The%20Shoot%20System%20I:%20The%20Stem

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The Shoot System I The Stem

• Chapter 5

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Organization of Shoot System

• Shoot system of flowering plant consists of
• Stem with attached leaves, buds, flowers, and
  fruits

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terminal bud contains SAM
bud

node
module
internode
leaf
Shoot system
Root system
primary root
lateral root
RAM
Fig. 5-1, p. 71
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Shoot System

• Functions
• Provide axis for attachment of leaves, buds,
  flowers
• To produce new cells, tissues, leaves, and buds
• Provide pathways for movement of water and
  dissolved minerals from roots to leaves
• Provide pathways for food synthesized in leaves
  to move into roots
• May be modified for different functions such as
  water storage

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Shoot System

• Modules
• Repeating units of the stem
• Consists of internode plus the leaf and bud
  attached to the stem
• Node
• Point of attachment

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Groups of Flowering Plants
Group Cotyledons Examples Descriptions
Monocotledonous plants (monocots) Produce embryos with one cotyledon (seed leaf) Corn, onion Stem has scattered vascular bundles, primary phloem usually positioned toward the outside
Dicotyledonous plants (dicots) Produce embryos with two cotyledons (seed leaves) Peas, oak Have pith surrounded by cylinder of vascular bundles, primary xylem toward inside, primary phloem toward outside
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SAM

• SAM
• Shoot apical meristem
• Composed of dividing cells
• Three primary meristems
• Protoderm
• Ground meristem
• procambium

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young leaf
SAM
procambium
protoderm
ground meristem
Fig. 5-3, p. 73
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Protoderm

• Outermost layer of cells in shoot tip
• When cells stop dividing and mature called
  epidermis

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Ground Meristem

• In center of shoot tip
• Just inside protoderm
• Cells slowly lose ability to divide

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Ground Meristem

• Differentiate into parenchyma cells of cortex and
  pith
• Parenchyma cells nearest outside of cortex may
  contain chloroplasts
• Parenchyma cells of cortex or pith may store
  starch
• Pith region may become hollow due to breakdown of
  parenchyma

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Procambium

• Forms as small bundles of long, thin cells with
  dense cytoplasm
• Bundles arranged in ring just inside outer
  cylinder of ground meristem and below SAM
• Cells divide
• At position down axis, cells stop dividing and
  differentiate into primary xylem and primary
  phloem

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Procambium

• Each bundle of procambium becomes vascular bundle
• Primary xylem toward inside of stem
• Primary phloem toward outside of stem
• Residual procambium
• Occurs in plants with secondary growth
• Procambium between primary xylem and phloem
• Remains undifferentiated

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Distribution of Primary Vascular Bundles in Dicot
Stem

• In vascular cylinder
• Leaf traces
• Bundles that network into attached leaves
• Organization of bundles in stems depends on
• Number and distribution of leaves
• Number of traces that branch into leaves and into
  buds

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Apical meristem
Three primary meristems
protoderm
ground meristem
procambium
primary phloem
residual procambium
primary xylem
epidermis
cortex
pith
vascular bundle
leaf trace
stem of primary plant body
Fig. 5-4, p. 73
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leaf traces
vascular bundle
small vascular bundle
internode
petiole
node
Fig. 5-5, p. 73
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Distribution of Primary Vascular Bundles in Dicot
Stem

• Number of vascular bundles in cylinder and number
  of leaf traces
• Varies by species
• Dependent on number and arrangement of leaves

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Leaf Arrangements
Pattern Leaves/node Angle of divergence
Alternate 1 leaf/node 180º
Opposite 2 leaves/node 90º
Whorled 3 or more leaves/node 60º
Spiral 1 leaf/node 137.5º
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Fig. 5-6, p. 74
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Monocot Stem Primary Growth

• Primary growth
• Scattered vascular bundles
• Terms pith and cortex usually not used when
  bundles are scattered
• Stem same diameter at apex and base
• Primary thickening meristem (PTM)
• Absent in dicot stems
• Contributes to both elongation and lateral growth

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vascular bundle
hollow center
cortex
epidermis
Fig. 5-7, p. 75
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Secondary Growth

• Most monocots show little or no secondary growth
• Herbaceous (nonwoody) plants
• Normally complete life cycle in one growing
  season
• Dicots and gymnosperms
• Display secondary growth starting first year of
  growth
• Woody plants

