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