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The Shoot System I: The Stem
Chapter 5
Organization of Shoot System
• Shoot system of flowering plant consists of– Stem with attached leaves, buds, flowers, and
fruits
Fig. 5-1, p. 71
bud
node
internode
RAM
lateral root
primary root
Root system
Shoot system
leaf
module
terminal budcontains SAM
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
Shoot System
• Modules– Repeating units of the stem– Consists of internode plus the leaf and bud
attached to the stem
• Node– Point of attachment
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
SAM
• SAM – Shoot apical meristem– Composed of dividing cells
• Three primary meristems– Protoderm– Ground meristem– procambium
Fig. 5-3, p. 73
groundmeristem
young leaf
SAM
procambium
protoderm
Protoderm
• Outermost layer of cells in shoot tip
• When cells stop dividing and mature called epidermis
Ground Meristem
• In center of shoot tip
• Just inside protoderm
• Cells slowly lose ability to divide
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
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
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
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
Fig. 5-4, p. 73
leaf trace
protoderm
Apical meristem
Three primary meristems
ground meristemprocambium
primary phloem
primary xylemresidual procambium
epidermiscortex
pithvascular bundle
stem ofprimary
plant body
Fig. 5-5, p. 73
small vascular bundle
leaf traces
node
petiole
vascular bundle
internode
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
Leaf Arrangements
Pattern Leaves/nodeAngle of divergence
Alternate 1 leaf/node 180º
Opposite 2 leaves/node 90º
Whorled3 or more leaves/node
60º
Spiral 1 leaf/node 137.5º
Fig. 5-6, p. 74
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
Fig. 5-7, p. 75
vascularbundle
hollowcentercortexepidermis
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
Fig. 5-9, p. 76
residual procambium
parenchymaprimary xylem
primary phloem
Cells begindividing
Vascularcambium
forms
Secondaryxylem and
phloemform
secondary xylem
secondary phloem
vascular cambium
secondary xylem
secondary phloem
vascularbundle
residual procambium
vascularbundle
primary xylemparenchyma
primaryphloem
interfascicularcambium
fascicularcambium
vascular cambium
secondary phloem
secondary xylem
secondary xylem
secondary phloem
vascular cambium
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
Fig. 5-10a, p. 77
interfascicularcambium
fascicularcambium
primaryphloem
primaryxylem
epidermis
Fig. 5-10b, p. 77vascular cambium
Formation of Secondary Xylem and Phloem
• Dividing residual procambium between bundles called interfascicular cambium
• Fascicular cambium + interfascicular cambium = vascular cambium
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
Fig. 5-11, p. 77
divisionsand
differentiationcontinues
cell ofvascularcambiumat start ofsecondarygrowth
surface ofstem or root
one celldifferentiatesinto xylem,one stays
meristematic
initial
division divisionone cell
differentiatesinto phloem,
one staysmeristematic
DIRECTION OF GROWTH
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
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
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
Fig. 5-12a, p. 78
Axialsystem
primary growth, somesecondary growth
year 1 2 3
barkvascular cambium
secondary growth
Ray system
Annual Rings
• Oldest known trees– Redwoods (Sequoia sempervirens)– Bristlecone pines (Pinus longaeva)
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
Annual Ring Components
• Ring porous – Large diameter vessels mainly in springwood
• Diffuse porous– Large diameter vessel members uniformly
distributed throughout springwood and summerwood
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
Fig. 5-16a, p. 80
secondary xylem
periderm
bark
vascular cambium
secondaryphloem
heartwood sapwood
Sapwood
• Lighter wood near periphery
• Secondary xylem – Has functional xylem cells
• Where actual transport of water and dissolved minerals takes place
Fig. 5-16b, p. 80
heartwood
sapwood
branch (knot)
Gymnosperm Structure
• Wood –simpler structure
• Mostly tracheids in axial system and simple rays
• May have resin ducts – Secretory structures that produce and
transport resin
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
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
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
Periderm
• Made up of – Phellem– Cork cambium– phelloderm
• Functions– Inhibits water evaporation– Protects against insect and pathogen invasion
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
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
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
Main Bark Patterns
Pattern Description Example
Ring bark Continuous rings Paper birch
Scale barkSmall, overlapping scales
Pine trees
Shag barkLong, overlapping, thin sheets
Eucalyptus
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
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
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
Secondary Growth in Monocot Stems
• Some monocots exhibit true secondary growth• Examples – Yucca, Agave (century plant),
Dracaena (dragon’s blood tree)• Produce stems that are thin at top, thick at base• Cambium primarily forms parenchyma cells• Xylem surrounds phloem in vascular bundle
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
Stem Modifications
• Tubers– Enlarged terminal portion of underground
rhizome– Example: potato plant– Eyes of tuber - lateral buds
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
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)
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 (Butcher’s
broom)
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
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
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
The Shoot System II: The Form and Structure of Leaves
Chapter 6
Functions of Leaves
• Photosynthesis– Release oxygen, synthesize sugars
• Transpiration– Evaporation of water from leaf surface
• Specialized functions– Water storage– Protection
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
Leaf Blade
• Broad, flat surface for capturing light and CO2
• Two types of leaves– Simple leaves– Compound leaves
Leaf Blade
• Simple leaves– Leaves with a single blade– Examples
• Poplar• Oak • Maple
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
Leaf Blade
– Advantages of compound leaves• Spaces between leaflets allow better air flow over
surface– May help cool leaf– May improve carbon dioxide uptake
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
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
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
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
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
Epidermal Cells
• Appear flattened in cross-sectional view
• Outer cell wall somewhat thickened
• Covered by waxy cuticle– Inhibits evaporation through outer epidermal
cell wall
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
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
Stomatal Apparatus
• Stomata usually more numerous on bottom of leaf
• Stomata also found in– Epidermis of young stem– Some flower parts
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
Trichomes
• Mat of branched hairs– Example: olive tree (Olea europea)– Act as heat insulators
• Specialized trichomes– Leaves modified to eat insects as food
Mesophyll
• Two distinct regions in dicot leaf– Palisade mesophyll– Spongy mesophyll
• Substomatal chamber– Air space just under stomata
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
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
Formation of New Leaves
• Originate from meristems
• Leaf primordia – early stages of development
Formation of New Leaves
• Steps in leaf formation– Initiated by chemical signal– Location in leaf depends on plant’s phyllotaxis– Cells at location begin dividing
• Becomes leaf primordium
– Shape of new leaf determined by how cells in primordium divide and enlarge
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
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
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
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
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
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)
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
Leaf Modifications
• Spines– Cells with hard cell wall– Pointed and dangerous to potential predators
• Tendrils– Modified leaflets– Wrap around things and support shoot
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
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)
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
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
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