Plant Organs: RootsPlant Organs: Roots

Chapter 6Chapter 6

KEY TERMSKEY TERMS

• TAPROOT SYSTEMTAPROOT SYSTEM • A root system consisting of one prominent A root system consisting of one prominent

main root with smaller lateral roots branching main root with smaller lateral roots branching from itfrom it

• FIBROUS ROOT SYSTEMFIBROUS ROOT SYSTEM • A root system consisting of several A root system consisting of several

adventitious roots of approximately equal size adventitious roots of approximately equal size that arise from the base of the stemthat arise from the base of the stem

Root SystemsRoot Systems

LEARNING OBJECTIVE 1LEARNING OBJECTIVE 1

• Describe the functions of Describe the functions of rootsroots

• Describe two features of roots that Describe two features of roots that shoots lackshoots lack

Functions of RootsFunctions of Roots

• Anchorage Anchorage • AbsorptionAbsorption• ConductionConduction• Storage Storage

Storage RootsStorage Roots

KEY TERMSKEY TERMS

• ROOT CAPROOT CAP • A covering of cells over the root tip that A covering of cells over the root tip that

protects delicate meristematic tissue directly protects delicate meristematic tissue directly behind itbehind it

• ROOT HAIRROOT HAIR • An extension of an epidermal cell of a root An extension of an epidermal cell of a root

that increases absorptive capacity of the rootthat increases absorptive capacity of the root

Root CapRoot Cap

• Each root tip has a Each root tip has a root caproot cap• A protective thimblelike layer A protective thimblelike layer • Many cells thickMany cells thick• Covers delicate root apical meristem Covers delicate root apical meristem • May orient root so it grows downward May orient root so it grows downward

Root HairsRoot Hairs

• Short-lived, unicellular extensions of Short-lived, unicellular extensions of epidermal cells near the growing root tip epidermal cells near the growing root tip

• Increase surface area of root in contact with Increase surface area of root in contact with moist soil, increasing root’s absorptive moist soil, increasing root’s absorptive capacitycapacity

Structures Unique to RootsStructures Unique to Roots

Root apicalmeristem(area ofcell division)

Root cap

(a) The root cap of an onion (Allium cepa)root. The root cap protects the root’s apicalmeristem.

Fig. 6-3a, p. 114

Root hairs

Soil airSoil water

Soil particles

Epidermis

(b) Root hairs on a radish (Raphanus sativus) seedling. Each delicate hair is aunicellular extension of the root epidermis. Root hairs increase the surface areain contact with the soil.

Fig. 6-3b, p. 114

LEARNING OBJECTIVE 2LEARNING OBJECTIVE 2

• Contrast the structure of a Contrast the structure of a primary primary eudicot rooteudicot root and a and a monocot rootmonocot root

• Describe the functions of each tissue Describe the functions of each tissue

Primary Eudicot Roots 1Primary Eudicot Roots 1

• Outer protective coveringOuter protective covering• EpidermisEpidermis

• Ground tissues Ground tissues • CortexCortex • PithPith (in certain roots) (in certain roots)

• Vascular tissuesVascular tissues• XylemXylem• PhloemPhloem

Primary Eudicot Roots 2Primary Eudicot Roots 2

• EpidermisEpidermis • Protects the rootProtects the root• Root hairs help absorb water and dissolved Root hairs help absorb water and dissolved

minerals minerals

• CortexCortex • Consists of parenchyma cellsConsists of parenchyma cells• Usually stores starch Usually stores starch

KEY TERMSKEY TERMS

• ENDODERMISENDODERMIS • Innermost layer of the cortex of the root that Innermost layer of the cortex of the root that

prevents water and dissolved materials from prevents water and dissolved materials from entering the xylem by passing between cellsentering the xylem by passing between cells

KEY TERMSKEY TERMS

• CASPARIAN STRIPCASPARIAN STRIP • A band of waterproof material around the A band of waterproof material around the

radial and transverse cells of the endodermisradial and transverse cells of the endodermis• Ensures that water and minerals enter the Ensures that water and minerals enter the

xylem only by passing through the xylem only by passing through the endodermal cellsendodermal cells

