Plant Structure and Growth

Plant Structure and Growth
28 Plant Structure and Growth

Overview: Are Plants Computers?
Romanesco grows according to a repetitive program The development of plants depends on the environment and is highly adaptive © 2014 Pearson Education, Inc. 2

Figure 28.1 Figure 28.1 Computer art? 3

Angiosperms are primary producers and are important in agriculture
Taxonomists split the angiosperms into two major clades: monocots and eudicots © 2014 Pearson Education, Inc. 4

Figure 28.2 A comparison of monocots and eudicots
Embryos One cotyledon Two cotyledons Leaf venation Veins usually netlike Veins usually parallel Stems Vascular tissue scattered Vascular tissue usually arranged in ring Roots Root system usually fibrous (no main root) Taproot (main root) usually present Figure 28.2 A comparison of monocots and eudicots Pollen Pollen grain with one opening Pollen grain with three openings Flowers Floral organs usually in multiples of three Floral organs usually in multiples of four or five 5

Monocots Eudicots Embryos Leaf venation One cotyledon Two cotyledons
Figure 28.2a Monocots Eudicots Embryos One cotyledon Two cotyledons Leaf venation Figure 28.2a A comparison of monocots and eudicots (part 1: embryos and leaf venation) Veins usually netlike Veins usually parallel 6

usually arranged in ring
Figure 28.2b Monocots Eudicots Stems Vascular tissue scattered Vascular tissue usually arranged in ring Figure 28.2b A comparison of monocots and eudicots (part 2: stems and roots) Roots Root system usually fibrous (no main root) Taproot (main root) usually present 7

Floral organs usually in multiples of four or five
Figure 28.2c Monocots Eudicots Pollen Pollen grain with one opening Pollen grain with three openings Flowers Figure 28.2c A comparison of monocots and eudicots (part 3: pollen and flowers) Floral organs usually in multiples of three Floral organs usually in multiples of four or five 8

Concept 28.1: 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 An organ consists of several types of tissues that together carry out particular functions A tissue is a group of cells consisting of one or more cell types that together perform a specialized function © 2014 Pearson Education, Inc. 9

The Three Basic Plant Organs: Roots, Stems, and Leaves
The basic 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 © 2014 Pearson Education, Inc. 10

The root system includes all of the plant’s roots
Three basic organs evolved to acquire these resources: roots, stems, and leaves The root system includes all of the plant’s roots The shoot system includes the stems, leaves, and (in angiosperms) flowers © 2014 Pearson Education, Inc. 11

Reproductive shoot (flower) Apical bud
Figure 28.3 Reproductive shoot (flower) Apical bud Node Internode Apical bud Shoot system Vegetative shoot Blade Leaf Petiole Axillary bud Stem Taproot Figure 28.3 An overview of a flowering plant Lateral (branch) roots Root system 12

Roots rely on sugar produced by photosynthesis in the shoot system
Shoots rely on water and minerals absorbed by the root system © 2014 Pearson Education, Inc. 13

Roots A root is an organ with important functions Anchoring the plant
Absorbing minerals and water Storing carbohydrates © 2014 Pearson Education, Inc. 14

Small or trailing plants have a fibrous root system, which consists of
Tall, erect plants with large shoot masses have a taproot system, which consists of A taproot, the main vertical root Lateral roots branching off the taproot Small or trailing plants have a fibrous root system, which consists of Adventitious roots that arise from stems Lateral roots that arise from the adventitious roots and form their own lateral roots © 2014 Pearson Education, Inc. 15

Figure 28.4 Figure 28.4 Root hairs of a radish seedling 17

Many plants have root adaptations with specialized functions
Most terrestrial plants form mycorrhizal associations, which increase mineral absorption Many plants have root adaptations with specialized functions © 2014 Pearson Education, Inc. 18

“Strangling” aerial roots
Figure 28.5 Storage roots Pneumatophores Figure 28.5 Evolutionary adaptations of roots “Strangling” aerial roots 19

Pneumatophores Figure 28.5a
Figure 28.5a Evolutionary adaptations of roots (part 1: pneumatophores) Pneumatophores 20

Figure 28.5b Figure 28.5b Evolutionary adaptations of roots (part 2: storage roots) Storage roots 21

“Strangling” aerial roots
Figure 28.5c Figure 28.5c Evolutionary adaptations of roots (part 3: "strangling" aerial roots) “Strangling” aerial roots 22

