1 Chapter 28 Plant Structure and Growth

2 Overview: Are Plants Computers?Romanesco grows according to a repetitive program The development of plants depends on the environment and is highly adaptive © 2011 Pearson Education, Inc.

3 Figure 35.1 Figure 35.1 Computer art?

4 Plants have a hierarchical organization consisting of organs, tissues, and cellsPlants 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 more cell types that together perform a specialized function An organ consists of several types of tissues that together carry out particular functions © 2011 Pearson Education, Inc.

5 The Three Basic Plant Organs: Roots, Stems, and LeavesBasic morphology of vascular plants reflects their evolution as organisms that draw nutrients from below ground and above ground Plants take up water and minerals from below ground Plants take up CO2 and light from above ground © 2011 Pearson Education, Inc.

6 Three basic organs evolved: roots, stems, and leaves They are organized into a root system and a shoot system © 2011 Pearson Education, Inc.

7 Reproductive shoot (flower)Figure 35.2 Reproductive shoot (flower) Apical bud Node Internode Apical bud Shoot system Vegetative shoot Axillary bud Blade Leaf Petiole Stem Figure 35.2 An overview of a flowering plant. Taproot Root system Lateral (branch) roots

8 Monocots and eudicots are the two major groups of angiospermsRoots rely on sugar produced by photosynthesis in the shoot system, and shoots rely on water and minerals absorbed by the root system Monocots and eudicots are the two major groups of angiosperms © 2011 Pearson Education, Inc.

9 Roots A root is an organ with important functions: Anchoring the plantAbsorbing minerals and water Storing carbohydrates © 2011 Pearson Education, Inc.

10 Most monocots have a fibrous root system, which consists of:Most eudicots and gymnosperms have a taproot system, which consists of: A taproot, the main vertical root Lateral roots, or branch roots, that arise from the taproot Most monocots have a fibrous root system, which consists of: Adventitious roots that arise from stems or leaves Lateral roots that arise from the adventitious roots © 2011 Pearson Education, Inc.

11 In most plants, absorption of water and minerals occurs near the root hairs, where vast numbers of tiny root hairs increase the surface area © 2011 Pearson Education, Inc.

12 Figure 35.3 Figure 35.3 Root hairs of a radish seedling.

13 Many plants have root adaptations with specialized functions© 2011 Pearson Education, Inc.

14 Storage roots Prop roots Buttress roots PneumatophoresFigure 35.4 “Strangling” aerial roots Storage roots Prop roots Buttress roots Figure 35.4 Evolutionary adaptations of roots. Pneumatophores

15 Stems A stem is an organ consisting ofAn alternating system of nodes, the points at which leaves are attached Internodes, the stem segments between nodes © 2011 Pearson Education, Inc.

16 Apical dominance helps to maintain dormancy in most axillary budsAn axillary bud is a structure that has the potential to form a lateral shoot, or branch An apical bud, or terminal bud, is located near the shoot tip and causes elongation of a young shoot Apical dominance helps to maintain dormancy in most axillary buds © 2011 Pearson Education, Inc.

17 Many plants have modified stems (e. gMany plants have modified stems (e.g., rhizomes, bulbs, stolons, tubers) © 2011 Pearson Education, Inc.

18 Rhizomes Rhizome Root Bulbs Storage leaves Stem Stolons Stolon TubersFigure 35.5 Rhizomes Rhizome Root Bulbs Storage leaves Stem Stolons Stolon Figure 35.5 Evolutionary adaptations of stems. Tubers

19 Leaves The leaf is the main photosynthetic organ of most vascular plants Leaves generally consist of a flattened blade and a stalk called the petiole, which joins the leaf to a node of the stem © 2011 Pearson Education, Inc.

20 Monocots and eudicots differ in the arrangement of veins, the vascular tissue of leavesMost monocots have parallel veins Most eudicots have branching veins In classifying angiosperms, taxonomists may use leaf morphology as a criterion © 2011 Pearson Education, Inc.