Endodermis and Mineral UptakeEndodermis and Mineral Uptake

Endodermis

Epidermis

Root cross section Casparian stripMovement ofwater throughthe endodermisto the centerof the root

Endodermis

Casparianstrip

Cortex

Fig. 6-5, p. 118

KEY TERMSKEY TERMS

• PERICYCLE PERICYCLE • A layer of cells just inside the endodermis of A layer of cells just inside the endodermis of

the root the root • Gives rise to Gives rise to lateral rootslateral roots

Lateral RootLateral Root

Fig. 6-7, p. 120

Willow

Pericycle

CortexLateral root

Ruptured epidermis

Fig. 6-7, p. 120

Primary Eudicot Roots 3Primary Eudicot Roots 3

• XylemXylem • conducts water and dissolved mineralsconducts water and dissolved minerals

• PhloemPhloem • conducts dissolved sugar conducts dissolved sugar

Herbaceous Eudicot RootHerbaceous Eudicot Root

Buttercup

Cortex

Stele

(a) Cross section of a buttercup (Ranunculus) root.Note that the bulk of the root is the cortex.

Epidermis

Fig. 6-4a, p. 116

Cortex cellsfilled withamyloplasts

Endodermiscell

Pericycle cell

Xylem vesselelements

Intercellularspace

(b) A close-up of the stele of the buttercup root.Note the solid core of vascular tissues.

Phloem cell

Fig. 6-4b, p. 116

Comparing Monocot and EudicotComparing Monocot and Eudicot

• Monocot roots often have a Monocot roots often have a pithpith in the in the center of the rootcenter of the root• In herbaceous eudicot roots, xylem and In herbaceous eudicot roots, xylem and

phloem form a solid mass in center of rootphloem form a solid mass in center of root

• Monocot roots lack a Monocot roots lack a vascular cambiumvascular cambium• Do not have secondary growthDo not have secondary growth

Monocot Monocot RootRoot

Monocot RootMonocot Root

Stele

Cortex

Epidermis

Greenbrier

Fig. 6-8a, p. 121

Pith

Vasculartissues

Xylem

Phloem

Pericycle

Endodermis

Fig. 6-8b, p. 121

LEARNING OBJECTIVE 3LEARNING OBJECTIVE 3

• Trace the pathway of water from the Trace the pathway of water from the soil through the various root tissuessoil through the various root tissues

Water Movement 1Water Movement 1

• In a primary eudicot root, water moves In a primary eudicot root, water moves from soil into center of root:from soil into center of root:• Root hair Root hair →→ epidermis epidermis →→ cortex ( cortex (symplastsymplast or or

apoplastapoplast pathway) pathway) →→ endodermis endodermis →→ pericycle pericycle →→ xylem of root xylem of root

• Water is transported upward through root Water is transported upward through root xylem into stem xylem and rest of plant xylem into stem xylem and rest of plant

KEY TERMSKEY TERMS

• SYMPLASTSYMPLAST • A continuum consisting of the cytoplasm of A continuum consisting of the cytoplasm of

many plant cells, connected from one cell to many plant cells, connected from one cell to the next by the next by plasmodesmataplasmodesmata

• APOPLASTAPOPLAST • A continuum consisting of the interconnected, A continuum consisting of the interconnected,

porous plant cell walls, along which water porous plant cell walls, along which water moves freelymoves freely

Symplast and ApoplastSymplast and Apoplast

Xylemvessels

Phloemcells

Pericycle

Plasma membrane

PlasmodesmaCell wall Apoplast:

interconnectedcell-wall spaces

Root hair

Water anddissolvednutrientminerals

Symplast:interconnectedcytoplasm ofliving cells

EpidermisCortex

Casparianstrip

EndodermisMovement upward

Fig. 6-6, p. 119

LEARNING OBJECTIVE 4LEARNING OBJECTIVE 4

• Describe several roots that are Describe several roots that are modified to perform unusual functionsmodified to perform unusual functions

KEY TERMSKEY TERMS

• PROP ROOT PROP ROOT • An adventitious root that arises from the stem An adventitious root that arises from the stem

and provides additional support for the plantand provides additional support for the plant

• CONTRACTILE ROOTCONTRACTILE ROOT • A specialized root, often found on bulbs or A specialized root, often found on bulbs or

corms, that contracts and pulls the plant to a corms, that contracts and pulls the plant to a desirable depth in the soildesirable depth in the soil

Prop RootsProp Roots

Contractile RootsContractile Roots

(a) Plants that produce corms or bulbs often havecontractile roots that lose much of their length asroot cells shorten and broaden.