Stems A stem is an organ consisting of
An alternating system of nodes, the points at which leaves are attached Internodes, the stem segments between nodes © 2014 Pearson Education, Inc. 23

Axillary buds are generally dormant
An apical bud, or terminal bud, is located near the shoot tip and causes elongation of a young shoot An axillary bud is located in the upper angle formed by the leaf and the stem and has the potential to form a lateral branch, thorn, or flower Axillary buds are generally dormant © 2014 Pearson Education, Inc. 24

Some plants have modified stems with additional functions
© 2014 Pearson Education, Inc. 25

Stolon Rhizome Root Rhizomes Stolons Tubers Figure 28.6
Figure 28.6 Evolutionary adaptations of stems Tubers 26

Rhizome Root Rhizomes Figure 28.6a
Figure 28.6a Evolutionary adaptations of stems (part 1: rhizomes) Rhizomes 27

Stolon Stolons Figure 28.6b
Figure 28.6b Evolutionary adaptations of stems (part 2: stolons) Stolons 28

Figure 28.6c Figure 28.6c Evolutionary adaptations of stems (part 3: tubers) Tubers 29

Leaves The leaf is the main photosynthetic organ of most vascular plants Leaves have other functions including gas exchange, dissipation of heat, and defense Leaves generally consist of a flattened blade and a stalk called the petiole, which joins the leaf to the stem © 2014 Pearson Education, Inc. 30

Monocots and eudicots differ in the arrangement of veins, the vascular tissue of leaves
Most monocots have parallel veins Most eudicots have branching veins © 2014 Pearson Education, Inc. 31

Some plant species have evolved modified leaves that serve various functions

Spines Tendrils Storage leaves Stem Reproductive leaves Storage leaves
Figure 28.7 Spines Tendrils Figure 28.7 Evolutionary adaptations of leaves Storage leaves Stem Reproductive leaves Storage leaves 33

Figure 28.7a Figure 28.7a Evolutionary adaptations of leaves (part 1: tendrils) Tendrils 34

Figure 28.7b Figure 28.7b Evolutionary adaptations of leaves (part 2: spines) Spines 35

Storage leaves Stem Storage leaves Figure 28.7c
Figure 28.7c Evolutionary adaptations of leaves (part 3: storage leaves) Stem Storage leaves 36

Reproductive leaves Figure 28.7d
Figure 28.7d Evolutionary adaptations of leaves (part 4: reproductive leaves) Reproductive leaves 37

Dermal, Vascular, and Ground Tissue Systems
Each 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 © 2014 Pearson Education, Inc. 38

Dermal tissue Ground tissue Vascular tissue Figure 28.8
Figure 28.8 The three tissue systems Dermal tissue Ground tissue Vascular tissue 39

The dermal tissue system is the outer protective covering
In nonwoody plants, the dermal tissue system consists of the epidermis, a layer of densely packed cells In leaves and most stems, a waxy coating called the cuticle helps prevent water loss from the epidermis © 2014 Pearson Education, Inc. 40

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 © 2014 Pearson Education, Inc. 41

The two vascular tissues are xylem and phloem
The vascular tissue system facilitates transport of materials through the plant and provides support The two vascular tissues are xylem and phloem Xylem conducts water and dissolved minerals upward from roots into the shoots Phloem transports organic nutrients from where they are made to where they are needed © 2014 Pearson Education, Inc. 42

The vascular tissue of a root or stem is collectively called the stele
In angiosperms, the root stele is a solid central vascular cylinder The stele of stems and leaves is divided into vascular bundles, strands of xylem and phloem © 2014 Pearson Education, Inc. 43

Tissues that are neither dermal nor vascular are the ground tissue system
Ground tissue internal to the vascular tissue is pith; ground tissue external to the vascular tissue is cortex Ground tissue includes cells specialized for photosynthesis, short-distance transport, storage, or support © 2014 Pearson Education, Inc. 44

Common Types of Plant Cells
Plant cells have structural adaptations that make their specific functions possible The major types of plant cells are Parenchyma Collenchyma Sclerenchyma Water-conducting cells of the xylem Sugar-conducting cells of the phloem © 2014 Pearson Education, Inc. 45

