Plant Organs: Stems Chapter 7. LEARNING OBJECTIVE 1 Describe three functions of stems Describe three functions of stems.

Plant Organs: StemsPlant Organs: Stems

Chapter 7Chapter 7

LEARNING OBJECTIVE 1LEARNING OBJECTIVE 1

• Describe three functions of Describe three functions of stemsstems

Stem Functions Stem Functions

• Support Support • leaves and reproductive structures leaves and reproductive structures

• Conduct Conduct • water, dissolved minerals, carbohydrates water, dissolved minerals, carbohydrates

• Produce new living tissues Produce new living tissues • at apical meristems at apical meristems • at lateral meristems (secondary growth)at lateral meristems (secondary growth)

Variation in StemsVariation in Stems

LEARNING OBJECTIVE 2LEARNING OBJECTIVE 2

• Relate the functions of each tissue in Relate the functions of each tissue in an an herbaceous stemherbaceous stem

Tissues in Herbaceous Stems 1Tissues in Herbaceous Stems 1

• EpidermisEpidermis • protective outer layer protective outer layer • covered by water-conserving covered by water-conserving cuticlecuticle

• Vascular tissuesVascular tissues• XylemXylem conducts water and dissolved minerals conducts water and dissolved minerals • phloemphloem conducts dissolved carbohydrates conducts dissolved carbohydrates

(sucrose)(sucrose)

Tissues in Herbaceous Stems 2Tissues in Herbaceous Stems 2

• Storage tissuesStorage tissues• Cortex Cortex andand pith pith• ground tissue ground tissue

LEARNING OBJECTIVE 3LEARNING OBJECTIVE 3

• Contrast the structures of an Contrast the structures of an herbaceous eudicotherbaceous eudicot stem and a stem and a monocot monocot stemstem

Herbaceous Eudicot StemsHerbaceous Eudicot Stems

• HaveHave vascular bundlesvascular bundles arranged in a arranged in a circle (in cross section)circle (in cross section)

• Have a distinct cortex and pith Have a distinct cortex and pith

Monocot StemsMonocot Stems

• Have scattered Have scattered vascular bundlesvascular bundles

• Have ground tissue instead of distinct Have ground tissue instead of distinct cortex and pithcortex and pith

Monocot StemsMonocot Stems

Epidermis Groundtissue

Vascularbundles

Cornseedling

(a) Cross section of a corn (Zea mays)stem, showing the scattered vascularbundles.

(b) Close-up of a vascular bundle. The air space is where the first xylem elements formed. The entire bundle is enclosed in a bundle sheath of sclerenchyma for additional support.

Bundlesheath

Airspace

Xylem

Phloem

Groundtissue

Fig. 7-4, p. 134

LEARNING OBJECTIVE 4LEARNING OBJECTIVE 4

• Distinguish between the structures of Distinguish between the structures of stems and rootsstems and roots

Differences Between Differences Between Stems and Roots 1Stems and Roots 1

• Unlike roots, stems have Unlike roots, stems have nodesnodes and and internodesinternodes, , leavesleaves and and budsbuds

• Unlike stems, roots have Unlike stems, roots have root capsroot caps and and root hairsroot hairs

KEY TERMSKEY TERMS

• NODE NODE • Area on a stem where one or more leaves is Area on a stem where one or more leaves is

attached; stems have nodes, but roots do notattached; stems have nodes, but roots do not

• INTERNODEINTERNODE • Stem area between two successive nodesStem area between two successive nodes

KEY TERMSKEY TERMS

• BUD BUD • An undeveloped shoot that contains an An undeveloped shoot that contains an

embryonic meristem embryonic meristem • May be terminal (at tip of stem) or axillary (on May be terminal (at tip of stem) or axillary (on

side of stem)side of stem)

A Woody TwigA Woody Twig

Bud scaleTerminalbud

One year’sgrowth

Terminal bud scale scars

Axillary budLeaf scar

Node

Node

Internode

Lenticels

Terminal budscale scars

Bundle scars

Fig. 7-2, p. 131

Differences Between Differences Between Stems and Roots 2Stems and Roots 2

• Internally Internally • herbaceous roots possess an endodermis herbaceous roots possess an endodermis

and pericycle and pericycle • stems lack a pericycle and rarely have an stems lack a pericycle and rarely have an

endodermisendodermis

Differences Between Differences Between Stems and RootsStems and Roots

LEARNING OBJECTIVE 5LEARNING OBJECTIVE 5

• Outline the transition from Outline the transition from primary primary growthgrowth to to secondary growthsecondary growth in a woody in a woody stem stem

• List the two List the two lateral meristemslateral meristems, and , and describe the tissues that arise from describe the tissues that arise from eacheach

Primary Primary Growth: Growth: EudicotEudicot

Sunflower

Vascularbundles

PithCortex

Epidermis

Epidermis

CortexPhloemfiber cap

PhloemVascularcambiumXylem

Pith ray

Pith

Vesselelement

Vascu

lar bu

nd

le

(a) Cross section of a sunflower (Helianthus annuus)stem, showing the organization of tissues. The vascularbundles are arranged in a circle.