21 Simple leaf Axillary bud Petiole Compound leaf Doubly compound leafFigure 35.6 Simple leaf Axillary bud Petiole Compound leaf Doubly compound leaf Leaflet Figure 35.6 Simple versus compound leaves. Petiole Axillary bud Axillary bud Petiole Leaflet

22 Some plant species have evolved modified leaves that serve various functions© 2011 Pearson Education, Inc.

23 Tendrils Spines Storage leaves Reproductive leaves Bracts Figure 35.7Figure 35.7 Evolutionary adaptations of leaves. Bracts

24 Dermal, Vascular, and Ground TissuesEach plant organ has dermal, vascular, and ground tissues Each of these three categories forms a tissue system Each tissue system is continuous throughout the plant © 2011 Pearson Education, Inc.

25 Dermal tissue Ground tissue Vascular tissue Figure 35.8Figure 35.8 The three tissue systems. Dermal tissue Ground tissue Vascular tissue

26 In nonwoody plants, the dermal tissue system consists of the epidermis A waxy coating called the cuticle helps prevent water loss from the epidermis In woody plants, protective tissues called periderm replace the epidermis in older regions of stems and roots Trichomes are outgrowths of the shoot epidermis and can help with insect defense © 2011 Pearson Education, Inc.

27 Very hairy pod (10 trichomes/ mm2) Figure 35.9 EXPERIMENT Very hairy pod (10 trichomes/ mm2) Slightly hairy pod (2 trichomes/ mm2) Bald pod (no trichomes) RESULTS Very hairy pod: 10% damage Slightly hairy pod: 25% damage Bald pod: 40% damage Figure 35.9 Inquiry: Do soybean pod trichomes deter herbivores?

28 Figure 35.9a Figure 35.9 Inquiry: Do soybean pod trichomes deter herbivores?

29 The two vascular tissues are xylem and phloem The vascular tissue system carries out long-distance transport of materials between roots and shoots The two vascular tissues are xylem and phloem Xylem conveys water and dissolved minerals upward from roots into the shoots Phloem transports organic nutrients from where they are made to where they are needed © 2011 Pearson Education, Inc.

30 The vascular tissue of a stem or root is collectively called the stele In angiosperms the stele of the root is a solid central vascular cylinder The stele of stems and leaves is divided into vascular bundles, strands of xylem and phloem © 2011 Pearson Education, Inc.

31 Tissues that are neither dermal nor vascular are the ground tissue systemGround tissue internal to the vascular tissue is pith; ground tissue external to the vascular tissue is cortex Ground tissue includes cells specialized for storage, photosynthesis, and support © 2011 Pearson Education, Inc.

32 Common Types of Plant CellsLike any multicellular organism, a plant is characterized by cellular differentiation, the specialization of cells in structure and function © 2011 Pearson Education, Inc.

33 The major types of plant cells are:Parenchyma Collenchyma Sclerenchyma Water-conducting cells of the xylem Sugar-conducting cells of the phloem © 2011 Pearson Education, Inc.

34 Parenchyma Cells Mature parenchyma cellsHave thin and flexible primary walls Lack secondary walls Are the least specialized Perform the most metabolic functions Retain the ability to divide and differentiate © 2011 Pearson Education, Inc.

35 Parenchyma cells in Elodea leaf, with chloroplasts (LM) 60 mFigure 35.10a Figure Exploring: Examples of Differentiated Plant Cells Parenchyma cells in Elodea leaf, with chloroplasts (LM) 60 m

36 Collenchyma Cells Collenchyma cells are grouped in strands and help support young parts of the plant shoot They have thicker and uneven cell walls They lack secondary walls These cells provide flexible support without restraining growth © 2011 Pearson Education, Inc.

37 Collenchyma cells (in Helianthus stem) (LM) 5 mFigure 35.10b Figure Exploring: Examples of Differentiated Plant Cells Collenchyma cells (in Helianthus stem) (LM) 5 m

38 Sclerenchyma Cells Sclerenchyma cells are rigid because of thick secondary walls strengthened with lignin They are dead at functional maturity There are two types: Sclereids are short and irregular in shape and have thick lignified secondary walls Fibers are long and slender and arranged in threads © 2011 Pearson Education, Inc.

39 Sclereid cells in pear (LM)Figure 35.10c 5 m Sclereid cells in pear (LM) 25 m Cell wall Figure Exploring: Examples of Differentiated Plant Cells Fiber cells (cross section from ash tree) (LM)

40 Water-Conducting Cells of the XylemThe two types of water-conducting cells, tracheids and vessel elements, are dead at maturity Tracheids are found in the xylem of all vascular plants © 2011 Pearson Education, Inc.

