Plant Structure, Growth, And Development -

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Concept 35.1: Plants have a hierarchicalorganization consisting of organs, tissues,and cellsChapter 35Plant Structure, Growth, andDevelopmentThe Three Basic Plant Organs: Roots,Stems, and Leaves Basic morphology of vascular plants reflects theirevolution as organisms that draw nutrients frombelow ground and above ground Plants take up water and minerals from belowground Plants take up CO2 and light from above groundFigure 35.2 Plants have organs composed of different tissues,which in turn are composed of different cell types A tissue is a group of cells consisting of one ormore cell types that together perform a specializedfunction An organ consists of several types of tissues thattogether carry out particular functions Three basic organs evolved: roots, stems, andleaves They are organized into a root system and ashoot systemReproductive shoot (flower)Apical budNodeInternodeApical budVegetative shootLeafAxillary budShootsystemBladePetioleStemTaprootLateral (branch)rootsRootsystem Roots rely on sugar produced byphotosynthesis in the shoot system, and shootsrely on water and minerals absorbed by the rootsystem Monocots and eudicots are the two majorgroups of angiosperms

Roots A root is an organ with important functions:– Anchoring the plant– Absorbing minerals and water– Storing carbohydrates Most eudicots and gymnosperms have a taprootsystem, which consists of: A taproot, the main vertical root Lateral roots, or branch roots, that arise from thetaproot Most monocots have a fibrous root system, whichconsists of: Adventitious roots that arise from stems or leaves Lateral roots that arise from the adventitious roots In most plants, absorption of water and mineralsoccurs near the root hairs, where vast numbers oftiny root hairs increase the surface area Many plants have root adaptations withspecialized functionsFigure 35.3Figure 35.4Stems A stem is an organ consisting of– An alternating system of nodes, the points atwhich leaves are attached– Internodes, the stem segments between nodes An axillary bud is a structure that has thepotential to form a lateral shoot, or branch An apical bud, or terminal bud, is located near theshoot tip and causes elongation of a young shoot Apical dominance helps to maintain dormancy inmost axillary buds

Many plants havemodified stems.Leaves The leaf is the main photosynthetic organ of mostvascular plants Leaves generally consist of a flattened blade anda stalk called the petiole, which joins the leaf to anode of the stemFigure 35.6 Modified leaves Monocots and eudicots differ in the arrangementof veins, the vascular tissue of leaves– Most monocots have parallel veins– Most eudicots have branching veins In classifying angiosperms, taxonomists may useleaf morphology as a criterionFigure 35.8Dermal, Vascular, and Ground Tissues Each plant organ has dermal, vascular, andground tissues Each of these three categories forms a tissuesystem Each tissue system is continuous throughout theplantDermaltissueGroundtissueVasculartissue

Figure 35.9 In nonwoody plants, the dermal tissue systemconsists of the epidermis A waxy coating called the cuticle helps preventwater loss from the epidermis In woody plants, protective tissues calledperiderm replace the epidermis in older regions ofstems and roots Trichomes are outgrowths of the shoot epidermisand can help with insect defense The vascular tissue system carries out longdistance transport of materials between roots andshoots The two vascular tissues are xylem and phloem Xylem conveys water and dissolved mineralsupward from roots into the shoots Phloem transports organic nutrients from wherethey are made to where they are neededEXPERIMENTVery hairy pod(10 trichomes/mm2)Slightly hairy pod(2 trichomes/mm2)Bald pod(no trichomes)Slightly hairy pod:25% damageBald pod:40% damageRESULTSVery hairy pod:10% damage The vascular tissue of a stem or root is collectivelycalled the stele In angiosperms the stele of the root is a solidcentral vascular cylinder The stele of stems and leaves is divided intovascular bundles, strands of xylem and phloemCommon Types of Plant Cells Tissues that are neither dermal nor vascular arethe ground tissue system Ground tissue internal to the vascular tissue ispith; ground tissue external to the vascular tissueis cortex Ground tissue includes cells specialized forstorage, photosynthesis, and support Like any multicellular organism, a plant ischaracterized by cellular differentiation, thespecialization of cells in structure and function

