Plant Structure, Growth, and Development

Plant Structure, Growth, and Development
Chapter 35 Plant Structure, Growth, and Development

Figure 35.1 Figure 35.1 Computer art?

The Three Basic Plant Organs: Roots, Stems, and Leaves
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 © 2011 Pearson Education, Inc.

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

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

Roots rely on sugar produced by photosynthesis in the shoot system
Shoots rely on water and minerals absorbed by the root system Monocots and eudicots (dicots) are the two major groups of angiosperms © 2011 Pearson Education, Inc.

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

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.

Figure 35.3 Figure 35.3 Root hairs of a radish seedling.

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

Figure 35.4a Figure 35.4 Evolutionary adaptations of roots. Prop roots

Figure 35.4b Storage roots Figure 35.4 Evolutionary adaptations of roots.

“Strangling” aerial roots
Figure 35.4c Figure 35.4 Evolutionary adaptations of roots. “Strangling” aerial roots

Pneumatophores Figure 35.4d
Figure 35.4 Evolutionary adaptations of roots. Pneumatophores

Buttress roots Figure 35.4e
Figure 35.4 Evolutionary adaptations of roots. Buttress roots

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

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 © 2011 Pearson Education, Inc.

Apical dominance helps to maintain dormancy in most axillary buds
An 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.

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

Rhizome Root Rhizomes Figure 35.5a
Figure 35.5 Evolutionary adaptations of stems. Rhizomes

Storage leaves Stem Bulbs Figure 35.5b
Figure 35.5 Evolutionary adaptations of stems. Stem Bulbs

Stolon Stolons Figure 35.5c
Figure 35.5 Evolutionary adaptations of stems. Stolons

Figure 35.5d Figure 35.5 Evolutionary adaptations of stems. Tubers

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

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.

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

Simple leaf Axillary bud Petiole Compound leaf Doubly compound leaf
Figure 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

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

Figure 35.7a Tendrils Figure 35.7 Evolutionary adaptations of leaves.

Figure 35.7b Figure 35.7 Evolutionary adaptations of leaves. Spines

Storage leaves Figure 35.7c
Figure 35.7 Evolutionary adaptations of leaves. Storage leaves

Reproductive leaves Figure 35.7d
Figure 35.7 Evolutionary adaptations of leaves. Reproductive leaves

Figure 35.7e Figure 35.7 Evolutionary adaptations of leaves. Bracts

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

Dermal, Vascular, and Ground Tissues
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 © 2011 Pearson Education, Inc.

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

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.

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?

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.

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 storage, photosynthesis, and support © 2011 Pearson Education, Inc.

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.

Parenchyma Cells Mature parenchyma cells
Have 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.

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

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.

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

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.

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)

Water-Conducting Cells of the Xylem
The 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.

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.

Tracheids and vessels (colorized SEM) Pits
Figure 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

Sugar-Conducting Cells of the Phloem
Sieve-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.

Sieve-tube elements: longitudinal view (LM) 3 m
Figure 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)

Sieve-tube elements: longitudinal view (LM)
Figure 35.10eb Sieve-tube elements: longitudinal view (LM) Sieve plate Companion cells Sieve-tube elements Figure Exploring: Examples of Differentiated Plant Cells 30 m

Nucleus of companion cell
Figure 35.10ed Sieve-tube elements Plasmodesma Sieve plate Nucleus of companion cell Figure Exploring: Examples of Differentiated Plant Cells Sieve-tube elements: longitudinal view

Sieve plate with pores (LM)
Figure 35.10ec Sieve plate 15 m Figure Exploring: Examples of Differentiated Plant Cells Sieve plate with pores (LM)

Concept 35.2: Meristems generate cells for primary and secondary growth
A 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.

Meristems are perpetually embryonic tissue and allow for indeterminate growth
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 © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Primary growth in stems
Figure 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

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.

This year’s growth (one year old) Leaf scar
Figure 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

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

Concept 35.3: Primary growth lengthens roots and shoots
Primary growth produces the parts of the root and shoot systems produced by apical meristems © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Zone of differentiation Ground Root hair Vascular
Figure 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

Mitotic cells 100 m Figure 35.13a
Figure Primary growth of a root. 100 m

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.