41 Vessel elements are common to most angiosperms and a few gymnosperms Vessel elements align end to end to form long micropipes called vessels © 2011 Pearson Education, Inc.

42 Tracheids and vessels (colorized SEM) PitsFigure 35.10d 100 m Vessel Tracheids Tracheids and vessels (colorized SEM) Pits Figure Exploring: Examples of Differentiated Plant Cells Perforation plate Vessel element Vessel elements, with perforated end walls Tracheids

43 Tracheids and vessels (colorized SEM)Figure 35.10da Vessel 100 m Tracheids Figure Exploring: Examples of Differentiated Plant Cells Tracheids and vessels (colorized SEM)

44 Sugar-Conducting Cells of the PhloemSieve-tube elements are alive at functional maturity, though they lack organelles Sieve plates are the porous end walls that allow fluid to flow between cells along the sieve tube Each sieve-tube element has a companion cell whose nucleus and ribosomes serve both cells © 2011 Pearson Education, Inc.

45 Sieve-tube elements: longitudinal view (LM) 3 mFigure 35.10e Sieve-tube elements: longitudinal view (LM) 3 m Sieve plate Sieve-tube element (left) and companion cell: cross section (TEM) Companion cells Sieve-tube elements Plasmodesma Figure Exploring: Examples of Differentiated Plant Cells Sieve plate 30 m Nucleus of companion cell 15 m Sieve-tube elements: longitudinal view Sieve plate with pores (LM)

46 Meristems generate cells for primary and secondary growthA plant can grow throughout its life; this is called indeterminate growth Some plant organs cease to grow at a certain size; this is called determinate growth © 2011 Pearson Education, Inc.

47 Meristems are perpetually embryonic tissue and allow for indeterminate growthApical meristems are located at the tips of roots and shoots and at the axillary buds of shoots Apical meristems elongate shoots and roots, a process called primary growth © 2011 Pearson Education, Inc.

48 Lateral meristems add thickness to woody plants, a process called secondary growthThere are two lateral meristems: the vascular cambium and the cork cambium The vascular cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem The cork cambium replaces the epidermis with periderm, which is thicker and tougher © 2011 Pearson Education, Inc.

49 Primary growth in stemsFigure 35.11 Primary growth in stems Epidermis Cortex Primary phloem Shoot tip (shoot apical meristem and young leaves) Primary xylem Pith Vascular cambium Secondary growth in stems Cork cambium Lateral meristems Cork cambium Axillary bud meristem Cortex Periderm Primary phloem Figure An overview of primary and secondary growth. Pith Secondary phloem Root apical meristems Primary xylem Vascular cambium Secondary xylem

50 Meristems give rise to:Initials, also called stem cells, which remain in the meristem Derivatives, which become specialized in mature tissues In woody plants, primary growth and secondary growth occur simultaneously but in different locations © 2011 Pearson Education, Inc.

51 This year’s growth (one year old) Leaf scarFigure 35.12 Apical bud Bud scale Axillary buds This year’s growth (one year old) Leaf scar Node Bud scar One-year-old side branch formed from axillary bud near shoot tip Internode Last year’s growth (two year old) Leaf scar Stem Figure Three years’ growth in a winter twig. Bud scar Growth of two years ago (three years old) Leaf scar

52 Flowering plants can be categorized based on the length of their life cycleAnnuals complete their life cycle in a year or less Biennials require two growing seasons Perennials live for many years © 2011 Pearson Education, Inc.

53 Primary growth lengthens roots and shootsPrimary growth produces the parts of the root and shoot systems produced by apical meristems © 2011 Pearson Education, Inc.

54 Primary Growth of RootsThe root tip is covered by a root cap, which protects the apical meristem as the root pushes through soil Growth occurs just behind the root tip, in three zones of cells: Zone of cell division Zone of elongation Zone of differentiation, or maturation © 2011 Pearson Education, Inc.

55 Zone of differentiation Ground Root hair VascularFigure 35.13 Cortex Vascular cylinder Key to labels Epidermis Dermal Zone of differentiation Ground Root hair Vascular Zone of elongation Figure Primary growth of a root. Zone of cell division (including apical meristem) Mitotic cells 100 m Root cap

56 In angiosperm roots, the stele is a vascular cylinder The primary growth of roots produces the epidermis, ground tissue, and vascular tissue In angiosperm roots, the stele is a vascular cylinder In most eudicots, the xylem is starlike in appearance with phloem between the “arms” In many monocots, a core of parenchyma cells is surrounded by rings of xylem then phloem © 2011 Pearson Education, Inc.