Animation: Tour of a Plant Cell
46

Parenchyma cells Have thin and flexible primary walls
Lack secondary walls Have a large central vacuole Perform the most metabolic functions Retain the ability to divide and differentiate © 2014 Pearson Education, Inc. 47

chloroplasts (in Elodea leaf) (LM) 60 m
Figure 28.9a Figure 28.9a Exploring examples of differentiated plant cells (part 1: parenchyma cells with chloroplasts in Elodea leaf, LM) Parenchyma cells with chloroplasts (in Elodea leaf) (LM) 60 m 48

Collenchyma cells Are grouped in strands
Help support young parts of the plant shoot Have thicker and uneven cell walls Lack secondary walls Provide flexible support without restraining growth © 2014 Pearson Education, Inc. 49

(in Helianthus stem) (LM) 5 m
Figure 28.9b Figure 28.9b Exploring examples of differentiated plant cells (part 2: collenchyma cells in Helianthus stem, LM) Collenchyma cells (in Helianthus stem) (LM) 5 m 50

They are dead at functional maturity There are two types
Sclerenchyma cells are rigid due to thick secondary walls containing lignin, a strengthening polymer 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 grouped in strands © 2014 Pearson Education, Inc. 51

Sclereid cells (in pear) (LM)
Figure 28.9c 5 m Sclereid cells (in pear) (LM) 25 m Cell wall Figure 28.9c Exploring examples of differentiated plant cells (part 3: sclerenchyma cells) Fiber cells (cross section from ash tree) (LM) 52

5 m Sclereid cells (in pear) (LM) Cell wall Figure 28.9ca
Figure 28.9ca Exploring examples of differentiated plant cells (part 3a: sclerenchyma cells; sclereid cells in pear, LM) Cell wall 53

Fiber cells (cross section from ash tree) (LM)
Figure 28.9cb 25 m Figure 28.9cb Exploring examples of differentiated plant cells (part 3b: sclerenchyma cells; fiber cells, cross section from ash tree, LM) Fiber cells (cross section from ash tree) (LM) 54

Water-conducting cells of the xylem are dead at functional maturity
There are two types of water-conducting cells Tracheids are long, thin cells with tapered ends that move water through pits Vessel elements align end to end to form long micropipes called vessels © 2014 Pearson Education, Inc. 55

Tracheids occur in the xylem of all vascular plants
Vessel elements are common to most angiosperms, a few gymnosperms, and a few seedless vascular plants © 2014 Pearson Education, Inc. 56

100 m Vessel Tracheids Pits Tracheids and vessels (colorized SEM)
Figure 28.9d 100 m Vessel Tracheids Pits Tracheids and vessels (colorized SEM) Figure 28.9d Exploring examples of differentiated plant cells (part 4: water-conducting cells of the xylem) Perforation plate Vessel element Vessel elements, with perforated end walls Tracheids 57

100 m Vessel Tracheids Tracheids and vessels (colorized SEM)
Figure 28.9da 100 m Vessel Tracheids Figure 28.9da Exploring examples of differentiated plant cells (part 4a: water-conducting cells of the xylem, colorized SEM) Tracheids and vessels (colorized SEM) 58

Sugar-conducting cells of the phloem are alive at functional maturity
In seedless vascular plants and gymnosperms, sugars are transported through sieve cells © 2014 Pearson Education, Inc. 59

In angiosperms, sugars are transported in sieve tubes, chains of cells called sieve-tube elements
Sieve plates are the porous end walls that allow fluid to flow between cells along the sieve tube Sieve-tube elements lack organelles, but each has a companion cell whose nucleus and ribosomes serve both cells © 2014 Pearson Education, Inc. 60

longitudinal view (LM) 3 m
Figure 28.9e 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 Sieve plate Figure 28.9e Exploring examples of differentiated plant cells (part 5: sugar-conducting cells of the phloem) 30 m Nucleus of companion cell 15 m Sieve-tube elements: longitudinal view Sieve plate with pores (LM) 61

Sieve-tube element (left) and companion cell: cross section (TEM)
Figure 28.9ea 3 m Sieve-tube element (left) and companion cell: cross section (TEM) Figure 28.9ea Exploring examples of differentiated plant cells (part 5a: sugar-conducting cells of the phloem; sieve-tube element (left) and companion cell, cross section, TEM) 62

longitudinal view (LM)
Figure 28.9eb Sieve-tube elements: longitudinal view (LM) Sieve plate Companion cells Sieve-tube elements Figure 28.9eb Exploring examples of differentiated plant cells (part 5b: sugar-conducting cells of the phloem; sieve-tube elements, longitudinal view, LM) 30 m 63