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residual procambium
residual procambium
vascular bundle
parenchyma
vascular bundle
primaryphloem
primary xylem
primary phloem
parenchyma
primary xylem
interfascicular cambium
Cells begin dividing
fascicular cambium
vascular cambium
Vascular cambium forms
secondary xylem
secondary phloem
vascular cambium
secondary phloem
secondary xylem
vascular cambium
secondary xylem
Secondary xylem and phloem form
secondary phloem
secondary phloem
secondary xylem
Fig. 5-9, p. 76
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Formation of Secondary Xylem and Phloem

• Formation of vascular cambium
• cell division occurs in residual procambium
  inside vascular bundles and parenchyma cells
  between bundles
• Plant hormone probably provides signal
• Dividing residual procambium within bundles
  called fascicular cambium

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fascicular cambium
epidermis
primary phloem
interfascicular cambium
primary xylem
Fig. 5-10a, p. 77
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vascular cambium
Fig. 5-10b, p. 77
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Formation of Secondary Xylem and Phloem

• Dividing residual procambium between bundles
  called interfascicular cambium
• Fascicular cambium interfascicular cambium
  vascular cambium

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Vascular Cambium

• Only one or two cells thick
• Divides in two directions
• Cells formed to outside form secondary phloem
• Cells formed to inside form secondary xylem
• Typically produces more xylem than phloem cells

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initial
surface of stem or root
division
cell of vascular cambium at start
of secondary growth
division
one cell differentiates into xylem, one
stays meristematic
one cell differentiates into phloem, one
stays meristematic
divisions and differentiation continues
DIRECTION OF GROWTH
Fig. 5-11, p. 77
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Vascular Cambium

• Fusiform initials
• Cambium cells
• Form into cells of axial system
• Ray initials
• Form cells of ray system
• Rays composed of ray parenchyma cells and ray
  tracheids
• Ray system transports water and minerals laterally

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Wood

• Composed of secondary xylem
• Planes of view
• Tangential section end view of rays
• Radial section side view of rays
• Transverse section end view of cells of axial
  system

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Annual Rings

• Concentric rings of cells of secondary xylem
• In temperate zones
• One ring/growing season
• Determine age of tree by counting rings
• In tropical rain forests
• Irregular growth rings
• Growth occurs year round

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primary growth, some secondary growth
secondary growth
year 1
2
3
Ray system
Axial system
bark
vascular cambium
Fig. 5-12a, p. 78
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Annual Rings

• Oldest known trees
• Redwoods (Sequoia sempervirens)
• Bristlecone pines (Pinus longaeva)

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Annual Ring Components

• Springwood or earlywood
• Cells in inner part of annual ring
• Cells larger in diameter
• Formed during first growth spurt of new season
• Summerwood or latewood
• Cells smaller in diameter
• Formed later in growing season

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Annual Ring Components

• Ring porous
• Large diameter vessels mainly in springwood
• Diffuse porous
• Large diameter vessel members uniformly
  distributed throughout springwood and summerwood

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Heartwood

• Heartwood
• Darker wood in center
• Cells blocked with resins and other materials
• No longer functions in transport
• Vessel members may be blocked by tyloses
• Form when cell wall of parenchyma cell grows
  through pit and into vessel member

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secondary xylem
secondary phloem
periderm
heartwood
sapwood
bark
vascular cambium
Fig. 5-16a, p. 80
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Sapwood

• Lighter wood near periphery
• Secondary xylem
• Has functional xylem cells
• Where actual transport of water and dissolved
  minerals takes place

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sapwood
heartwood
branch (knot)
Fig. 5-16b, p. 80
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Gymnosperm Structure

• Wood simpler structure
• Mostly tracheids in axial system and simple rays
• May have resin ducts
• Secretory structures that produce and transport
  resin

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Resin

• Synthesized and secreted by lining of epithelial
  cells
• Sap
• Resin flowing through resin ducts to outside of
  stem
• Rosin
• Hardened resin
• Amber
• Fossilized rosin

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Bark

• Protective covering over wood of tree
• Everything between vascular cambium and outside
  of woody stem
• Composition varies, depending on age of tree
• Young tree
• Secondary phloem, few cortex cells, 1 or 2
  increments of periderm
• Old tree
• Layers of secondary phloem and several layers of
  periderm

