Plant Form
Chapter 36
2
Plant Body Organization
A vascular plant consists of:
1. Root system, which is underground
-Anchors the plant, and is used to absorb water and minerals
2. Shoot system, which is above ground
-Consists of supporting stems, photosynthetic leaves and reproductive flowers
Each has an apex that extends growth
3
Shoot apex
Flower
Stipule
AxillarybudInternode
NodeVascular system
Primary root
Lateral root
Root apex
Root
Shoot
Petiole
Vein
BladeLeaflet Leaf
Tendril
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4
Plant Body Organization
Plant cell walls consist of cellulose
-Primary cell wall
-Found in all cells
-Cellulose fibers parallel to microtubules
-Secondary cell wall
-Found in some cells
-Additional layers of cellulose and lignin
-Increase mechanical strength of wall
5
a. b.
Cellulose fiberCell membrane
Cytosol
Microtubule
Cellulose-formingrosettes
Cytosol
Cell membrane
Parallelcellulosefibers
Primary cell wall
Time
Primary cell wallremains outside as inner layersare laid down
Secondarycell wall 1
Secondarycell wall 2
Plant Body Organization
6
Plant Body Organization
Roots, shoots and leaves contain three basic tissue systems:-Dermal tissue – For protection
-Wax and bark-Ground tissue – For storage, photosynthesis and secretion-Vascular tissue – For conduction
-Xylem – Water and dissolved minerals
-Phloem – Nutrient-containing solution
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Plant Body Organization
Meristems are clumps of small cells with dense cytoplasm and large nuclei
They act as stem cells do in animals
-One cell divides producing a differentiating cell and another that remains meristematic
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Meristem cell
Meristem cell
Differentiated cell
Cell division
Meristem cell
Cell division
Meristem cell Differentiated cell
Cell division
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Differentiated cell
9
Plant Body Organization
Apical meristems are located at the tips of stems and roots
-Give rise to primary tissues which are collectively called the primary plant body
-Three primary meristems
-Protoderm Epidermis
-Procambium 1o vascular tissue
-Ground meristem Ground tissue
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Plant Body Organization
11
Plant Body Organization
Lateral meristems are found in plants that exhibit secondary growth
-Give rise to secondary tissues which are collectively called the secondary plant body
-Woody plants have two types
-Cork cambium Outer bark
-Vascular cambium 2o vascular tissue
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Apicalgrowth
Ground meristem
Procambium
Primaryxylem
Primaryphloem
Lateralgrowth
Secondaryxylem
Secondaryphloem
Primaryphloem
Primaryphloem
Primaryxylem
Primaryxylem
Vascular cambium
Cork cambium
Lateralgrowth
Vascular cambium
Secondaryxylem
Secondaryphloem
Apicalgrowth
Primaryxylem
Primaryphloem
Ground meristem
Procambium
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Plant Tissues
As mentioned earlier, plants contain three main types of tissue
-Dermal
-Ground
-Vascular
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Dermal Tissue
Forms the epidermis, which is usually one cell layer thick
Covered with a fatty cutin layer constituting the cuticle
Contains special cells, including guard cells, trichomes and root hairs
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Dermal Tissue
Guard cells are paired sausage-shaped cells
-Flank a stoma, which is the passageway for oxygen and carbon dioxide
Guard cell formation is the result of an asymmetrical cell division that produces:
-A guard cell
-A subsidiary cell
-Aids in stoma opening and closing
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Dermal Tissue
17
Dermal Tissue
Trichomes are cellular or multicellular hairlike outgrowths of the epidermis
-Keep leaf surfaces cool and reduce evaporation by covering stomatal openings
-Some are glandular, secreting substances that deter herbivory
Trichome patterning is under genetic control
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Dermal Tissue
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Dermal Tissue
Roots hairs are tubular extensions of individual epidermal cells
-Greatly increase the root’s surface area and efficiency of absorption
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Ground Tissue
Consist of three types of cells
-Parenchyma
-Collenchyma
-Sclerenchyma
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Ground Tissue
Parenchyma cells are the most common type of plant cell -May live for many years, functioning in storage, photosynthesis and secretion-Some contain chloroplasts and are called chlorenchyma
Collenchyma cells provide support for plant organs, allowing bending but not breaking-Have living protoplasts and may live for many years
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Ground Tissue
Sclerenchyma cells have tough thick walls
-Lack living walls at maturity
-Two general types
-Fibers: Long, slender cells that are usually grouped in strands
-Sclereids: Variable shape; branched; may occur singly or in groups
-Both strengthen tissues
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Ground Tissue
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Vascular Tissue
Xylem
-Constitutes the main water- and mineral-conducting tissue
-Vessels: Continuous tubes of dead cylindrical cells arranged end-to-end
-Tracheids: Dead cells that taper at the end and overlap one another
-Vessels are shorter & wider than tracheids
-And conduct water more efficiently
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Vascular Tissue
Xylem
-Also conducts inorganic ions such as nitrates, and supports the plant body
-Typically includes parenchyma cells in horizontal rows called rays
-Function in lateral conduction and food storage
Note: The diffusion of water vapor from a plant is termed transpiration
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Vascular Tissue
Phloem -Constitutes the main food-conducting tissue in vascular plants-Contains two types of elongated cells: sieve cells and sieve tube members
-Living cells that contain clusters of pores called sieve areas or sieve plates-Sieve-tube members are more
specialized -Associated with companion cells
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29
Roots
Roots have a simpler pattern of organization and development than stems
Four regions are commonly recognized:
-Root cap
-Zone of cell division
-Zone of elongation
-Zone of maturation
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Zone ofmaturation
Zone ofelongation
Zone ofcell division
Epidermis
Ground meristemProcambium
Columella cells
Protoderm
Endodermis
GroundtissueVasculartissue
Quiescentcenter
Root incross-section
Apical meristem Root cap
dermal tissueground tissuevascular tissue
Root hair
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31
Roots
Root cap
-Contains two types of cells that are formed continuously by the root apical meristem
-Columella cells: Inner
-Root cap cells: Outer and lateral
-Functions mainly in protection of the delicate tissues behind it
-Also in the perception of gravity
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Roots
Zone of cell division
-Contains mostly cuboidal cells, with small vacuoles and large central nuclei
-Derived from rapid divisions of the root apical meristem
-Quiescent center cells divide very infrequently
-Apical meristem daughter cells soon subdivide into the three primary tissues
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Roots
Zone of cell division
-Patterning of these tissues begins in this zone
-WEREWOLF (WER) gene
-Suppresses root hair development
-SCARECROW (SCR) gene
-Necessary for differentiation of endodermal and ground cells
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a.