(b) Close-up of a vascular bundle. The xylem is toward the stem’sinterior, and the phloem toward the outside. Each vascular bundleis “capped” by a batch of fibers for additional support. Fig. 7-3, p. 132

KEY TERMSKEY TERMS

• VASCULAR CAMBIUMVASCULAR CAMBIUM • A lateral meristem that produces secondary A lateral meristem that produces secondary

xylem (wood) to the inside and secondary xylem (wood) to the inside and secondary phloem (inner bark) to the outsidephloem (inner bark) to the outside

Secondary GrowthSecondary Growth

• Occurs in woody eudicots and conifers Occurs in woody eudicots and conifers

• Produced byProduced by vascular cambium vascular cambium • between primary xylem and primary phloembetween primary xylem and primary phloem

Vascular Cambium 1Vascular Cambium 1

• Is not initially a solid cylinder of cellsIs not initially a solid cylinder of cells• becomes continuous when production of becomes continuous when production of

secondary tissues beginssecondary tissues begins

Vascular Cambium 2Vascular Cambium 2

• Certain parenchyma cells between Certain parenchyma cells between vascular bundlesvascular bundles• retain ability to divide retain ability to divide • connect to vascular cambium cells in each connect to vascular cambium cells in each

vascular bundlevascular bundle• form a complete ring of vascular cambiumform a complete ring of vascular cambium

Dividing Vascular CambiumDividing Vascular Cambium

Tim

e

Secondary xylem Secondary phloem

Second division of vascularcambium forms a phloem cell.

Division of vascular cambium formstwo cells, one xylem cell and onevascular cambium cell.

Vascular cambium cell whensecondary growth begins.

Vascular cambium cell

1X

1X

1X

1X

1X

1X2X

2X

2X

2X

1P

1P

1P

1P

1P

2P

2P

2P

3X

3X4X

Fig. 7-6, p. 136

KEY TERMSKEY TERMS

• CORK CAMBIUMCORK CAMBIUM • A lateral meristem that produces cork A lateral meristem that produces cork

parenchyma to the inside and cork cells to the parenchyma to the inside and cork cells to the outsideoutside

• Cork cambium and the tissues it produces Cork cambium and the tissues it produces make up the outer bark of a woody plantmake up the outer bark of a woody plant

Cork CambiumCork Cambium

• Arises near the stem’s surface Arises near the stem’s surface

• Is either a continuous cylinder of dividing Is either a continuous cylinder of dividing cells or a series of overlapping arcs of cells or a series of overlapping arcs of meristematic cells that form from meristematic cells that form from parenchyma cells in successively deeper parenchyma cells in successively deeper layers of the cortex and, eventually, layers of the cortex and, eventually, secondary phloem secondary phloem

Development: Development: Woody Woody EudicotEudicot

(a) At the onset of secondary growth, vascular cambium arises in theparenchyma between the vascular bundles (that is, in the pith rays), forminga cylinder of meristematic tissue (blue circle in cross section).

Pith

Vascular cambium

Cortex

EpidermisPrimaryxylem

Primaryphloem

Fig. 7-5a, p. 135

(b) Vascular cambium begins to divide, forming secondary xylem on the insideand secondary phloem on the outside.

Remnant ofprimary xylem

Remnant of pith

Vascularcambium

Periderm(outer bark)

Secondary xylem(wood)

Secondary phloem(inner bark)

Remnantof epidermis

Remnantof cortexRemnant

of primaryphloem

Fig. 7-5b, p. 135

Secondary xylem(wood)

Remnant ofprimary xylem

Remnant ofpith

Vascularcambium

Secondary phloem(inner bark)

Periderm(outer bark; remnants ofprimary phloem, cortex,and epidermis are graduallycrushed or torn apart andslough off)

(c) A young woody stem. Vascular cambium produces more secondary xylemthan secondary phloem.

Fig. 7-5c, p. 135

3-Year-Old Stem3-Year-Old Stem

(a) LM of cross section of basswood (Tilia americana) stem. Note thelocation of the vascular cambium between the secondary xylem (wood) and secondary phloem (inner bark).