57 Core of parenchyma cellsFigure 35.14 Epidermis Cortex Endodermis Vascular cylinder Pericycle Core of parenchyma cells Xylem 100 m Phloem 100 m (a) Root with xylem and phloem in the center (typical of eudicots) (b) Root with parenchyma in the center (typical of monocots) 50 m Key to labels Figure Organization of primary tissues in young roots. Endodermis Pericycle Dermal Xylem Ground Phloem Vascular

58 The innermost layer of the cortex is called the endodermis The ground tissue, mostly parenchyma cells, fills the cortex, the region between the vascular cylinder and epidermis The innermost layer of the cortex is called the endodermis The endodermis regulates passage of substances from the soil into the vascular cylinder © 2011 Pearson Education, Inc.

59 Lateral roots arise from within the pericycle, the outermost cell layer in the vascular cylinder© 2011 Pearson Education, Inc.

60 100 m Epidermis Emerging lateral root Lateral root CortexFigure 100 m Epidermis Emerging lateral root Lateral root Cortex Vascular cylinder Pericycle Figure The formation of a lateral root. 1 2 3

61 Primary Growth of ShootsA shoot apical meristem is a dome-shaped mass of dividing cells at the shoot tip Leaves develop from leaf primordia along the sides of the apical meristem Axillary buds develop from meristematic cells left at the bases of leaf primordia © 2011 Pearson Education, Inc.

62 Developing vascular strandFigure 35.16 Shoot apical meristem Leaf primordia Young leaf Developing vascular strand Figure The shoot tip. Axillary bud meristems 0.25 mm

63 Tissue Organization of StemsLateral shoots develop from axillary buds on the stem’s surface In most eudicots, the vascular tissue consists of vascular bundles arranged in a ring © 2011 Pearson Education, Inc.

64 Sclerenchyma (fiber cells) Ground tissue Figure 35.17 Phloem Xylem Sclerenchyma (fiber cells) Ground tissue Ground tissue connecting pith to cortex Pith Epidermis Key to labels Epidermis Cortex Vascular bundles Figure Organization of primary tissues in young stems. Vascular bundle Dermal 1 mm 1 mm Ground (a) Cross section of stem with vascular bundles forming a ring (typical of eudicots) (b) Vascular Cross section of stem with scattered vascular bundles (typical of monocots)

65 In most monocot stems, the vascular bundles are scattered throughout the ground tissue, rather than forming a ring © 2011 Pearson Education, Inc.

66 Tissue Organization of LeavesThe epidermis in leaves is interrupted by stomata, which allow CO2 and O2 exchange between the air and the photosynthetic cells in a leaf Each stomatal pore is flanked by two guard cells, which regulate its opening and closing The ground tissue in a leaf, called mesophyll, is sandwiched between the upper and lower epidermis © 2011 Pearson Education, Inc.

67 The mesophyll of eudicots has two layers:The palisade mesophyll in the upper part of the leaf The spongy mesophyll in the lower part of the leaf; the loose arrangement allows for gas exchange © 2011 Pearson Education, Inc.

68 Each vein in a leaf is enclosed by a protective bundle sheathThe vascular tissue of each leaf is continuous with the vascular tissue of the stem Veins are the leaf’s vascular bundles and function as the leaf’s skeleton Each vein in a leaf is enclosed by a protective bundle sheath © 2011 Pearson Education, Inc.

69 Surface view of a spiderwort (Tradescantia) leaf (LM) Cuticle StomaFigure 35.18 Guard cells Key to labels Stomatal pore Dermal Ground Epidermal cell 50 m Vascular Sclerenchyma fibers (b) Surface view of a spiderwort (Tradescantia) leaf (LM) Cuticle Stoma Upper epidermis Palisade mesophyll Spongy mesophyll Bundle- sheath cell Figure Leaf anatomy. Lower epidermis 100 m Xylem Vein Cuticle Phloem Guard cells Guard cells Vein Air spaces (a) Cutaway drawing of leaf tissues (c) Cross section of a lilac (Syringa) leaf (LM)

70 (a) Cutaway drawing of leaf tissuesFigure 35.18a Key to labels Sclerenchyma fibers Dermal Cuticle Stoma Ground Vascular Upper epidermis Palisade mesophyll Spongy mesophyll Bundle- sheath cell Figure Leaf anatomy. Lower epidermis Xylem Vein Cuticle Phloem Guard cells (a) Cutaway drawing of leaf tissues

71 Surface view of a spiderwort (Tradescantia) leaf (LM)Figure 35.18b Guard cells Stomatal pore Epidermal cell 50 m (b) Surface view of a spiderwort (Tradescantia) leaf (LM) Figure Leaf anatomy.

