Chapter 35: Plant Structure, Growth & Development

Chapter 35:Plant Structure, Growth &Development1. Vascular Plant Structure2. Vascular Plant Growth3. Vascular Plant Development

1. Vascular Plant Structure

“Roots & Shoots”Reproductive shoot (flower)Apical budSHOOT SystemNode(above ground)InternodeApical budShootsystemVegetativeshootLeaf stems leaves flowers, fruitsBladePetioleAxillary budStemROOT SystemTaproot(below ground)Lateral(branch)rootsRootsystem taproot (if present) lateral roots

Overall Organization ofVascular PlantsPlants have a hierarchical organizationconsisting of organs, tissues, and cells:ORGANS distinct functional structure consisting ofmultiple types of tissuesTISSUES collection of 1 or more cell types thatperforms a specific function within an organ

3 Basic Plant OrgansPlant organs evolved to obtain nutrients, waterand energy on land – below & above groundROOTS absorb water, minerals and other nutrients fromthe soilanchor & support plant in the groundSTEMS structural support of plant above groundtransport of water & nutrients throughout theplantLEAVES harvesting light & CO2 for photosynthesis

Root FunctionRoots supply the plantwith:CO2 anchorage in the soil water mineral nutrients carbohydrate storage roots rely on shootsystem for carbohydrates roots also need accessto O2*over-watering can suffocate a plant!MineralsH 2OO2

Root StructuresThe first root to emerge during plant development,the primary root, will then give rise to: lateral roots to increase absorption and anchorage tiny root hairs to maximizesurface area for absorptionIn may plants, the primaryroot develops into aprominent tap root whichprovides: support for a large,vertical (tall) shoot system storage for carbohydrates

Fibrous Root StructuresIn some plants, usually monocots, the primaryroot disappears and a fibrous root systemforms which: increases survival fromgrazing animals sinceplant can grow back fromremaining roots retains topsoil

Evolutionary Adaptations of RootsIn other plants, adventitious roots develop fromunusual sources (stems, leaves) which may provide: greater O2 access in wateryenvironments greater structuralsupportProp rootsButtress rootsPneumatophores

Stem Structure and FunctionStems support and position the photosyntheticstructures (leaves) and reproductive structures (e.g.,flowers, cones) to maximize their success.Reproductive shoot (flower)Apical budStem structures include:Node points of leaf attachment callednodes internodes – the stemsbetween each nodeInternodeApical ary bud apical buds at the shoot tipswhere growth occurs axillary buds which give rise tolateral branches, thorns orflowersStemTaprootLateralRoot(branch)systemroots

Evolutionary Adaptations of StemsStems can be modified to serve a variety offunctions: rhizomes which grow just beneaththe soil surface and give rise tovertical shoots from axillary buds stolons that function as “runners”along the soil surface giving rise tonew plantletsRhizomeRootRhizomesStolo tubers that serveas storage “sinks”for carbohydratesTubersStolons

Leaf Structure & FunctionLeaves are the primary photosynthetic organs.Leaf structures include:Simple leaf one or more blades simple leaves have 1 blade compound leaves havemultiple blades called leafletsAxillarybudPetioleCompound leaf a stalk called a petiole thatconnects the leaf to a stem veins that have a branched(dicots) or parallel (monocots)arrangementLeafletAxillarybudPetiole

Evolutionary Adaptations of LeavesLeaves can be modified for a variety of functions: reproductive leavesthat detach and giverise to a new plant(asexual) tendrils which clingto larger supportstructuresTendrilsReproductive leaves spines to repelherbivores bulbs that storenutrientsStorage leavesStemSpines

3 Basic Plant Tissue TypesDermal tissue outer, protective coveringof the plantVascular tissue transports water, nutrients& gives structural supporteach of these tissues formsa continuous tissue systemthroughout the plantDermaltissueGroundtissueVasculartissueGround tissue everything else!There is also a type ofundifferentiated tissue calledmeristem which we willaddress later in this chapter.