Figure 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

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

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

100 m Emerging lateral root Cortex Vascular cylinder Pericycle 1
Figure 100 m Emerging lateral root Cortex Vascular cylinder Pericycle Figure The formation of a lateral root. 1

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

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

Tissue Organization of Stems
Figure 35.17 Tissue Organization of Stems 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)

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 sandwiched between the upper and lower epidermis © 2011 Pearson Education, Inc.

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.

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 and function as the leaf’s skeleton Each vein in a leaf is enclosed by a protective bundle sheath © 2011 Pearson Education, Inc.

Surface view of a spiderwort (Tradescantia) leaf (LM) Cuticle Stoma
Figure 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)

(a) Cutaway drawing of leaf tissues
Figure 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

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.

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)

Concept 35.4: Secondary growth increases the diameter of stems and roots in woody plants
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 Secondary growth is characteristic of gymnosperms and many eudicots, but not monocots © 2011 Pearson Education, Inc.

Primary and secondary growth in a two-year-old woody stem
Figure 35.19a-1 Pith (a) Primary and secondary growth in a two-year-old woody stem Primary xylem Vascular cambium Epidermis Primary phloem Cortex Cortex Primary phloem Epidermis Vascular cambium Primary xylem Pith Periderm (mainly cork cambia and cork) Figure Primary and secondary growth of a woody stem. Secondary phloem Secondary xylem

Figure 35.19a-2 Pith (a) Primary and secondary growth in a two-year-old woody stem Primary xylem Vascular cambium Epidermis Primary phloem Cortex Cortex Primary phloem Epidermis Vascular cambium Growth Vascular ray Primary xylem Secondary xylem Pith Secondary phloem First cork cambium Cork Periderm (mainly cork cambia and cork) Figure Primary and secondary growth of a woody stem. Secondary phloem Secondary xylem

Figure 35.19a-3 Pith (a) Primary and secondary growth in a two-year-old woody stem Primary xylem Vascular cambium Epidermis Primary phloem Cortex Cortex Primary phloem Epidermis Vascular cambium Growth Vascular ray Primary xylem Secondary xylem Pith Secondary phloem First cork cambium Cork Periderm (mainly cork cambia and cork) Growth Figure Primary and secondary growth of a woody stem. Most recent cork cambium Cork Secondary phloem Bark Layers of periderm Secondary xylem

Cross section of a three-year- old Tilia (linden) stem (LM)
Figure 35.19b Secondary phloem Bark Vascular cambium Cork cambium Late wood Secondary xylem Periderm Early wood Cork 0.5 mm Figure Primary and secondary growth of a woody stem. Vascular ray Growth ring (b) Cross section of a three-year- old Tilia (linden) stem (LM) 0.5 mm

After one year of growth After two years of growth
Figure 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

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

Figure 35.23 Figure Is this tree living or dead?

The Cork Cambium and the Production of Periderm
Cork 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.

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.

Evolution of Secondary Growth
In 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.

Concept 35.5: Growth, morphogenesis, and cell differentiation produce the plant body
Cells 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.

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

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.

Model Organisms: Revolutionizing the Study of Plants
New 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.

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

Figure 35.28 Figure Establishment of axial polarity.

Orientation of Cell Expansion
Plant 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.

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

Morphogenesis and Pattern Formation
Pattern 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.

KNOTTED-1 is important in the development of leaf morphology
Hox 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.

Figure 35.30 Figure Overexpression of a Hox-like gene in leaf formation.

Genetic Control of Flowering
Flower 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.

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.

(a) Normal Arabidopsis flower Pe
Figure 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)

Researchers have identified three classes of floral organ identity genes
The 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.

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

B  C gene activity A  B gene activity
Figure 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

(b) Side view of flowers with organ identity mutations
Figure 35.34b 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: Carpel Stamen Petal Sepal Figure The ABC hypothesis for the functioning of organ identity genes in flower development. Wild type Mutant lacking A Mutant lacking B Mutant lacking C (b) Side view of flowers with organ identity mutations

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