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Secondary Phloem

• Forms to outside of vascular cambium
• Cell types
• Sieve-tube members, companion cells, phloem,
  parenchyma, phloem fibers, sclereids in axial
  system, ray parenchyma in ray system
• Cannot count phloem rings to determine age of
  tree
• Phloem rays
• Phloem ray parenchyma cells

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Periderm

• Made up of
• Phellem
• Cork cambium
• phelloderm
• Functions
• Inhibits water evaporation
• Protects against insect and pathogen invasion

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Periderm

• Cork cambium (phellogen)
• New cork cambium usually produced each spring
• Divides in two directions to produce
• Phellem cells (cork cells)
• Produced toward the outside
• Phelloderm cells
• Produced toward the inside

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Periderm

• Phellem cells
• Regular rows
• Cell walls contain suberin
• Usually dead by time periderm is functional
• Phelloderm cells
• Form regular rows
• Cells live longer and resemble parenchyma cells

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Periderm

• Lenticels
• In bark of young, woody tree branches
• Loosely packed parenchyma cells
• Provide area for gas exchange
• Girdling
• Removal of continuous strip around tree
  circumference kills tree
• Nutrient transporting secondary phloem severed in
  process

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Main Bark Patterns
Pattern Description Example
Ring bark Continuous rings Paper birch
Scale bark Small, overlapping scales Pine trees
Shag bark Long, overlapping, thin sheets Eucalyptus
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Buds

• Short, compressed branches
• Covered with hard, modified leaves called bud
  scales
• Types of buds
• Terminal bud
• At end of branch
• Lateral bud
• At base of petioles of leaves on side of a branch
• Flower bud
• Produces flower parts

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Buds

• Bud scale scar
• Leaf scar
• Bundle scar
• Can identify plants in winter by
• Structure of leaf scar
• Number and distribution pattern of bundle scars

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Secondary Growth in Monocot Stems

• Most monocots do not form secondary xylem and
  secondary phloem
• Palm trees
• Exhibit diffuse secondary growth
• Some thickening of stem from division and
  enlargement of parenchyma cells
• Not true secondary growth because cambium is
  lacking

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Secondary Growth in Monocot Stems

• Some monocots exhibit true secondary growth
• Examples Yucca, Agave (century plant), Dracaena
  (dragons blood tree)
• Produce stems that are thin at top, thick at base
• Cambium primarily forms parenchyma cells
• Xylem surrounds phloem in vascular bundle

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Stem Modifications

• Rhizomes
• Underground stem
• Internodes and nodes
• Sometimes small, scale-like leaves
• Leaves do not grow
• Leaves are not photosynthetic
• Buds in axils of scale leaves elongate, produce
  new branches which form new plants

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Stem Modifications

• Tubers
• Enlarged terminal portion of underground rhizome
• Example potato plant
• Eyes of tuber - lateral buds

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Stem Modifications

• Corms and bulbs
• Corm
• Short, thickened underground stem with thin,
  papery leaves
• Central portion accumulates stored food to be
  used at time of flowering
• New corms can form from lateral buds on main corm
• Example Gladiolus

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Stem Modifications

• Corms and bulbs
• Bulbs
• Small stem portion
• At least one terminal bud (produces new, upright
  leafy stem)
• Lateral bud (produces new bulb)
• Stores food in specialized fleshy leaves
• Food used during initial growth spurt
• Example Allium cepa (table onion)

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Stem Modifications

• Cladophylls
• Also called cladodes
• Flattened, photosynthetic stems that function as
  and resemble leaves
• Develop from buds in axils of small, scale-like
  leaves
• Example Ruscus aculeatus (Butchers broom)

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Stem Modifications

• Thorns
• Originate from axils of leaves
• Help protect plant from predators
• May have leaves growing on them
• Spines and prickles
• Not modified stems
• Spines
• Modified leaves
• Prickles
• modified clusters of epidermal hairs

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Economic Value of Woody Stems

• Forests
• Home to many plants and animals
• Source of raw materials for many useful products
• Purify air
• Keep soil from washing away
• Affect weather patterns

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Economic Value of Woody Stems

