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

2. Concept 35.3: Primary growth lengthens roots and shoots
Primary growth produces the parts of the root and shoot systems produced by apical meristems
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3. 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
Video: Root Growth in a Radish Seed (Time Lapse)
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4. 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

5. 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
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6. Core of parenchyma cells
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

7. 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
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8. Lateral roots arise from within the pericycle, the outermost cell layer in the vascular cylinder
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9. 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

10. 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

11. 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

12. 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 along the sides of the apical meristem
Axillary buds develop from meristematic cells left at the bases of leaf primordia
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13. Developing vascular strand
Figure 35.16
Shoot apical meristem
Leaf primordia
Young leaf
Developing vascular strand
Figure The shoot tip.
Axillary bud meristems
0.25 mm

14. Tissue Organization of Stems
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
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15. 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)

16. In most monocot stems, the vascular bundles are scattered throughout the ground tissue, rather than forming a ring
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17. 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
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18. 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
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19. 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
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20. 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)

21. 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
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22. 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

23. The Vascular Cambium and Secondary Vascular Tissue
The vascular cambium is a cylinder of meristematic cells one cell layer thick
It develops from undifferentiated parenchyma cells
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24. 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
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25. 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

26. 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
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27. 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
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28. 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
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29. RESULTS 2 1.5 Ring-width indexes 1 0.5 1600 1700 1800 1900 2000 Year
Figure 35.21
RESULTS 2
1.5
Ring-width indexes 1
0.5
1600
1700
1800
1900
2000
Figure Research Method: Using Dendrochronology to Study Climate
Year

30. Older secondary phloem sloughs off and does not accumulate
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
Older secondary phloem sloughs off and does not accumulate
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31. 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

32. Figure 35.23
Figure Is this tree living or dead?

33. 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
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34. 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
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35. 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
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36. 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
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37. Figure 35.24
Figure Developmental plasticity in the aquatic plant Cabomba caroliniana.

38. 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
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39. 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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40. The Plane and Symmetry of Cell Division
New cell walls form in a plane (direction) perpendicular to the main axis of cell expansion
The plane in which a cell divides is determined during late interphase
Microtubules become concentrated into a ring called the preprophase band that predicts the future plane of cell division
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41. Preprophase band 7 m Figure 35.25
Figure The preprophase band and the plane of cell division.
7 m

42. Leaf growth results from a combination of transverse and longitudinal cell divisions
It 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
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43. Leaf epidermal cells of wild-type maize
Figure 35.26
Figure Cell division patterns in wild-type versus mutant maize plants.
30 m
Leaf epidermal cells of wild-type maize
Leaf epidermal cells of tangled-1 maize mutant

44. Asymmetrical cell division signals a key event in development
The 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
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45. Asymmetrical cell division
Figure 35.27
Asymmetrical cell division
Guard cell “mother cell”
Unspecialized epidermal cell
Figure Asymmetrical cell division and stomatal development.
Developing guard cells

46. Asymmetrical cell divisions play a role in establishing polarity
Polarity 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
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47. 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
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48. Figure 35.28
Figure Establishment of axial polarity.

49. 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
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50. Cellulose microfibrils
Figure 35.29
Cellulose microfibrils
Elongation
Nucleus
Vacuoles
Figure The orientation of plant cell expansion.
5 m

51. 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
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52. 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
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53. Shifts in Development: Phase Changes
Plants 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
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54. Leaves produced by adult phase of apical meristem
Figure 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

55. 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
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56. 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
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57. (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)

58. 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
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59. 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

60. 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

61. (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