Sieve plate with pores (LM)
Figure 28.9ec Sieve plate 15 m Figure 28.9ec Exploring examples of differentiated plant cells (part 5c: sugar-conducting cells of the phloem; sieve plate with pores, LM) Sieve plate with pores (LM) 64

Concept 28.2: Meristems generate new cells for growth and control the developmental phases and life spans of plants A plant can grow throughout its life; this is called indeterminate growth Indeterminate growth is enabled by meristems, which are perpetually undifferentiated tissues Some plant organs cease to grow at a certain size; this is called determinate growth © 2014 Pearson Education, Inc. 65

Different Meristems Produce Primary and Secondary Growth
There are two main types of meristems Apical meristems Lateral meristems © 2014 Pearson Education, Inc. 66

Primary growth in stems Epidermis
Figure 28.10 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 Lateral meristems Cork cambium Cork cambium Axillary bud meristem Periderm Pith Cortex Figure An overview of primary and secondary growth Primary phloem Secondary phloem Primary xylem Root apical meristems Secondary xylem Vascular cambium 67

Shoot tip (shoot apical meristem and young leaves) Vascular cambium
Figure 28.10a Shoot tip (shoot apical meristem and young leaves) Vascular cambium Lateral meristems Cork cambium Axillary bud meristem Figure 28.10a An overview of primary and secondary growth (part 1: meristems) Root apical meristems 68

Primary growth in stems
Figure 28.10b Primary growth in stems Epidermis Cortex Primary phloem Primary xylem Figure 28.10b An overview of primary and secondary growth (part 2: primary stem growth) Pith 69

Secondary growth in stems
Figure 28.10c Secondary growth in stems Cork cambium Periderm Pith Cortex Primary phloem Secondary phloem Primary xylem Figure 28.10c An overview of primary and secondary growth (part 3: secondary stem growth) Secondary xylem Vascular cambium 70

Apical 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 © 2014 Pearson Education, Inc. 71

Lateral meristems add thickness to woody plants, a process called secondary growth
There 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 © 2014 Pearson Education, Inc. 72

Apical bud Bud scale Axillary buds This year’s growth Leaf
Figure 28.11 Apical bud Bud scale Axillary buds This year’s growth (one year old) Leaf scar Bud scar Node One-year-old branch formed from axillary bud near shoot tip Internode Last year’s growth (two years old) Leaf scar Stem Figure Three years’ growth in a winter twig Bud scar Growth of two years ago (three years old) Leaf scar 74

Gene Expression and Control of Cell Differentiation
Cells 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 © 2014 Pearson Education, Inc. 76

Gene activation or inactivation depends on cell-to-cell communication
For example, Arabidopsis root epidermis forms root hairs or hairless cells depending on the number of cortical cells it is touching © 2014 Pearson Education, Inc. 77

and the cell remains hairless. Cortical cells
Figure 28.12 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 master regulatory gene (LM) 20 m The root cap cells will be sloughed off before root hairs emerge. 78

Meristematic Control of the Transition to Flowering and the Life Spans of Plants
Flower formation involves a transition from vegetative growth to reproductive growth It is triggered by a combination of environmental cues and internal signals Reproductive growth is determinate; the production of a flower stops primary growth of that shoot © 2014 Pearson Education, Inc. 79

Flowering plants can be categorized based on the timing and completeness of the switch from vegetative to reproductive growth Annuals complete their life cycle in a year or less Biennials require two growing seasons Perennials live for many years © 2014 Pearson Education, Inc. 80

Concept 28.3: Primary growth lengthens roots and shoots
Primary growth arises from the apical meristems and produces parts of the root and shoot systems © 2014 Pearson Education, Inc. 81

Primary Growth of Roots
The 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 © 2014 Pearson Education, Inc. 82

Video: Root Time Lapse 83

Cortex Vascular cylinder Dermal Ground Epidermis Vascular Zone of
Figure 28.13 Cortex Vascular cylinder Dermal Ground Epidermis Vascular Zone of differentiation Root hair Zone of elongation Figure Primary growth of a typical eudicot root Zone of cell division (including root apical meristem) Mitotic cells 100 m Root cap 84

Mitotic cells 100 m Figure 28.13a
Figure 28.13a Primary growth of a typical eudicot root (LM) 100 m 85

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 © 2014 Pearson Education, Inc. 86