• Renewable resources
• Harvesting of product from plant without
  destroying plant
• Natural rubber, chewing gum, turpentine
• Nonrenewable resources
• actual harvesting and use of entire plant
• Recycling
• Example recycling paper products
• Helps preserve natural tree resources

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The Shoot System II The Form and Structure of
Leaves

• Chapter 6

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Functions of Leaves

• Photosynthesis
• Release oxygen, synthesize sugars
• Transpiration
• Evaporation of water from leaf surface
• Specialized functions
• Water storage
• Protection

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Comparison of Monocot and Dicot Leaves
Type Shape of blade Venation Description
Monocot Strap-shaped blade Parallel vascular bundles Leaf bases usually wrap around stem
Dicot Thin, flat blade Netted pattern of vascular bundles Petiole holds blade away from stem
blade portion of leaf that absorbs light energy
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Leaf Blade

• Broad, flat surface for capturing light and CO2
• Two types of leaves
• Simple leaves
• Compound leaves

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Leaf Blade

• Simple leaves
• Leaves with a single blade
• Examples
• Poplar
• Oak
• Maple

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Leaf Blade

• Compound leaves
• Blade divided into leaflets
• Two types
• Palmately compound
• Leaflets diverge from a single point
• Example red buckeye
• Pinnately compound
• Leaflets arranged along an axis
• Examples black locust, honey locust

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Leaf Blade

• Advantages of compound leaves
• Spaces between leaflets allow better air flow
  over surface
• May help cool leaf
• May improve carbon dioxide uptake

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Petiole

• Narrow base of most dicot leaves
• Leaf without petiole sessile
• Vary in shape
• Improves photosynthesis
• Reduces extent to which leaf is shaded by other
  leaves
• Allows blade to move in response to air currents

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Sheath

• Formed by monocot leaf base wrapping around stem
• Ligule
• Keeps water and dirt from getting between stem
  and leaf sheath
• Auricles
• In some grass species
• Two flaps of leaf tissue
• Extend around stem at juncture of sheath and blade

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Sheath

• Why does grass need mowing so often?
• Grass grows from base of sheath
• Intercalary meristem
• Allows for continued growth of mature leaf
• Stops dividing when leaf reaches certain age or
  length

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Leaf Veins

• Vascular bundles composed of xylem and phloem

Type of venation Example Description
Parallel Monocots Several major veins running parallel from base to tip of leaf Minor veins perpendicular to major veins
Netted Dicots Major vein (midvein or midrib) runs up middle of leaf Lateral veins branch from midvein
Open dichotomous Ferns and some gymnosperms Y-branches with no small interconnecting veins
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Epidermis

• Covers entire surface of blade, petiole, and leaf
  sheath
• Continuous with stem epidermis
• Usually a single layer of cells
• Cell types
• Epidermal cells
• Guard cells
• Subsidiary cells
• Trichomes

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Epidermal Cells

• Appear flattened in cross-sectional view
• Outer cell wall somewhat thickened
• Covered by waxy cuticle
• Inhibits evaporation through outer epidermal cell
  wall

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Stomatal Apparatus

• Cuticle blocks most evaporation
• Opening needed in epidermis for controlled gas
  exchange
• Two guard cells pore stoma
• Subsidiary cells
• Surround guard cells
• May play role in opening and closing pore

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Stomatal Apparatus

• Guard cells subsidiary cells stomatal
  apparatus
• Functions of stoma
• Allows entry of CO2 for photosynthesis
• Allows loss of water vapor by transpiration
• Cools leaf by evaporation
• Pulls water up from roots

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Stomatal Apparatus

• Stomata usually more numerous on bottom of leaf
• Stomata also found in
• Epidermis of young stem
• Some flower parts

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Trichomes

• Secretory
• Stalk with multicellular or secretory head
• Secretion often designed to attract pollinators
  to flowers
• Short hairs
• Example saltbush (Atriplex)
• Hairs store water, reflect sunlight, insulate
  leaf against extreme desert heat

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Trichomes

• Mat of branched hairs
• Example olive tree (Olea europea)
• Act as heat insulators
• Specialized trichomes
• Leaves modified to eat insects as food

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Mesophyll

• Two distinct regions in dicot leaf
• Palisade mesophyll
• Spongy mesophyll
• Substomatal chamber
• Air space just under stomata