Parenchyma Cells The major types of plant cells are:–––––Mature parenchyma ymaWater-conducting cells of the xylemSugar-conducting cells of the phloemHave thin and flexible primary wallsLack secondary wallsAre the least specializedPerform the most metabolic functionsRetain the ability to divide and differentiateParenchyma cells in Elodealeaf, with chloroplasts (LM)Collenchyma CellsSclerenchyma Cells Collenchyma cells aregrouped in strands andhelp support young partsof the plant shoot They have thicker anduneven cell walls They lack secondarywalls These cells provideflexible support withoutrestraining growthCollenchyma cells Sclerenchyma cells are rigid because of thicksecondary walls strengthened with lignin They are dead at functional maturity There are two types:– Sclereids are short and irregular in shape andhave thick lignified secondary walls– Fibers are long and slender and arranged inthreads(in Helianthus stem) (LM)Figure 35.10c5 µmSclereid cells in pear (LM)25 µmCell wallFiber cells (cross section from ash tree) (LM)Water-Conducting Cells of the Xylem The two types of water-conducting cells,tracheids and vessel elements, are dead atmaturity Tracheids are found in the xylem of all vascularplants Vessel elements are common to mostangiosperms and a few gymnosperms Vessel elements align end to end to form longmicropipes called vessels

Figure 35.10dVesselTracheids100 µmSugar-Conducting Cells of the PhloemTracheids and vessels(colorized SEM) Sieve-tube elements are alive at functionalmaturity, though they lack organelles Sieve plates are the porous end walls that allowfluid to flow between cells along the sieve tube Each sieve-tube element has a companion cellwhose nucleus and ribosomes serve both cellsPitsPerforationplateVesselelementVessel elements, withperforated end wallsTracheidsFigure 35.10e3 µmSieve-tube elements:longitudinal view (LM)Concept 35.2: Meristems generate cells forprimary and secondary growthSieve plateSieve-tube element (left)Companionand companion cell:cellscross section (TEM) A plant can grow throughout its life; this is calledindeterminate growth Some plant organs cease to grow at a certain size;this is called determinate growthSieve-tubeelementsPlasmodesmaSieveplate30 µmNucleus ofcompanioncell15 µmSieve-tube elements:longitudinal viewSieve plate with pores (LM) Meristems are perpetually embryonic tissue andallow for indeterminate growth Apical meristems are located at the tips of rootsand shoots and at the axillary buds of shoots Apical meristems elongate shoots and roots, aprocess called primary growth Lateral meristems add thickness to woody plants,a process called secondary growth There are two lateral meristems: the vascularcambium and the cork cambium The vascular cambium adds layers of vasculartissue called secondary xylem (wood) andsecondary phloem The cork cambium replaces the epidermis withperiderm, which is thicker and tougher

Figure 35.11Primary growth in stemsEpidermisCortexPrimary phloemShoot tip (shootapical meristemand young leaves)Axillary budmeristemPrimary xylemPithVascular cambiumLateral Secondary growth in stemsCorkmeristemscambiumCork ithRoot apicalmeristemsPrimaryxylemFigure 35.12Secondaryxylem Meristems give rise to:– Initials, also called stem cells, which remain in themeristem– Derivatives, which become specialized in maturetissues In woody plants, primary growth and secondarygrowth occur simultaneously but in differentlocationsVascularcambiumApical budBud scaleAxillary budsThis year’s growth(one year old) Flowering plants can be categorized based on thelength of their life cycleLeafscarBudscarNodeInternodeLast year’s growth(two year old)One-year-old sidebranch formedfrom axillary budnear shoot tip– Annuals complete their life cycle in a year or less– Biennials require two growing seasons– Perennials live for many yearsLeaf scarStemBud scarGrowth of twoyears ago(three years old)Leaf scarConcept 35.3: Primary growth lengthensroots and shoots Primary growth produces the parts of the root andshoot systems produced by apical meristemsPrimary Growth of Roots The root tip is covered by a root cap, whichprotects the apical meristem as the root pushesthrough soil Growth occurs just behind the root tip, in threezones of cells:– Zone of cell division– Zone of elongation– Zone of differentiation, or maturation