(a) Root with xylem and phloem in the center (typical of eudicots)
Figure 28.14 Epidermis Cortex Endodermis Vascular cylinder Pericycle Core of parenchyma cells Xylem 100 m Phloem (a) Root with xylem and phloem in the center (typical of eudicots) 100 m (b) Root with parenchyma in the center (typical of monocots) Endodermis Figure Organization of primary tissues in young roots Pericycle Xylem Phloem Dermal Ground Vascular 70 m 87

(a) Root with xylem and phloem in the center (typical of eudicots)
Figure 28.14a Epidermis Cortex Endodermis Vascular cylinder Pericycle Xylem Figure 28.14a Organization of primary tissues in young roots (part 1: eudicot, LM) Phloem 100 m Dermal (a) Root with xylem and phloem in the center (typical of eudicots) Ground Vascular 88

(b) Root with parenchyma in the center (typical of monocots) Ground
Figure 28.14b Epidermis Cortex Endodermis Vascular cylinder Pericycle Core of parenchyma cells Xylem Figure 28.14b Organization of primary tissues in young roots (part 2: monocot, LM) Phloem 100 m Dermal (b) Root with parenchyma in the center (typical of monocots) Ground Vascular 89

Endodermis Pericycle Xylem Phloem Dermal Ground Vascular 70 m
Figure 28.14c Endodermis Pericycle Xylem Phloem Dermal Figure 28.14c Organization of primary tissues in young roots (part 3: eudicot, LM, detail) Ground Vascular 70 m 90

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 © 2014 Pearson Education, Inc. 91

Lateral roots arise from within the pericycle, the outermost cell layer in the vascular cylinder

Emerging lateral root 100 m Cortex Vascular Pericycle cylinder
Figure Emerging lateral root 100 m Cortex Figure The formation of a lateral root (step 1) Vascular cylinder Pericycle 93

Emerging Epidermis lateral root 100 m Lateral root Cortex Vascular
Figure Emerging lateral root Epidermis 100 m Lateral root Cortex Figure The formation of a lateral root (step 2) Vascular cylinder Pericycle 94

Emerging Epidermis lateral root 100 m Lateral root Cortex Vascular
Figure Emerging lateral root Epidermis 100 m Lateral root Cortex Figure The formation of a lateral root (step 3) Vascular cylinder Pericycle 95

Emerging lateral root 100 m Cortex Vascular Pericycle cylinder
Figure 28.15a Emerging lateral root 100 m Cortex Figure 28.15a The formation of a lateral root (part 1: origin in pericycle, LM) Vascular cylinder Pericycle 96

Epidermis Lateral root Figure 28.15b
Figure 28.15b The formation of a lateral root (part 2: growth through cortex, LM) 97

Epidermis Lateral root Figure 28.15c
Figure 28.15c The formation of a lateral root (part 3: growth through epidermis, LM) 98

Primary Growth of Shoots
A shoot apical meristem is a dome-shaped mass of dividing cells at the shoot tip Leaves develop from leaf primordia, projections along the sides of the apical meristem Shoot elongation results from lengthening of internode cells below the shoot tip © 2014 Pearson Education, Inc. 99

Shoot apical meristem Leaf primordia Young leaf Developing vascular
Figure 28.16 Shoot apical meristem Leaf primordia Young leaf Developing vascular strand Figure The shoot tip (LM) Axillary bud meristems 0.25 mm 100

Axillary buds are dormant due to apical dominance, inhibition from an active apical bud
When released from dormancy, axillary buds give rise to lateral shoots (branches) © 2014 Pearson Education, Inc. 101

Tissue Organization of Leaves
The 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 composed of parenchyma cells sandwiched between the upper and lower epidermis © 2014 Pearson Education, Inc. 102

Figure 28.17 Guard cells Stomatal pore 50 m Dermal Epidermal cell Ground Cuticle Sclerenchyma fibers Vascular Stoma (b) Surface view of a spiderwort (Tradescantia) leaf (LM) Upper epidermis Palisade mesophyll Spongy mesophyll Figure Leaf anatomy Lower epidermis 100 m Bundle- sheath cell Cuticle Xylem Vein Phloem Guard cells Vein Air spaces Guard cells (a) Cutaway drawing of leaf tissues (c) Cross section of a lilac (Syringa) leaf (LM) 103