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Mesophyll
Type Cell type Location Description
Palisade mesophyll Palisade parenchyma, tightly packed, column shaped, oriented at right angles to leaf surface Usually on upper surface Cells tightly packed, absorb sunlight more efficiently
Spongy mesophyll Spongy parenchyma cells, irregularly shaped, abundant air spaces Usually located on bottom surface Irregular cell shape, abundant air spaces allow more efficient air exchange
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Mesophyll

• Dicot midrib (midvein)
• Xylem in upper part of bundle
• Phloem in lower part of bundle
• Bundle sheath
• Single layer of cells surrounding vascular bundle
• Loads sugars into phloem
• Unloads water and minerals out of xylem

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Formation of New Leaves

• Originate from meristems
• Leaf primordia early stages of development

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Formation of New Leaves

• Steps in leaf formation
• Initiated by chemical signal
• Location in leaf depends on plants phyllotaxis
• Cells at location begin dividing
• Becomes leaf primordium
• Shape of new leaf determined by how cells in
  primordium divide and enlarge

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Cotyledons

• Seed leaves
• Primarily storage organs
• Slightly flattened, often oval shaped
• Usually wither and die during seedling growth
• Example of exception bean plant
• Cotyledons enlarge and conduct photosynthesis

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Heterophylly

• Different leaf shapes on a single plant
• Types of heterophylly
• Related to age of plant
• Example ivy (Hedera helix)
• Juvenile ivy leaves three lobes to leaves
• Adult ivy leaves leaves are not lobed

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Heterophylly

• Environment to which shoot apex is exposed during
  leaf development
• Example marsh plants
• Water leaves
• Leaves developing underwater are thin with deep
  lobes
• Air leaves
• Shoot tip above water in summertime develops
  thicker leaves with reduced lobing

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Heterophylly

• Position of leaf on tree
• Shade leaves
• Develop on bottom branches of tree
• Mainly exposed to shade
• Leaves are thin with large surface area
• Sun leaves
• Develop near top of same tree
• Exposed to more direct sunlight
• Leaves are thicker and smaller

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Adaptations for Environmental Extremes

• Xerophytes
• Grow in dry climates
• Leaves designed to conserve water, store water,
  insulate against heat
• Sunken stomata
• Thick cuticle
• Sometimes multiple layers to epidermis

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Adaptations for Environmental Extremes

• Xerophytes
• Abundance of fibers in leaves
• Help support leaves
• Help leaf hold shape when it dries
• Examples
• Oleander (Nerium oleander)
• Fig (Ficus)
• Jade plant (Crassula argentea)

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Adaptations for Environmental Extremes

• Hydrophytes
• Grow in moist environments
• Lack characteristics to conserve water
• Leaves
• Thin
• Thin cuticle
• Often deeply lobed
• Mesophytes
• Grow in moderate climates

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Leaf Modifications

• Spines
• Cells with hard cell wall
• Pointed and dangerous to potential predators
• Tendrils
• Modified leaflets
• Wrap around things and support shoot

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Leaf Modifications

• Bulbs
• Thick leaves sometimes referred to as bulb scales
• Store food and water
• Modified branches with short, thick stem and
  short, thick storage leaves

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Leaf Modifications

• Plantlets
• Leaves have notches along margins
• Meristem develops in bottom of each notch that
  produce a new plantlet
• Plantlet falls off leaf and roots in soil
• Form of vegetative (asexual) reproduction
• Example
• Air-plant (Kalanchoe pinnata)

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Leaf Abscission

• Abscission separation
• Result of differentiation and specialization at
  region at base of petiole called abscission zone
• Weak area due to
• Parenchyma cells in abscission zone are smaller
  and may lack lignin in cell walls
• Xylem and phloem cells are shorter in vascular
  bundles at base of petiole
• Fibers often absent in abscission zone

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Leaf Abscission

• Abscission zone weakens
• Cells in vascular bundles become plugged
• Leaf falls off
• Leaf scar
• Scar that remains when leaf falls off
• Sealed over with waxy materials which block
  entrance of pathogens

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Environmental Abscission Controls

• Cold temperatures
• Short days
• Induce hormonal changes that affect formation of
  abscission zone
• Leaves move nutrients back into stem
• Leaves lose color
• Leaves fall off tree
• Leaves decompose and recycle nutrients