Figure 35.13CortexVascular cylinderKeyto labelsEpidermisRoot hairZone ofdifferentiationDermalGroundVascularZone ofelongationMitoticcellsZone of celldivision(includingapicalmeristem)100 µmRoot capFigure 35.14 The primary growth of roots produces theepidermis, ground tissue, and vascular tissue In angiosperm roots, the stele is a vascularcylinder In most eudicots, the xylem is starlike inappearance with phloem between the “arms” In many monocots, a core of parenchyma cells issurrounded by rings of xylem then icycleCore ofparenchymacellsXylem100 µm(a) Root with xylem andphloem in the center(typical of eudicots)50 µmPhloemEndodermisPericycleXylemPhloem100 µm(b) Root with parenchyma in thecenter (typical of monocots)Keyto labelsDermal The ground tissue, mostly parenchyma cells, fillsthe cortex, the region between the vascularcylinder and epidermis The innermost layer of the cortex is called theendodermis The endodermis regulates passage ofsubstances from the soil into the vascularcylinderGroundVascularPrimary Growth of Shoots Lateral roots arise from within the pericycle, theoutermost cell layer in the vascular cylinderFigure 35.15 A shoot apical meristem is a dome-shaped massof dividing cells at the shoot tip Leaves develop from leaf primordia along thesides of the apical meristem Axillary buds develop from meristematic cells leftat the bases of leaf primordia

Figure 35.16Shoot apical meristemLeaf primordiaTissue Organization of StemsYoungleafDevelopingvascularstrandAxillary budmeristems Lateral shootsdevelop from axillarybuds on the stem’ssurface In most eudicots, thevascular tissueconsists of vascularbundles arranged ina ringFigure 35.170.25 mm In most monocot stems, the vascular bundles arescattered throughout the ground tissue, ratherthan forming a ringTissue Organization of Leaves The epidermis in leaves is interrupted bystomata, which allow CO2 and O2 exchangebetween the air and the photosynthetic cells in aleaf Each stomatal pore is flanked by two guardcells, which regulate its opening and closing The ground tissue in a leaf, called mesophyll, issandwiched between the upper and lowerepidermisFigure 35.17 The mesophyll of eudicots has two layers: The palisade mesophyll in the upper part of theleaf The spongy mesophyll in the lower part of theleaf; the loose arrangement allows for gasexchange The vascular tissue of each leaf is continuous withthe vascular tissue of the stem Veins are the leaf’s vascular bundles and functionas the leaf’s skeleton Each vein in a leaf is enclosed by a protectivebundle sheath

Figure 35.18GuardcellsKeyto labelsGround50 µ l(b) Surface view ofa spiderwort(Tradescantia)leaf (LM)SpongymesophyllLowerepidermisXylemVein CuticleGuard cellsPhloemGuardVein Air spacescells(c) Cross section of a lilac(a) Cutaway drawing of leaf tissues(Syringa) leaf (LM)100 µ mBundlesheathcellFigure 35.19a-3Primary xylemPith Secondary growth occurs in stems and roots ofwoody plants but rarely in leaves The secondary plant body consists of the tissuesproduced by the vascular cambium and corkcambium Secondary growth is characteristic ofgymnosperms and many eudicots, but notmonocotsFigure 35.19bPithPrimary xylemVascular cambiumPrimary phloemCortexEpidermis(a) Primary and secondary growthin a two-year-old woody stemEpidermisCortexPrimary phloemVascular cambiumConcept 35.4: Secondary growth increasesthe diameter of stems and roots in woodyplantswGrothVascular raySecondary xylemSecondary xylemSecondary phloemVascular cambiumLate woodEarly woodBarkCorkcambiumPeridermCorkSecondary phloemFirst cork cambiumPeriderm (mainlycork cambiaand cork)SecondaryphloemSecondaryxylemw thGro0.5 mmCorkMost recent cork cambiumVascular rayCorkBark0.5 mmGrowth ring(b) Cross section of a three-yearold Tilia (linden) stem (LM)Layers ofperidermFigure 35.20The Vascular Cambium and SecondaryVascular Tissue The vascular cambium is a cylinder ofmeristematic cells one cell layer thick It develops from undifferentiated parenchyma cells In cross section, the vascular cambium appears asa ring of initials (stem cells) The initials increase the vascular cambium’scircumference and add secondary xylem to theinside and secondary phloem to the outsideVascularcambiumGrowthSecondaryxylemAfter one yearof growthVascularcambiumSecondaryphloemAfter two yearsof growth