More on Dermal Tissue In nonwoody plants and structures (e.g., leaves) thedermal tissue is epidermis.epidermis is frequently covered with a waxy cuticle tominimize water loss some plants also have trichomes in epidermal tissuewhich provide protection from water loss, intense lightand insectsTrichomesIn woody plants the epidermis develops into aprotective laver called periderm (part of the bark).300 μm

More on Vascular Tissue Plant vascular tissue consists of phloem & xylem.Xylem transports water & minerals upward from the rootsystem to the organs and tissues of the shoot systemPhloem transports photosyntheticproducts (e.g., sugars)downward to the roots andother parts of the plantPhloem & xylem are organizedinto vascular bundles orcylinders called steles.

More on Ground Tissue Tissues that are not dermal or vascular are groundtissue which come in 2 general types.Pith ground tissue foundinternal to thevascular tissueCortex ground tissue foundbetween the dermaland vascular tissueGround tissues include cells involved in storage,transport, structural support and photosynthesis.

Basic Plant Cell TypesPlant cells fall into one of 5 general types: Parenchyma Collenchyma Sclerenchyma Water-conducting cells of xylem Water-conducting cells of xylem

Parenchyma Cells have thin primary (1o) cell walls without asecondary (2o) cell wall the least differentiated plant cell type the mostmetabolically activeplant cell type are capable ofundergoing celldivision and furtherdifferentiationParenchyma cells in a privet(Ligustrum) leaf (LM)25 μm

Collenchyma Cells provide flexible support in newly formedshoot structures without restraininggrowth flexible 1o cellwalls withirregular2o wallthickeningCollenchyma cells(in Helianthus stem) (LM)5 μm

Sclerenchyma Cells provide rigid support due to thick 2o cell wallscontaining lignin that are dead at maturity 2 types ofsclerenchymacells: 5 μmSclereid cells in pear (LM)sclereid cellswith very thick2o cell walls25 μmCell wall long andslender fibercells arrangedin threadsFiber cells (cross section from ash tree) (LM)

Water-Conducting Xylem CellsVessel Tracheids 100 μm2 types of xylemcells, both ofwhich are dead atmaturity:TRACHEIDS found in all xylemvessels long, thin withtapered endsVESSEL ELEMENTS wider, less tapered perforated endsPitsTracheids and vessels(colorized SEM)PerforationplateVesselelementVessel elements, withperforated end wallsTracheids

Sugar-Conducting Phloem Cells2 types of phloem cells, both of which arealive at maturity:SIEVE CELLS found in seedlessvascular plants &gymnospermsSIEVE-TUBE ELEMENTS cells that form sievetubes in angiosperms have sieve platesbetween elements &supportingcompanion cells

2. Vascular Plant Growth

Meristem TissueUnlike animals, plants are capable of indeterminategrowth – growth throughout the life of the plant.This unlimited growth potential is due to meristemtissue – a special, undifferentiated tissue withunlimited replicative potential. in contrast, animals and some plant structures (e.g.,flowers, thorns) exhibit determinate growth in whichthey stop growing when they reach a certain sizeThere are 2 types of meristems: APICAL MERISTEM LATERAL MERISTEM

Apical MeristemShoot tip(shoot apicalmeristem andyoung leaves)Axillary budmeristemRoot apicalmeristemsApical meristem is located atthe tips of roots and shootsand is responsible for growthin length – what is calledprimary growth. in non-woody (herbaceous)plants, most if not all growth isdue to apical meristem in woody plants (e.g., trees),there is also growth in width,what is referred to assecondary growth

Lateral MeristemSecondarygrowth in width is due toPrimary growth in stems2 types of lateral meristem:EpidermisCortex adds new vascular cambium whichPrimary phloemlayers of phloem & xylemxylem cork cambium which PrimaryreplacesthePithepidermis with protective peridermVascularcambiumCork cambiumLateralSecondary growth in stemsmeristemsCork xPrimaryphloemSecondaryphloemVascularcambium