72 Cross section of a lilac (Syringa) leaf (LM)Figure 35.18c Upper epidermis Palisade mesophyll Spongy mesophyll 100 m Lower epidermis Guard cells Figure Leaf anatomy. Vein Air spaces (c) Cross section of a lilac (Syringa) leaf (LM)

73 Secondary growth increases the diameter of stems and roots in woody plantsSecondary growth occurs in stems and roots of woody plants but rarely in leaves The secondary plant body consists of the tissues produced by the vascular cambium and cork cambium Secondary growth is characteristic of gymnosperms and many eudicots, but not monocots © 2011 Pearson Education, Inc.

74 Figure 35.19 Primary and secondary growth of a woody stem.Primary and secondary growth in a two-year-old woody stem Epidermis Pith Cortex Primary xylem Primary phloem Vascular cambium Epidermis Primary phloem Cortex Vascular cambium Primary xylem Growth Vascular ray Pith Primary xylem Secondary xylem Vascular cambium Secondary phloem Primary phloem First cork cambium Cork Periderm (mainly cork cambia and cork) Growth Figure Primary and secondary growth of a woody stem. Secondary phloem Bark Vascular cambium Primary phloem Secondary xylem Late wood Cork cambium Early wood Periderm Secondary phloem Cork Secondary xylem (two years of production) Vascular cambium 0.5 mm Secondary xylem Vascular cambium Bark Secondary phloem Primary xylem Vascular ray Most recent cork cambium Layers of periderm Growth ring Cork (b) Cross section of a three-year- old Tilia (linden) stem (LM) Pith 0.5 mm

75 The Vascular Cambium and Secondary Vascular TissueThe vascular cambium is a cylinder of meristematic cells one cell layer thick It develops from undifferentiated parenchyma cells © 2011 Pearson Education, Inc.

76 In cross section, the vascular cambium appears as a ring of initials (stem cells)The initials increase the vascular cambium’s circumference and add secondary xylem to the inside and secondary phloem to the outside © 2011 Pearson Education, Inc.

77 After one year of growth After two years of growthFigure 35.20 Vascular cambium Growth Vascular cambium Secondary phloem Secondary xylem Figure Secondary growth produced by the vascular cambium. After one year of growth After two years of growth

78 Elongated initials produce tracheids, vessel elements, fibers of xylem, sieve-tube elements, companion cells, axially oriented parenchyma, and fibers of the phloem Shorter initials produce vascular rays, radial files of parenchyma cells that connect secondary xylem and phloem © 2011 Pearson Education, Inc.

79 Secondary xylem accumulates as wood and consists of tracheids, vessel elements (only in angiosperms), and fibers Early wood, formed in the spring, has thin cell walls to maximize water delivery Late wood, formed in late summer, has thick-walled cells and contributes more to stem support In temperate regions, the vascular cambium of perennials is inactive through the winter © 2011 Pearson Education, Inc.

80 Tree rings are visible where late and early wood meet, and can be used to estimate a tree’s ageDendrochronology is the analysis of tree ring growth patterns and can be used to study past climate change © 2011 Pearson Education, Inc.

81 Older secondary phloem sloughs off and does not accumulateAs a tree or woody shrub ages, the older layers of secondary xylem, the heartwood, no longer transport water and minerals The outer layers, known as sapwood, still transport materials through the xylem Older secondary phloem sloughs off and does not accumulate © 2011 Pearson Education, Inc.

82 Growth ring Vascular ray Heartwood Secondary xylem SapwoodFigure 35.22 Growth ring Vascular ray Heartwood Secondary xylem Sapwood Figure Anatomy of a tree trunk. Vascular cambium Secondary phloem Bark Layers of periderm

83 Figure 35.23 Figure Is this tree living or dead?

84 The Cork Cambium and the Production of PeridermCork cambium gives rise to two tissues: Phelloderm is a thin layer of parenchyma cells that forms to the interior of the cork cambium Cork cells accumulate to the exterior of the cork cambium Cork cells deposit waxy suberin in their walls, then die Periderm consists of the cork cambium, phelloderm, and cork cells it produces © 2011 Pearson Education, Inc.