(a) Cutaway drawing of leaf tissues
Figure 28.17a Dermal Ground Sclerenchyma fibers Cuticle Vascular Stoma Upper epidermis Palisade mesophyll Spongy mesophyll Figure 28.17a Leaf anatomy (part 1: cutaway drawing of leaf tissues) Lower epidermis Bundle- sheath cell Cuticle Xylem Vein Phloem Guard cells (a) Cutaway drawing of leaf tissues 104

(b) Surface view of a spiderwort (Tradescantia) leaf (LM) Dermal
Figure 28.17b Guard cells Stomatal pore 50 m Epidermal cell (b) Surface view of a spiderwort (Tradescantia) leaf (LM) Figure 28.17b Leaf anatomy (part 2: surface view of a leaf of a spiderwort, Tradescantia, LM) Dermal 105

(c) Cross section of a lilac (Syringa) leaf (LM)
Figure 28.17c Upper epidermis Palisade mesophyll Spongy mesophyll Lower epidermis 100 m Figure 28.17c Leaf anatomy (part 3: cross section of a leaf of a lilac, Syringa, LM) Dermal Vein Air spaces Guard cells Ground (c) Cross section of a lilac (Syringa) leaf (LM) 106

The mesophyll of eudicots has two layers
The palisade mesophyll in the upper part of the leaf consists of elongated cells for photosynthesis The spongy mesophyll in the lower part of the leaf consists of loosely arranged cells for gas exchange © 2014 Pearson Education, Inc. 107

Each vein in a leaf is enclosed by a protective bundle sheath
The vascular tissue of each leaf is continuous with the vascular tissue of the stem Veins are the leaf’s vascular bundles, which function in fluid transport and provide structure to the leaf Each vein in a leaf is enclosed by a protective bundle sheath © 2014 Pearson Education, Inc. 108

Tissue Organization of Stems
Stems are covered in epidermis and contain vascular bundles that run the length of the stem Lateral shoots develop from axillary buds on the stem’s surface In most eudicots, the vascular tissue consists of vascular bundles arranged in a ring © 2014 Pearson Education, Inc. 109

(a) Cross section of stem with vascular
Figure 28.18 Sclerenchyma (fiber cells) Phloem Xylem Ground tissue connecting pith to cortex Ground tissue Pith Epidermis Dermal Figure Organization of primary tissues in young stems Epidermis Cortex Ground Vascular bundles Vascular bundle Vascular 1 mm 1 mm (a) Cross section of stem with vascular bundles forming a ring (typical of eudicots) (b) Cross section of stem with scattered vascular bundles (typical of monocots) 110

(a) Cross section of stem with vascular
Figure 28.18a Sclerenchyma (fiber cells) Phloem Xylem Ground tissue connecting pith to cortex Pith Figure 28.18a Organization of primary tissues in young stems (part 1: eudicot, LM) Dermal Epidermis Cortex Ground Vascular bundle Vascular 1 mm (a) Cross section of stem with vascular bundles forming a ring (typical of eudicots) 111

(b) Cross section of stem with scattered
Figure 28.18b Ground tissue Epidermis Figure 28.18b Organization of primary tissues in young stems (part 2: monocot, LM) Dermal Ground Vascular bundles Vascular 1 mm (b) Cross section of stem with scattered vascular bundles (typical of monocots) 112

Concept 28.4: Secondary growth increases the diameter of stems and roots in woody plants
Secondary growth is characteristic of gymnosperms and many eudicots, but not monocots Secondary 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 © 2014 Pearson Education, Inc. 114

In woody plants, primary growth and secondary growth occur simultaneously but in different locations

Figure 28.19 (a) Primary and secondary growth in a two-year-old woody stem Pith Primary xylem Vascular cambium Primary phloem Epidermis Cortex Cortex Epidermis Primary phloem Vascular cambium Vascular ray Growth Periderm Primary xylem Secondary phloem Cork cambium Vascular cambium Cork Pith Bark Secondary xylem Late wood Secondary xylem Early wood Secondary phloem Cork First cork cambium 1 mm Periderm (mainly cork cambia and cork) Growth Vascular ray Growth ring Figure Primary and secondary growth of a woody stem 1.4 mm (b) Cross section of a three-year- old Tilia (linden) stem (LM) Secondary phloem Most recent cork cambium Layers of periderm Secondary xylem Cork Bark 116