Primary Growth of RootsRoot tips have a protective, non-dividing root cap.CortexVascular cylinderEpidermisRoot hairDermalGroundVascularJust underneath theroot cap is theZone of Cell Divisionwhich contains theapical meristem cells.Zone ofdifferentiationBeyond the Zone ofCell Division are 2zones in successivedevelopmental stages:Zone ofelongationZone of celldivision(includingapicalmeristem)Root capMitoticcellsZone of Elongation pushes root into soilZone of Differentiation100 μm cells adopt specific fates

EpidermisCortexEndodermisVascular cylinderPericycleCore ofparenchymacells100 μm(a) Root with xylem and phloem inthe center (typical of 70 μmDermalGroundVascularEudicotRootsIn most eudicotroots, there is acentral vascularcylinder (stele)with a “X-shaped”arrangement ofxylem as seen incross section withphloem filling inbetween the“arms” of the X.

MonocotRootsEpidermisCortexEndodermisVascular cylinderPericycleIn most monocotroots, there is acore of parenchymacells surrounded bya ring of alternatingphloem and xylemvessels.Core ofparenchymacellsXylemPhloem100 μm(b) Root with parenchyma in thecenter (typical of monocots)DermalGroundVascular

Lateral Root GrowthEmerginglateralroot100 μmEpidermisLateral rootCortexVascularcylinder1Pericycle23Lateral root growth occurs from the meristematicpericycle, the outermost layer of cells in thevascular cylinder just inside the endodermis, theinnermost layer of cortex.

Primary Growth of ShootsPrimary growth of shoot structures occurs from: apical meristemwhich lengthensthe stem andgives rise to leafprimordiaLeaf primordiaYoung leafShoot apicalmeristemDevelopingvascularstrand axial meristemwhich gives riseto new branchesfrom the mainstemAxillary budmeristems0.25 mm

Organization of Eudicot StemsPhloemSclerenchyma(fiber cells)XylemGround tissueconnectingpith to cortexPithCortexEpidermisVascularbundle(a) Cross section of stem withvascular bundles forming aring (typical of eudicots) (LM)1 mmDermalGroundVascularIn most eudicotstems, the vasculartissue consists ofbundles of phloemand xylem arrangedin a ring around thecentral pith tissue. the xylem is alwayslocated inside thephloem adjacent tothe pith

Organization of Monocot StemsGroundtissueIn most monocotstems, the vasculartissue consists ofbundles of phloemand xylem scatteredthroughout theground tissue.EpidermisVascularbundles1 mmDermalGroundVascular(b) Cross section of stem withscattered vascular bundles(typical of monocots) (LM)

Leaf StructureGuardcellsCuticle50 a(b) Surface view of a spiderwort(Tradescantia) leaf (LM)UpperepidermisPalisademesophyll100 ticleVeinXylemPhloem(a) Cutaway drawing of leaf tissuesGuardcellsDermalGroundVascularVeinAir spacesGuard cells(c) Cross section of a lilac(Syringa) leaf (LM)

Epidermis outer cell layer on both sides of leaf secrete waxy cuticle to waterproof the leafMesophyll (ground tissue of leaf) loosely packed photosynthetic parenchyma cells palisade or spongy arrangementVascular Bundles phloem & xylem surrounded by bundle sheath cellsStomata (singular “stoma”) openings for gas exchange, transpiration regulated by guard cells

(a) Primary and secondary growthin a two-year-old woody stemPithPrimary xylemVascular cambiumPrimary phloemEpidermisCortexCortexSecondary(2o) Growthof Stems r rayPrimaryxylemSecondary xylemPithSecondary phloemFirst cork cambiumCorkMost recentcork cambiumPeriderm(mainlycorkcambiaand cork)Secondary phloemVascular cambiumBarkCorkCorkcambiumLate woodEarly woodLayers ofperidermCork1 mmSecondaryphloemSecondaryxylemBarkVascular raySecondaryxylem1.4 mmGrowth ring(b) Cross section of a three-yearold Tilia (linden) stem (LM)Periderm