85 Lenticels in the periderm allow for gas exchange between living stem or root cells and the outside air Bark consists of all the tissues external to the vascular cambium, including secondary phloem and periderm © 2011 Pearson Education, Inc.

86 Evolution of Secondary GrowthIn the herbaceous plant Arabidopsis, the addition of weights to the plants triggered secondary growth This suggests that stem weight is the cue for wood formation © 2011 Pearson Education, Inc.

87 Growth, morphogenesis, and cell differentiation produce the plant bodyCells form specialized tissues, organs, and organisms through the process of development Developmental plasticity describes the effect of environment on development For example, the aquatic plant fanwort forms different leaves depending on whether or not the apical meristem is submerged © 2011 Pearson Education, Inc.

88 Figure 35.24 Figure Developmental plasticity in the aquatic plant Cabomba caroliniana.

89 Growth is an irreversible increase in size Development consists of growth, morphogenesis, and cell differentiation Growth is an irreversible increase in size Morphogenesis is the development of body form and organization Cell differentiation is the process by which cells with the same genes become different from each other © 2011 Pearson Education, Inc.

90 Model Organisms: Revolutionizing the Study of PlantsNew techniques and model organisms are catalyzing explosive progress in our understanding of plants Arabidopsis is a model organism and the first plant to have its entire genome sequenced Arabidopsis has 27,000 genes divided among 5 pairs of chromosomes © 2011 Pearson Education, Inc.

91 Table 35.1 Table 35.1 Arabidopsis thaliana Gene Functions

92 Arabidopsis is easily transformed by introducing foreign DNA via genetically altered bacteriaStudying the genes and biochemical pathways of Arabidopsis will provide insights into plant development, a major goal of systems biology © 2011 Pearson Education, Inc.

93 Growth: Cell Division and Cell ExpansionBy increasing cell number, cell division in meristems increases the potential for growth Cell expansion accounts for the actual increase in plant size © 2011 Pearson Education, Inc.

94 The Plane and Symmetry of Cell DivisionNew cell walls form in a plane (direction) perpendicular to the main axis of cell expansion The plane in which a cell divides is determined during late interphase Microtubules become concentrated into a ring called the preprophase band that predicts the future plane of cell division © 2011 Pearson Education, Inc.

95 Preprophase band 7 m Figure 35.25Figure The preprophase band and the plane of cell division. 7 m

96 Leaf growth results from a combination of transverse and longitudinal cell divisionsIt was previously thought that the plane of cell division determines leaf form A mutation in the tangled-1 gene that affects longitudinal divisions does not affect leaf shape © 2011 Pearson Education, Inc.

97 Asymmetrical cell division signals a key event in developmentThe symmetry of cell division, the distribution of cytoplasm between daughter cells, determines cell fate Asymmetrical cell division signals a key event in development For example, the formation of guard cells involves asymmetrical cell division and a change in the plane of cell division © 2011 Pearson Education, Inc.

98 Asymmetrical cell divisionFigure 35.27 Asymmetrical cell division Guard cell “mother cell” Unspecialized epidermal cell Figure Asymmetrical cell division and stomatal development. Developing guard cells

99 Asymmetrical cell divisions play a role in establishing polarityPolarity is the condition of having structural or chemical differences at opposite ends of an organism For example, plants have a root end and a shoot end Asymmetrical cell divisions play a role in establishing polarity © 2011 Pearson Education, Inc.

100 The first division of a plant zygote is normally asymmetrical and initiates polarization into the shoot and root The gnom mutant of Arabidopsis results from a symmetrical first division © 2011 Pearson Education, Inc.

101 Figure 35.28 Figure Establishment of axial polarity.

102 Orientation of Cell ExpansionPlant cells grow rapidly and “cheaply” by intake and storage of water in vacuoles Plant cells expand primarily along the plant’s main axis Cellulose microfibrils in the cell wall restrict the direction of cell elongation © 2011 Pearson Education, Inc.

