35
Plant Chapter Structure, Growth, and Development
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
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Concept 35.1: The plant body has a hierarchy of organs, tissues, and cells Plants, like multicellular animals
Have organs composed of different tissues, which are in turn composed of cells
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The Three Basic Plant Organs: Roots, Stems, and Leaves
The basic morphology of vascular plants
Reflects their evolutionary history as terrestrial organisms that draw nutrients from two very different environments: below-ground and above-ground
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Three basic organs evolved: roots, stems, and leaves They are organized into a root system and a shoot system
Reproductive shoot (flower) Terminal bud Node Internode Terminal bud Shoot system Vegetative shoot Leaf Blade Petiole Axillary bud Stem Taproot
Lateral roots
Root system
Figure 35.2
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Roots A root
Is an organ that anchors the vascular plant Absorbs minerals and water Often stores organic nutrients
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In most plants
The absorption of water and minerals occurs near the root tips, where vast numbers of tiny root hairs increase the surface area of the root
Figure 35.3
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Many plants have modified roots
(a) Prop roots
(b) Storage roots(c) "Strangling" aerial roots
Figure 35.4ae
(d) Buttress roots
(e) Pneumatophores
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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
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An axillary bud
Is a structure that has the potential to form a lateral shoot, or branch
A terminal bud
Is located near the shoot tip and causes elongation of a young shoot
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Many plants have modified stems
(a) Stolons. Shown here on a strawberry plant, stolons are horizontal stems that grow along the surface. These "runners" enable a plant to reproduce asexually, as plantlets form at nodes along each runner. Storage leaves
Stem Root (b) Bulbs. Bulbs are vertical, underground shoots consisting (c) Tubers. Tubers, such as these mostly of the enlarged bases red potatoes, are enlarged of leaves that store food. You ends of rhizomes specialized can see the many layers of for storing food. The "eyes" modified leaves attached arranged in a spiral pattern to the short stem by slicing an around a potato are clusters onion bulb lengthwise. of axillary buds that mark the nodes.
(d) Rhizomes. The edible base of this ginger plant is an example of a rhizome, a horizontal stem that grows just below the surface or emerges and grows along the surface. Node
Rhizome
Root
Figure 35.5ad
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Leaves The leaf
Is the main photosynthetic organ of most vascular plants
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Leaves generally consist of
A flattened blade and a stalk The petiole, which joins the leaf to a node of the stem
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Monocots and dicots
Differ in the arrangement of veins, the vascular tissue of leaves
Most monocots
Have parallel veins
Most dicots
Have branching veins
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In classifying angiosperms
Taxonomists may use leaf morphology as a Simple leaf. A simple leaf criterion (a) is a single, undivided blade.
Some simple leaves are deeply lobed, as in an oak leaf. Petiole (b) Compound leaf. In a compound leaf, the blade consists of multiple leaflets. Notice that a leaflet has no axillary bud at its base. (c) Doubly compound leaf. In a doubly compound leaf, each leaflet is divided into smaller leaflets. Axillary bud Leaflet
Petiole Axillary bud
Figure 35.6ac
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Leaflet Petiole Axillary bud
Some plant species
Have evolved modified leaves that serve various functions
(a) Tendrils. The tendrils by which this pea plant clings to a support are modified leaves. After it has "lassoed" a support, a tendril forms a coil that brings the plant closer to the support. Tendrils are typically modified leaves, but some tendrils are modified stems, as in grapevines.
(b) Spines. The spines of cacti, such as this prickly pear, are actually leaves, and photosynthesis is carried out mainly by the fleshy green stems.
(c) Storage leaves. Most succulents, such as this ice plant, have leaves modified for storing water. (d) Bracts. Red parts of the poinsettia are often mistaken for petals but are actually modified leaves called bracts that surround a group of flowers. Such brightly colored leaves attract pollinators. (e) Reproductive leaves. The leaves of some succulents, such as Kalanchoe daigremontiana, produce adventitious plantlets, which fall off the leaf and take root in the soil.