Contractileroots

Corm

Fig. 6-13a, p. 124

Contractileroots

Fig. 6-13b, p. 124

KEY TERMSKEY TERMS

• PNEUMATOPHORE PNEUMATOPHORE • A specialized aerial root produced by certain A specialized aerial root produced by certain

trees living in swampy habitatstrees living in swampy habitats• May facilitate gas exchange between the May facilitate gas exchange between the

atmosphere and submerged roots atmosphere and submerged roots

PneumatophoresPneumatophores

Other Modified Roots 1Other Modified Roots 1

• Buttress rootsButtress roots • Swollen bases or braces that hold trees upright Swollen bases or braces that hold trees upright • Aid in extensive distribution of shallow rootsAid in extensive distribution of shallow roots• Found in some tropical rainforest treesFound in some tropical rainforest trees

Buttress Buttress RootsRoots

Other Modified Roots 2Other Modified Roots 2

• SuckersSuckers • Aboveground stems that develop from Aboveground stems that develop from

adventitious buds on the rootsadventitious buds on the roots• Asexual reproduction method of some rootsAsexual reproduction method of some roots

• Certain epiphytes have roots that are Certain epiphytes have roots that are modified to photosynthesizemodified to photosynthesize

Parasitic Parasitic EpiphytesEpiphytes

(b) Micrograph of a parasitized juniper (Juniperus)branch, showing a mistletoe root penetrating thewood (secondary xylem) of the juniper.

Juniperwood

Mistletoeroot

Juniperbark

Fig. 6-12b, p. 123

LEARNING OBJECTIVE 5LEARNING OBJECTIVE 5

• Discuss the significance of roots to Discuss the significance of roots to humanshumans

Important FoodsImportant Foods

• Roots which store the products of Roots which store the products of photosynthesis are important sources of photosynthesis are important sources of food for human consumption food for human consumption

• Some roots are used as flavoringsSome roots are used as flavorings• Example:Example: root beer flavoring (dried greenbrier root beer flavoring (dried greenbrier

roots)roots)

Root CropsRoot Crops

• Predominantly Predominantly taprootstaproots • carrots, beets, sugar beets, parsnips, turnips, carrots, beets, sugar beets, parsnips, turnips,

rutabagas, radishes rutabagas, radishes

• Some Some fibrous rootsfibrous roots • sweet potatoes, cassavasweet potatoes, cassava

KEY TERMSKEY TERMS

• MYCORRHIZAMYCORRHIZA • A mutually beneficial association between a A mutually beneficial association between a

fungus and a root that helps the plant absorb fungus and a root that helps the plant absorb essential minerals from the soilessential minerals from the soil

• NODULENODULE • A small swelling on the root of a leguminous A small swelling on the root of a leguminous

plant in which beneficial nitrogen-fixing plant in which beneficial nitrogen-fixing bacteria (bacteria (RhizobiumRhizobium) live) live

MycorrhizaeMycorrhizae

Fungal hyphabetween plantcells

Sheath offungal hyphaeencircles root

(a) Cross section of root showing ectomycorrhizae,fungal associations that form a sheath around theroot. The fungal hyphae penetrate the root betweencortical cells but do not enter the cells.

Fig. 6-14a, p. 124

Fungal hyphaewithin plantcortical cells

(b) Cells of a root cortex showing endomycorrhizae,fungal associations in which the fungal hyphae penetrateroot cells of the cortex to aid in delivering and receivingnutrients. Endomycorrhizae colonize roots of most vascularplant species. Fig. 6-14b, p. 124

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Animation: Water AbsorptionAnimation: Water Absorption

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RootRoot

Animation: Root Cross-Animation: Root Cross-sectionsection

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