plant organs: roots chapter 6. key terms taproot system taproot system a root system consisting of...
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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
Animation: Root SystemsAnimation: Root Systems
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Animation: Water AbsorptionAnimation: Water Absorption
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Animation: Root FunctioningAnimation: Root Functioning
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RootRoot
Animation: Root Cross-Animation: Root Cross-sectionsection
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