Figure 35.6ae
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The Three Tissue Systems: Dermal, Vascular, and Ground
Each plant organ
Has dermal, vascular, and ground tissues
Dermal tissue Ground tissue Vascular tissue
Figure 35.8
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The dermal tissue system
Consists of the epidermis and periderm
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The vascular tissue system
Carries out long-distance transport of materials between roots and shoots Consists of two tissues, xylem and phloem
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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
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Ground tissue
Includes various cells specialized for functions such as storage, photosynthesis, and support
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Common Types of Plant Cells Like any multicellular organism
A plant is characterized by cellular differentiation, the specialization of cells in structure and function
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Some of the major types of plant cells include
Parenchyma Collenchyma Sclerenchyma Water-conducting cells of the xylem Sugar-conducting cells of the phloem
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Parenchyma, collenchyma, and sclerenchyma cells
PARENCHYMA CELLS 80 m
COLLENCHYMA CELLS Cortical parenchyma cells
SCLERENCHYMA CELLS 5 m Sclereid cells in pear 25 m
Cell wall Parenchyma cells
60 m
Collenchyma cells Fiber cells
Figure 35.9
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Concept 35.2: Meristems generate cells for new organs Apical meristems
Are located at the tips of roots and in the buds of shoots Elongate shoots and roots through primary growth
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Lateral meristems
Add thickness to woody plants through secondary growth
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An overview of primary and secondary growth
Shoot apical meristems (in buds) In woody plants, there are lateral meristems that add secondary growth, increasing the girth of roots and stems. Primary growth in stems Epidermis Cortex Primary phloem Primary xylem Lateral meristems Pith
Vascular cambium Cork cambium
Apical meristems add primary growth, or growth in length. Pith
Secondary growth in stems Periderm Cork cambium The cork cambium adds secondary dermal tissue. Cortex Primary phloem The vascular cambium adds Secondary secondary phloem xylem and Vascular cambium phloem.
Primary xylem
Root apical meristems
Secondary xylem
Figure. 35.10
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In woody plants
Primary and secondary growth occur simultaneously but in different locations
Terminal bud Bud scale Axillary buds Leaf scar This year's growth (one year old) Node Stem Internode One-year-old side branch formed from axillary bud near shoot apex Leaf scar Last year's growth (two years old) Scars left by terminal bud scales of previous winters
Figure 35.11
Growth of two years ago (three years old)
Leaf scar
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Concept 35.3: Primary growth lengthens roots and shoots Primary growth produces the primary plant body, the parts of the root and shoot systems produced by apical meristems
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Primary Growth of Roots The root tip is covered by a root cap, which protects the delicate apical meristem as the root pushes through soil during primary growth
Cortex Epidermis Key Dermal Ground Vascular Root hair Zone of maturation Vascular cylinder
Zone of elongation
Apical meristem Root cap
Zone of cell division
Figure 35.12
100 m
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The primary growth of roots
Produces the epidermis, ground tissue, and vascular tissue
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Organization of primary tissues in young roots
Epidermis Cortex Vascular cylinder Endodermis Pericycle Core of parenchyma cells Xylem
100 m (a) Transverse section of a typical root. In the roots of typical gymnosperms and eudicots, as well as some monocots, the stele is a vascular cylinder consisting of a lobed core of xylem with phloem between the lobes. Endodermis Pericycle
Phloem
100 m (b) Transverse section of a root with parenchyma in the center. The stele of many monocot roots is a vascular cylinder with a core of parenchyma surrounded by a ring of alternating xylem and phloem. Key Dermal Ground Vascular
Xylem Phloem
Figure 35.13a, b
50 m
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Lateral roots
Arise from within the pericycle, the outermost cell layer in the vascular cylinder
100 m Emerging lateral root
Cortex
1
Vascular cylinder Epidermis
2
Lateral root
Figure 35.14
3
4
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Primary Growth of Shoots A shoot apical meristem
Is a dome-shaped mass of dividing cells at the tip of the terminal bud Gives rise to a repetition of internodes and Apical meristem Leaf primordia leaf-bearing nodes
Developing vascular strand
Axillary bud meristems
Figure. 35.15
0.25 mm
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Tissue Organization of Stems In gymnosperms and most eudicots