Elongated initials produce tracheids, vesselelements, fibers of xylem, sieve-tube elements,companion cells, axially oriented parenchyma, andfibers of the phloem Shorter initials produce vascular rays, radial filesof parenchyma cells that connect secondary xylemand phloem Tree rings are visible where late and early woodmeet, and can be used to estimate a tree’s age Dendrochronology is the analysis of tree ringgrowth patterns and can be used to study pastclimate change Secondary xylem accumulates as wood andconsists of tracheids, vessel elements (only inangiosperms), and fibers Early wood, formed in the spring, has thin cellwalls to maximize water delivery Late wood, formed in late summer, has thickwalled cells and contributes more to stem support In temperate regions, the vascular cambium ofperennials is inactive through the winter As a tree or woody shrub ages, the older layers ofsecondary xylem, the heartwood, no longertransport water and minerals The outer layers, known as sapwood, stilltransport materials through the xylem Older secondary phloem sloughs off and does notaccumulateFigure 35.21Figure 35.22The Cork Cambium and the Production ylemSapwoodVascular cambiumSecondary phloemBarkLayers of periderm Cork cambium gives rise to two tissues: Phelloderm is a thin layer of parenchyma cellsthat forms to the interior of the cork cambium Cork cells accumulate to the exterior of the corkcambium Cork cells deposit waxy suberin in their walls, thendie Periderm consists of the cork cambium,phelloderm, and cork cells it produces

Lenticels in the periderm allow for gas exchangebetween living stem or root cells and the outsideair Bark consists of all the tissues external to thevascular cambium, including secondary phloemand peridermConcept 35.5: Growth, morphogenesis, andcell differentiation produce the plant body Cells form specialized tissues, organs, andorganisms through the process of development Developmental plasticity describes the effect ofenvironment on development– For example, the aquatic plant fanwort formsdifferent leaves depending on whether or not theapical meristem is submergedFigure 35.24 Development consists of growth, morphogenesis,and cell differentiation Growth is an irreversible increase in size Morphogenesis is the development of body formand organization Cell differentiation is the process by which cellswith the same genes become different from eachotherTable 35.1Model Organisms: Revolutionizing theStudy of Plants New techniques and model organisms arecatalyzing explosive progress in ourunderstanding of plants Arabidopsis is a model organism and the firstplant to have its entire genome sequenced Arabidopsis has 27,000 genes divided among5 pairs of chromosomes

Growth: Cell Division and Cell Expansion Arabidopsis is easily transformed by introducingforeign DNA via genetically altered bacteria Studying the genes and biochemical pathwaysof Arabidopsis will provide insights into plantdevelopment, a major goal of systems biology By increasing cell number, cell division inmeristems increases the potential for growth Cell expansion accounts for the actual increase inplant sizeThe Plane and Symmetry of Cell Division New cell walls form in a plane (direction)perpendicular to the main axis of cell expansion The plane in which a cell divides is determinedduring late interphase Microtubules become concentrated into a ringcalled the preprophase band that predicts thefuture plane of cell division Leaf growth results from a combination oftransverse and longitudinal cell divisions It was previously thought that the plane of celldivision determines leaf form A mutation in the tangled-1 gene that affectslongitudinal divisions does not affect leaf shapeFigure 35.27 The symmetry of cell division, the distribution ofcytoplasm between daughter cells, determines cellfate Asymmetrical cell division signals a key event indevelopment– For example, the formation of guard cells involvesasymmetrical cell division and a change in theplane of cell divisionAsymmetricalcell divisionUnspecializedepidermal cellGuard cell“mother cell”Developingguard cells