(a) Primary and secondary growth in a two-year-old woody stem Pith
Figure 28.19a-1 (a) Primary and secondary growth in a two-year-old woody stem Pith Primary xylem Vascular cambium Epidermis Primary phloem Cortex Cortex Epidermis Primary phloem Vascular cambium Primary xylem Pith Periderm (mainly cork cambia and cork) Figure 28.19a-1 Primary and secondary growth of a woody stem (part 1: a two-year-old woody stem, step 1) Secondary phloem Secondary xylem 117

Figure 28.19a-2 (a) Primary and secondary growth in a two-year-old woody stem Pith Primary xylem Vascular cambium Epidermis Primary phloem Cortex Cortex Epidermis Primary phloem Vascular cambium Growth Primary xylem Vascular ray Pith Secondary xylem Secondary phloem First cork cambium Cork Periderm (mainly cork cambia and cork) Figure 28.19a-2 Primary and secondary growth of a woody stem (part 1: a two-year-old woody stem, step 2) Secondary phloem Secondary xylem 118

Figure 28.19a-3 (a) Primary and secondary growth in a two-year-old woody stem Pith Primary xylem Vascular cambium Epidermis Primary phloem Cortex Cortex Epidermis Primary phloem Vascular cambium Growth Primary xylem Vascular ray Pith Secondary xylem Secondary phloem First cork cambium Cork Periderm (mainly cork cambia and cork) Growth Most recent cork cambium Figure 28.19a-3 Primary and secondary growth of a woody stem (part 1: a two-year-old woody stem, step 3) Cork Bark Secondary phloem Layers of periderm Secondary xylem 119

(b) Cross section of a three-year- old Tilia (linden) stem (LM)
Figure 28.19b Secondary phloem Periderm Cork cambium Vascular cambium Cork Bark Secondary xylem Late wood Early wood 1 mm Figure 28.19b Primary and secondary growth of a woody stem (part 2: cross section of a three-year-old stem of a linden, Tilia, LM) Vascular ray Growth ring 1.4 mm (b) Cross section of a three-year- old Tilia (linden) stem (LM) 120

The Vascular Cambium and Secondary Vascular Tissue
The vascular cambium is a cylinder of meristematic cells one cell layer thick 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 © 2014 Pearson Education, Inc. 121

Vascular cambium Growth Vascular cambium Secondary phloem Secondary
Figure 28.20 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 122

Elongated, parallel-oriented initials produce tracheids, vessel elements, fibers of xylem, sieve-tube elements, companion cells, axially oriented parenchyma, and fibers of the phloem Shorter, perpendicularly oriented initials produce vascular rays, radial files of parenchyma cells that connect secondary xylem and phloem © 2014 Pearson Education, Inc. 123

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 © 2014 Pearson Education, Inc. 124

Tree rings are visible where late and early wood meet and can be used to estimate a tree’s age
Dendrochronology is the analysis of tree ring growth patterns and can be used to study past climate change © 2014 Pearson Education, Inc. 125

As 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 Only young secondary phloem transports sugar; older secondary phloem sloughs off and does not accumulate © 2014 Pearson Education, Inc. 126

Growth ring Vascular ray Heartwood Secondary xylem Sapwood
Figure 28.21 Growth ring Vascular ray Heartwood Secondary xylem Sapwood Figure Anatomy of a tree trunk Vascular cambium Secondary phloem Bark Layers of periderm 127

The Cork Cambium and the Production of Periderm
The cork cambium gives rise to cork cells that accumulate to the exterior of the cork cambium Cork cells deposit waxy suberin in their walls and then die Periderm consists of the cork cambium and the cork cells it produces Periderm functions as a protective barrier, impermeable to water and gasses © 2014 Pearson Education, Inc. 128

Bark consists of all the tissues external to the vascular cambium, including secondary phloem and periderm Lenticels in the periderm allow for gas exchange between living stem or root cells and the outside air © 2014 Pearson Education, Inc. 129

Figure 28.UN01 Figure 28.UN01 Skills exercise: using bar graphs to interpret data 130

Shoot tip (shoot apical meristem and young leaves) Vascular cambium
Figure 28.UN02 Shoot tip (shoot apical meristem and young leaves) Vascular cambium Lateral meristems Cork cambium Axillary bud meristems Figure 28.UN02 Summary of key concepts: meristems Root apical meristems 131

Figure 28.UN03 Figure 28.UN03 Test your understanding, question 6 132

Plant Structure and Growth

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Plant Structure and Growth

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