The vascular tissue consists of vascular bundles arranged in a ring
Phloem Sclerenchyma (fiber cells) Xylem Ground tissue connecting pith to cortex
Pith
Key Epidermis Vascular bundle 1 mm Cortex Dermal Ground Vascular
Figure 35.16a
(a) A eudicot stem. A eudicot stem (sunflower), with vascular bundles forming a ring. Ground tissue toward the inside is called pith, and ground tissue toward the outside is called cortex. (LM of transverse section)
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In most monocot stems
The vascular bundles are scattered throughout the ground tissue, rather than forming a ring
Ground tissue
Epidermis
Vascular bundles 1 mm
Figure 35.16b (b)
A monocot stem. A monocot stem (maize) with vascular bundles scattered throughout the ground tissue. In such an arrangement, ground tissue is not partitioned into pith and cortex. (LM of transverse section)
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Tissue Organization of Leaves The epidermal barrier in leaves
Is interrupted by stomata, which allow CO2 exchange between the surrounding air and the photosynthetic cells within a leaf
The ground tissue in a leaf
Is sandwiched between the upper and lower epidermis
The vascular tissue of each leaf
Is continuous with the vascular tissue of the stem
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Leaf anatomy
Key to labels Dermal Ground Vascular
Guard cells Stomatal pore Epidermal cell 50 m (b) Surface view of a spiderwort (Tradescantia) leaf (LM) Upper epidermis Palisade mesophyll
Cuticle
Sclerenchyma fibers Stoma
Bundlesheath cell
Spongy mesophyll Lower epidermis Cuticle Xylem Phloem Vein Guard cells Vein Air spaces Guard cells
Guard cells
Figure 35.17ac
(a) Cutaway drawing of leaf tissues
(c) Transverse section of a lilac 100 m (Syringa) leaf (LM)
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Concept 35.4: Secondary growth adds girth to stems and roots in woody plants Secondary growth
in Occurs 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
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The Vascular Cambium and Secondary Vascular Tissue
The vascular cambium
Is a cylinder of meristematic cells one cell thick Develops from parenchyma cells
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Primary and secondary growth of a stem
(a) Primary and secondary growth in a two-year-old stem 1 1 In the youngest part of the stem, you can see the primary plant body, as formed by the apical meristem during primary growth. The vascular cambium is beginning to develop. 2 As primary growth continues to elongate the stem, the portion of the stem formed earlier the same year has already started its secondary growth. This portion increases in girth as fusiform initials of the vascular cambium form secondary xylem to the inside and secondary phloem to the outside. 3 The ray initials of the vascular cambium give rise to the xylem and phloem rays. 4 As the diameter of the vascular cambium increases, the secondary phloem and other tissues external to the cambium cannot keep pace with the expansion because the cells no longer divide. As a result, these tissues, including the epidermis, rupture. A second lateral meristem, the cork cambium, develops from parenchyma cells in the cortex. The cork cambium produces cork cells, which replace the epidermis. 5 In year 2 of secondary growth, the vascular cambium adds to the secondary xylem and phloem, and the cork cambium produces cork. 6 6 As the diameter of the stem continues to increase, the outermost tissues exterior to the cork cambium rupture and slough off from the stem. 7 Cork cambium re-forms in progressively deeper layers of the cortex. When none of the original cortex is left, the cork cambium develops from parenchyma cells in the secondary phloem. 8 Each cork cambium and the tissues it produces form a layer of periderm. 9 Bark 7 Cork 9 Bark consists of all tissues exterior to the vascular cambium. 8 Layers of periderm Epidermis Cortex Primary phloem Vascular cambium Primary xylem Pith Pith Primary xylem Vascular cambium Primary phloem Cortex Epidermis 2 Phloem ray 3 rowth G Xylem ray
Periderm (mainly cork cambia and cork) Primary phloem Secondary phloem Vascular cambium Secondary xylem Primary xylem Pith
Primary xylem Secondary xylem Vascular cambium Secondary phloem Cork Primary phloem 4 First cork cambium th Grow
Secondary xylem (two years of production) Vascular cambium Secondary phloem 5 Most recent cork cambium
Figure 35.18a
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Secondary Late wood Early wood xylem
Secondary phloem Vascular cambium
Cork cambium Cork
Periderm
(b) Transverse section of a three-yearold stem (LM) Xylem ray Bark 0.5 mm Figure 35.18b 0.5 mm
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Viewed in transverse section, the vascular cambium
Appears as a ring, with interspersed regions of dividing cells called fusiform initials and ray initials
Vascular cambium
CC
X C
C
P
(a) Types of cell division. An initial can divide
transversely to form two cambial initials (C) or radially to form an initial and either a xylem (X) or phloem (P) cell.