Polarity is the condition of having structural orchemical differences at opposite ends of anorganism– For example, plants have a root end and a shootend The first division of a plant zygote is normallyasymmetrical and initiates polarization into theshoot and root The gnom mutant of Arabidopsis results from asymmetrical first division Asymmetrical cell divisions play a role inestablishing polarityFigure 35.29CellulosemicrofibrilsOrientation of Cell Expansion Plant cells grow rapidly and “cheaply” by intakeand storage of water in vacuoles Plant cells expand primarily along the plant’s mainaxis Cellulose microfibrils in the cell wall restrict thedirection of cell elongationElongationNucleusVacuoles5 µmMorphogenesis and Pattern Formation Pattern formation is the development of specificstructures in specific locations Two types of hypotheses explain the fate of plantcells– Lineage-based mechanisms propose that cell fateis determined early in development and passedon to daughter cells– Position-based mechanisms propose that cell fateis determined by final position Hox genes in animals affect the number andplacement of appendages in embryos A plant homolog of Hox genes called KNOTTED-1does not affect the number or placement of plantorgans KNOTTED-1 is important in the development ofleaf morphology

Figure 35.30Gene Expression and Control of CellDifferentiation Cells of a developing organism synthesizedifferent proteins and diverge in structure andfunction even though they have a commongenome Cellular differentiation depends on geneexpression, but is determined by position Positional information is communicated throughcell interactionsFigure 35.31Corticalcells Gene activation or inactivation depends on cell-tocell communicationGLABRA-2 is expressed, andthe cell remains hairless.– For example, Arabidopsis root epidermis formsroot hairs or hairless cells depending on thenumber of cortical cells it is touching20 µ mGLABRA-2 isnot expressed,and the cellwill developa root hair.The root cap cells will be sloughed offbefore root hairs emerge.Figure 35.32Shifts in Development: Phase Changes Plants pass through developmental phases, calledphase changes, developing from a juvenile phaseto an adult phase Phase changes occur within the shoot apicalmeristem The most obvious morphological changes typicallyoccur in leaf size and shapeLeaves producedby adult phaseof apical meristemLeaves producedby juvenile phaseof apical meristem

Genetic Control of Flowering Flower formation involves a phase change fromvegetative growth to reproductive growth It is triggered by a combination of environmentalcues and internal signals Transition from vegetative growth to flowering isassociated with the switching on of floralmeristem identity genes In a developing flower, the order of eachprimordium’s emergence determines its fate:sepal, petal, stamen, or carpel Plant biologists have identified several organidentity genes (plant homeotic genes) thatregulate the development of floral pattern These are MADS-box genes A mutation in a plant organ identity gene cancause abnormal floral developmentFigure 35.33PeCaStSe Researchers have identified three classes of floralorgan identity genes The ABC hypothesis of flower formation identifieshow floral organ identity genes direct the formationof the four types of floral organs An understanding of mutants of the organ identitygenes depicts how this model accounts for floralphenotypesPeSePe(a) Normal Arabidopsis flowerPeSe(b) Abnormal ArabidopsisflowerFigure 35.34SepalsPetalsStamensAB(a) A schematic diagram of the ABChypothesisCarpelsCA BgeneactivityB CgeneactivityC geneactivityCarpelPetalA geneactivityStamenSepalActivegenes:BBB BA A CCCC A AB BB BC C CCCC C CAA C C C C A AA AAAAB BA AB B AMutant lacking AMutant lacking BMutant lacking CWhorls:CarpelPetalStamenSepalWild type(b) Side view of flowers with organ identity mutations

Plant Structure, Growth, and Development Chapter 35 Concept 35.1: Plants have a hierarchical organization consisting of organs, tissues, and cells Plants have organs composed of different tissues, which in turn are composed of different cell types A tissue is a group of cells consisting of one or

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