103 Cellulose microfibrilsFigure 35.29 Cellulose microfibrils Elongation Nucleus Vacuoles Figure The orientation of plant cell expansion. 5 m

104 Morphogenesis and Pattern FormationPattern formation is the development of specific structures in specific locations Two types of hypotheses explain the fate of plant cells Lineage-based mechanisms propose that cell fate is determined early in development and passed on to daughter cells Position-based mechanisms propose that cell fate is determined by final position © 2011 Pearson Education, Inc.

105 In contrast, cell fate in animals is largely lineage-dependentExperiments suggest that plant cell fate is established late in development and depends on cell position In contrast, cell fate in animals is largely lineage-dependent © 2011 Pearson Education, Inc.

106 KNOTTED-1 is important in the development of leaf morphologyHox genes in animals affect the number and placement of appendages in embryos A plant homolog of Hox genes called KNOTTED-1 does not affect the number or placement of plant organs KNOTTED-1 is important in the development of leaf morphology © 2011 Pearson Education, Inc.

107 Gene Expression and Control of Cell DifferentiationCells of a developing organism synthesize different proteins and diverge in structure and function even though they have a common genome Cellular differentiation depends on gene expression, but is determined by position Positional information is communicated through cell interactions © 2011 Pearson Education, Inc.

108 Gene activation or inactivation depends on cell-to-cell communicationFor example, Arabidopsis root epidermis forms root hairs or hairless cells depending on the number of cortical cells it is touching © 2011 Pearson Education, Inc.

109 GLABRA-2 is expressed, and the cell remains hairless. Cortical cellsFigure 35.31 GLABRA-2 is expressed, and the cell remains hairless. Cortical cells GLABRA-2 is not expressed, and the cell will develop a root hair. Figure Control of root hair differentiation by a homeotic gene (LM). 20 m The root cap cells will be sloughed off before root hairs emerge.

110 Shifts in Development: Phase ChangesPlants pass through developmental phases, called phase changes, developing from a juvenile phase to an adult phase Phase changes occur within the shoot apical meristem The most obvious morphological changes typically occur in leaf size and shape © 2011 Pearson Education, Inc.

111 Leaves produced by adult phase of apical meristemFigure 35.32 Leaves produced by adult phase of apical meristem Figure Phase change in the shoot system of Acacia koa. Leaves produced by juvenile phase of apical meristem

112 Genetic Control of FloweringFlower formation involves a phase change from vegetative growth to reproductive growth It is triggered by a combination of environmental cues and internal signals Transition from vegetative growth to flowering is associated with the switching on of floral meristem identity genes © 2011 Pearson Education, Inc.

113 These are MADS-box genes In a developing flower, the order of each primordium’s emergence determines its fate: sepal, petal, stamen, or carpel Plant biologists have identified several organ identity genes (plant homeotic genes) that regulate the development of floral pattern These are MADS-box genes A mutation in a plant organ identity gene can cause abnormal floral development © 2011 Pearson Education, Inc.

114 (a) Normal Arabidopsis flower PeFigure 35.33 Pe Ca St Se Pe Se (a) Normal Arabidopsis flower Pe Pe Figure Organ identity genes and pattern formation in flower development. Se Abnormal Arabidopsis flower (b)

115 Researchers have identified three classes of floral organ identity genesThe ABC hypothesis of flower formation identifies how floral organ identity genes direct the formation of the four types of floral organs An understanding of mutants of the organ identity genes depicts how this model accounts for floral phenotypes © 2011 Pearson Education, Inc.

116 Figure 35.34 Sepals Petals Stamens (a) A A schematic diagram of the ABC hypothesis B Carpels C C gene activity B  C gene activity Carpel A  B gene activity Petal A gene activity Stamen Sepal Active genes: B B B B B B B B A A A A A A C C C C A A C C C C C C C C A A C C C C A A A B B A A B B A Whorls: Figure The ABC hypothesis for the functioning of organ identity genes in flower development. Carpel Stamen Petal Sepal Wild type Mutant lacking A Mutant lacking B Mutant lacking C (b) Side view of flowers with organ identity mutations

117 B  C gene activity A  B gene activityFigure 35.34a Sepals (a) A schematic diagram of the ABC hypothesis Petals Stamens A Carpels B C C gene activity B  C gene activity Carpel A  B gene activity Petal A gene activity Figure The ABC hypothesis for the functioning of organ identity genes in flower development. Stamen Sepal