X
X C X C
C P
X C P
XX C P
P
(b) Accumulation of secondary growth. Although shown here
Figure 35.19a, b
as alternately adding xylem and phloem, a cambial initial usually produces much more xylem.
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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
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Growth ring
Vascular ray
Heartwood Secondary xylem
Sapwood
Vascular cambium Secondary phloem Bark Layers of periderm Figure 35.20
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Cork Cambia and the Production of Periderm The cork cambium
Gives rise to the secondary plant body's protective covering, or periderm
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Periderm
Consists of the cork cambium plus the layers of cork cells it produces
Bark
Consists of all the tissues external to the vascular cambium, including secondary phloem and periderm
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Concept 35.5: Growth, morphogenesis, and differentiation produce the plant body The three developmental processes of growth, morphogenesis, and cellular differentiation
Act in concert to transform the fertilized egg into a plant
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Molecular Biology: Revolutionizing the Study of Plants
New techniques and model systems
Are catalyzing explosive progress in our understanding of plants
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Arabidopsis
Is the first plant to have its entire genome sequenced
Unknown (36.6%) Cell organization and biogenesis (1.7%) DNA metabolism (1.8%) Carbohydrate metabolism (2.4%) Signal transduction (2.6%) Protein biosynthesis (2.7%) Electron transport (3%) Protein modification (3.7%) Protein metabolism (5.7%) Transcription (6.1%)
Other metabolism (6.6%) Other biological processes (18.6%)
Figure 35.21
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Transport (8.5%)
Growth: Cell Division and Cell Expansion By increasing cell number
Cell division in meristems increases the potential for growth
Cell expansion
Accounts for the actual increase in plant size
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The Plane and Symmetry of Cell Division The plane (direction) and symmetry of cell division
Are immensely important in determining plant form
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If the planes of division of cells are parallel to the plane of the first division
A single file of cells will be produced
Division in same plane Plane of cell division Division in three planes Cube forms Nucleus Single file of cells forms
(a) Cell divisions in the same plane produce a single file of cells, whereas cell divisions in three planes give rise to a cube.
Figure 35.22a
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If the planes of division vary randomly
Asymmetrical cell division occurs
Asymmetrical cell division Unspecialized epidermal cell Unspecialized epidermal cell Guard cell "mother cell" Unspecialized epidermal cell
Developing guard cells
(b) An asymmetrical cell division precedes the development of epidermal guard cells, the cells that border stomata (see Figure 35.17).