Pith Primary xylemAnnual ring ofsecondary xylem

Secondary xylem (wood)

Vascular cambium

Secondary phloem

Periderm and remnants of primaryphloem, cortex, and epidermis

Expanded phloem ray

Xylem ray

Remains ofepidermis

Cork Corkcambium

Expandedphloem ray

CortexSecondary phloemVascular cambium

Secondary xylem (third year)

Xylem rays

Secondary xylem (first year)Primary xylemPith

(b) Sketch of a pie-shaped segment of the cross section. The primary phloem is not labeled because it is crushed beyondrecognition.

Fig. 7-7, p. 137

Onset of Secondary GrowthOnset of Secondary Growth

Cross sectionof twig Vascular

cambiumPith

Primary xylem

Secondary xylem

Vascular cambiumSecondary phloem

Primary phloem

Phloemfiber cap

Cortex

Fig. 7-8, p. 138

Variation in BarkVariation in Bark

LenticelsLenticels

Calico flower

Lenticel

Cork cambium and cork parenchyma(phelloderm)

Corkcells

Fig. 7-10, p. 141

Heartwood and SapwoodHeartwood and Sapwood

Sapwood

Heartwood

Fig. 7-11, p. 141

Tree-Ring DatingTree-Ring Dating

Tilia(basswood)

Long, slender core of woodextracted by a boring tool

Bark

Annual ringsPith

Outerbark

Vascularcambium Annual rings Pith

Outermost ring is the year when the tree was cut.

Matching and overlapping older and older wood sections extends dates

back in time

Sample from an old building in the same area as the forest

Sample from a dead treein the same forest

Sample from a living tree

1931 1926 1920 1918

1940 1932 1931 1926

193219401950

p. 143

Annual RingsAnnual Rings

Cross sectionof 3-year-oldTilia stem Late summerwood

of preceding year

Springwood

Annualring ofxylem

Latesummerwood

Vascular cambium

Secondaryphloem

Fig. 7-12, p. 144

SectionsSections

Annualrings

RaysRays Annual

rings

(c) Radial section

Ray(a) Cross sectionAnnualrings

(b) Tangential section

Fig. 7-13, p. 144

KEY TERMSKEY TERMS

• DEFORESTATIONDEFORESTATION • The temporary or permanent clearance of The temporary or permanent clearance of

large expanses of forests for agriculture or large expanses of forests for agriculture or other usesother uses

Rain Forest DistributionRain Forest Distribution

KEY TERMSKEY TERMS

• VINE VINE • A plant with a long, thin, often climbing stem A plant with a long, thin, often climbing stem

LEARNING OBJECTIVE 6LEARNING OBJECTIVE 6

• Contrast the various stems that are Contrast the various stems that are specialized for specialized for asexual reproductionasexual reproduction

KEY TERMSKEY TERMS

• RHIZOMERHIZOME • A horizontal underground stem that often A horizontal underground stem that often

serves as a storage organ and a means of serves as a storage organ and a means of sexual reproductionsexual reproduction

• Example:Example: iris iris

A RhizomeA Rhizome

Adventitiousroots

Rhizome

Fig. 7-14a, p. 147

KEY TERMSKEY TERMS

• TUBER TUBER • The thickened end of a rhizome that is fleshy The thickened end of a rhizome that is fleshy

and enlarged for food storageand enlarged for food storage • Example:Example: white potato white potato

A TuberA Tuber

Roots

Tuber

Rhizome

Fig. 7-14b, p. 147

KEY TERMSKEY TERMS

• BULB BULB • A rounded, fleshy underground bud that A rounded, fleshy underground bud that

consists of a short stem with fleshy leavesconsists of a short stem with fleshy leaves• Example:Example: onion onion

A BulbA Bulb

Bulb Fleshyleaves

Stem

Adventitiousroots

Fig. 7-14c, p. 147

KEY TERMSKEY TERMS

• CORM CORM • A short, thickened underground stem A short, thickened underground stem

specialized for food storage and asexual specialized for food storage and asexual reproductionreproduction

• Example:Example: crocus crocus

A A CormCorm

Adventitiousroots

Old corm(last year’s)

Corm(modifiedstem)

Axillary bud

Leaf scars

Fig. 7-14d, p. 147

KEY TERMSKEY TERMS

• STOLON STOLON • An aerial horizontal stem with long internodes; An aerial horizontal stem with long internodes;

often forms buds that develop into separate often forms buds that develop into separate plantsplants

• Example:Example: strawberry strawberry

StolonsStolons

Newshoot

Scale leaf(at node)

Adventitiousroots

Stolon(runner)

Fig. 7-14e, p. 147

Animation: Layers in a Woody Animation: Layers in a Woody StemStem

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Animation: Growth in a Animation: Growth in a Walnut TwigWalnut Twig

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Animation: Annual RingsAnimation: Annual Rings

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