expanded leaf blade. A higher
magnification of the midvein region shows the
enlarged, turgid bulliform cells.
Mesophyll cell
Midvein
Fig. 8-7b, p. 159
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LEARNING OBJECTIVE 3

• Outline the physiological changes that accompany
  stomatal opening and closing

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Variation in Guard Cells
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Closed
Open
Guard cells
Subsidiary cells
(a) Guard cells of eudicots and many monocots are
bean shaped.
Fig. 8-8a, p. 160
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Closed
Open
Subsidiary cells
Guard cells
(b) Some monocot guard cells (those of grasses,
reeds, and sedges) are narrow in the center and
thicker at each end.
Fig. 8-8b, p. 160
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Fig. 8-8d, p. 160
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Animation Stomata
CLICKTO PLAY
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Stomatal Opening 1

• 1. Blue light activates proton pumps
• in guard-cell plasma membrane
• 2. Protons (H) are pumped out of guard cells,
  forming a proton gradient
• Charge and concentration difference on two sides
  of the guard-cell plasma membrane

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KEY TERMS

• PROTON GRADIENT
• Difference in concentration of protons on the two
  sides of a cell membrane
• Contains potential energy that can be used to
  form ATP or do work in the cell

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Stomatal Opening 2

• 3. Gradient drives facilitated diffusion of
  potassium ions into guard cells
• 4. Chloride ions also enter guard cells through
  ion channels
• Ions accumulate in vacuoles of guard cells
• Solute concentration becomes greater than that of
  surrounding cells

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KEY TERMS

• FACILITATED DIFFUSION
• Diffusion of materials from a region of higher
  concentration to a region of lower concentration
  through special passageways in the membrane

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Stomatal Opening 3

• 5. Water enters guard cells from surrounding
  epidermal cells by osmosis
• Increased turgidity changes the shape of guard
  cells, causing stoma to open

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Stomatal Opening
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Blue light activates proton pumps.
Protons are pumped out of guard cells, forming
proton gradient.
Potassium ions enter guard cells
through voltage-activated ion channels.
Chloride ions also enter guard cells through
ion channels.
Water enters guard cells by osmosis,and stoma
opens.
1
4
5
3
2
Fig. 8-9, p. 162
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Stomatal Closing

• As evening approaches, sucrose concentration in
  guard cells declines
• Sucrose is converted to starch (osmotically
  inactive)
• Water leaves by osmosis, guard cells lose their
  turgidity, pore closes

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Adaptations to Environment
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Blade
Petiole
Fig. 8-10, p. 163
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Guard cells of sunken stoma
Epidermis and cuticle
Resin duct
Endodermis
Xylem
Vascular bundle
Phloem
Mesophyll cell (photosynthetic parenchyma cell)
Fig. 8-11, p. 164
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LEARNING OBJECTIVE 4

• Discuss transpiration and its effects on the
  plant

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KEY TERMS

• TRANSPIRATION
• Loss of water vapor from a plants aerial parts

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Transpiration

• Occurs primarily through stomata
• Rate of transpiration is affected by
  environmental factors
• temperature, wind, relative humidity
• Both beneficial and harmful to the plant

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Transpiration
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75 Water recycled by transpiration and evaporatio
n
25 Water seeps into ground or runs off to
rivers, streams, and lakes
p. 165
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Wilting
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Guttation
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LEARNING OBJECTIVE 5

• Define leaf abscission
• Explain why it occurs and what physiological and
  anatomical changes precede it

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KEY TERMS

• ABSCISSION
• Normal (usually seasonal) falling off of leaves
  or other plant parts, such as fruits or flowers

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Leaf Abscission

• In temperate climates, most woody plants with
  broad leaves shed leaves in fall
• Helps them survive low temperatures of winter
• Involves physiological and anatomical changes

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Processes of Abscission 1

• As autumn approaches, plant reabsorbs sugar
• essential minerals are transported out of leaves
• Chlorophyll is broken down
• red water-soluble pigments are synthesized and
  stored in vacuoles of leaf cells (in some
  species)

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Processes of Abscission 2

• A protective layer of cork cells develops on the
  stem side of the abscission zone
• Area where leaf petiole detaches from stem,
  composed primarily of thin-walled parenchyma cells

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Processes of Abscission 3

• Enzymes dissolve middle lamella in abscission
  zone
• (cement that holds primary cell walls of
  adjacent cells together)
• After leaf detaches, protective layer of cork
  seals off the area, forming a leaf scar

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Abscission Zone
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Axillary bud
Bud scales
Petiole
Abscission zone
Stem
Fig. 8-14, p. 167
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LEARNING OBJECTIVE 6

• List at least five modified leaves, and give the
  function of each

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KEY TERMS

• BUD SCALE
• Modified leaf that covers and protects delicate
  meristematic tissue of winter buds
• SPINE
• Leaf modified for protection, such as a cactus
  spine

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KEY TERMS

• BRACT
• Modified leaf associated with a flower or
  inflorescence but not part of the flower itself
• TENDRIL
• Leaf or stem that is modified for holding on or
  attaching to objects
• Supports weak stems

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KEY TERMS

• BULB
• A rounded, fleshy, underground bud that consists
  of a short stem with fleshy leaves
• Specialized for storage

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Leaf Modifications
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Fig. 8-15a, p. 168
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Fig. 8-15b, p. 168
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Fig. 8-15c, p. 168
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Fig. 8-15d, p. 168
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Fig. 8-15e, p. 168
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Fig. 8-15f, p. 168
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Epiphytes

• Flowerpot Plant

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Stem
Pot (modified leaf)
(a) The leaves of the flowerpot plant
(Dischidia rafflesiana) are modified to hold
water and organic material carried in by ants.
Fig. 8-16a, p. 169
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Root
(b) A cutaway view of a pot removed from a plant
reveals the special root that absorbs water and
dissolved minerals inside the pot.
Fig. 8-16b, p. 169
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Carnivorous Plants

• Leaves modified to trap insects