Figure 35.22b
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The plane in which a cell divides
Is determined during late interphase
Microtubules in the cytoplasm
Become concentrated into a ring called the preprophase band
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Preprophase bands of microtubules
10 m
Nuclei Cell plates
Figure 35.23
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Orientation of Cell Expansion Plant cells
Rarely expand equally in all directions
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The orientation of the cytoskeleton
Affects the direction of cell elongation by controlling the orientation of cellulose microfibrils within the cell wall
Cellulose microfibrils
Vacuoles Nucleus 5 m
Figure 35.24
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Microtubules and Plant Growth Studies of fass mutants of Arabidopsis
Have confirmed the importance of cytoplasmic microtubules in cell division and expansion
(b) fass seedling
Figure 35.25ac (a) Wild-type seedling (c) Mature fass mutant
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Morphogenesis and Pattern Formation Pattern formation
Is the development of specific structures in specific locations Is determined by positional information in the form of signals that indicate to each cell its location
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Polarity
Is one type of positional information
In the gnom mutant of Arabidopsis
The establishment of polarity is defective
Figure 35.26
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Morphogenesis in plants, as in other multicellular organisms
Is often under the control of homeotic genes
Figure 35.27
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Gene Expression and Control of Cellular Differentiation
In cellular differentiation
Cells of a developing organism synthesize different proteins and diverge in structure and function even though they have a common genome
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Cellular differentiation
To a large extent depends on positional information Is affected by homeotic genes
When epidermal cells border a single cortical cell, the homeotic gene GLABRA-2 is selectively expressed, and these cells will remain hairless. (The blue color in this light micrograph indicates cells in which GLABRA-2 is expressed.) Cortical cells Here an epidermal cell borders two cortical cells. GLABRA-2 is not expressed, and the cell will develop a root hair.
20 m
The ring of cells external to the epidermal layer is composed of root cap cells that will be sloughed off as the root hairs start to differentiate.
Figure 35.28
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Location and a Cell's Developmental Fate A cell's position in a developing organ
Determines its pathway of differentiation
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Shifts in Development: Phase Changes Plants pass through developmental phases, called phase changes
Developing from a juvenile phase to an adult vegetative phase to an adult reproductive phase
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The most obvious morphological changes
Typically occur in leaf size and shape
Leaves produced by adult phase of apical meristem
Leaves produced by juvenile phase of apical meristem Figure 35.29
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Genetic Control of Flowering Flower formation
Involves a phase change from vegetative growth to reproductive growth Is triggered by a combination of environmental cues and internal signals
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The transition from vegetative growth to flowering
Is associated with the switching-on of floral meristem identity genes
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Plant biologists have identified several organ identity genes
That regulate the development of floral pattern
Ca St Pe Pe Se
Se
(a) Normal Arabidopsis flower. Arabidopsis normally has four whorls of flower parts: sepals (Se), petals (Pe), stamens (St), and carpels (Ca).
Pe Pe
Figure
(b) Abnormal Arabidopsis flower. Reseachers have identified several mutations of organ identity genes that cause abnormal flowers to develop. This flower has an extra set of petals in place of stamens and an internal flower where normal 35.30a, bplants have carpels.
Se
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The ABC model of flower formation
Identifies how floral organ identity genes direct the formation of the four types of floral organs
Sepals Petals Stamens A B Carpels C (a) A schematic diagram of the ABC hypothesis. Studies of plant mutations reveal that three classes of organ identity genes are responsible for the spatial pattern of floral parts. These genes are designated A, B, and C in this schematic diagram of a floral meristem in transverse view. These genes regulate expression of other genes responsible for development of sepals, petals, stamens, and carpels. Sepals develop from the meristematic region where only A genes are active. Petals develop where both A and B genes are expressed. Stamens arise where B and C genes are active. Carpels arise where only C genes are expressed.
A+B B+C gene gene activity activity
C gene activity
Figure 35.31a
A gene activity
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An understanding of mutants of the organ identity genes
Depicts how this model accounts for floral phenotypes
Active genes: Whorls: BB B B A A CCCC AA BB BB CCCCCCCC A A CCCCA A AA AA ABBAABBA
Carpel Stamen Petal
Sepal Wild type Mutant lacking A Mutant lacking B Mutant lacking C
(b) Side view of organ identity mutant flowers. Combining the model shown in part (a) with the rule that if A gene or C gene activity is
missing, the other activity spreads through all four whorls, we can explain the phenotypes of mutants lacking a functional A, B, or C organ identity gene.
Figure 35.31b
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