WER (wild type) wer (mutant)
Root tip Epidermal cell
WER
NoWER
NoWER
NoWER
Nonhair
Hair will develop inzone of maturation
Hair will develop inzone of maturation
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a.
SCR (wild type) scr (mutant)
Root tip
Endodermalcell
SCR
Cell withground andendodermaltraitsAsymmetrical
division
Groundcell
Groundmeristem
cell
2 layers of cells
Root tip
Groundmeristem
cell
SCR
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Roots
Zone of elongation
-Roots lengthen because cells become several times longer than wide
-No further increase occurs above this zone
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Roots
Zone of maturation
-The elongated cells become differentiated into specific cell types
-Epidermal cells: Have very thin cuticle
-Include root hair and nonhair cells
-Cortex: Interior to the epidermis
-Parenchyma cells used for storage
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Roots
Zone of maturation
-Endodermis: Single-layered cylinder
-Impregnated with bands of suberin called the Casparian strips
-Stele: All tissues interior to endodermis
-Pericycle: Multiple-layered cylinder
-Gives rise to lateral (branch) roots or the two lateral meristems
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Casparian strip
Endodermal cellH2O
Xylem
Phloem
Cortex
Pericycle
H2O
Roots
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41
Zygote Embryo
Shoot apical meristem
Root apical meristem
Cork cambium
Vascular cambium
Leaf primordia
Bud primordia
Shoot elongation
Outer barkPhloemXylem
Inner bark
Wood
Bark
Bark
Leaves
Lateral shoots
Cork cambium
Vascular cambium
Pericycle
Phloem
Xylem
Lateral roots
Root elongation
Outer bark
Inner bark
Wood
Plant Tissue Differentiation
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Modified Roots
Most plants produce either/or:
-Taproot system: Single large root with small branch roots
-Fibrous root system: Many small roots of similar diameter
Some plants, however, produce modified roots with specific functions
-Adventitious roots arise from any place other than the plant’s root
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Modified Roots
Prop roots: Keep the plant upright
Aerial roots: Obtain water from the air
Pneumatophores: Facilitate oxygen uptake
Contractile roots: Pull plant deeper into soil
Parasitic roots: Penetrate host plants
Food storage roots: Store carbohydrates
Water storage roots: Weigh 50 or more kg
Buttress roots: Provide considerable stability
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Prop roots Aerial roots
Pneumatophores Water storage roots Buttress roots
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Stems
Like roots, stems contain the three types of plant tissue
-Also undergo growth from cell division in apical and lateral stems
Shoot apical meristem initiates stem tissue and intermittently produces primordia
-Develop into leaves, other shoots and even flowers
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Stems
47
Stems
Leaves may be arranged in one of three ways
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Stems
The spiral (alternate) arrangement is the most common
-Sequential leaves tend to be placed 137.5o apart
-This is termed phyllotaxy
-May optimize the exposure of leaves to the sun
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External Stem Structure
Node = Point of attachment of leaf to stem Internode = Area of stem between two nodesBlade = Flattened part of leafPetiole = Stalk of leafAxil = Angle between petiole/blade and stemAxillary bud = Develops into branches with
leaves or may form flowersTerminal bud = Extends the shoot system
during the growing season
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Internal Stem Structure
Monocot vascular bundles are usually scattered throughout ground tissue system
Eudicot vascular tissue is arranged in a ring with internal ground tissue (pith) and external ground tissue (cortex)
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Internal Stem Structure
Vascular tissue arrangement is directly related to the stem’s ability for secondary growth
-In eudicots, a vascular cambium develops between the primary xylem and phloem
-Connects the ring of primary vascular bundles
-In monocots, there is no vascular cambium
-Therefore, no secondary growth
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a.
b.
c.
Primary xylemSecondary xylem
Primary phloemSecondary phloem
Vascular cambium(lateral meristem)
Vascular cambium(lateral meristem)
Primary xylemSecondary xylem
Primary phloemSecondary phloem
Annual growth layers
EpidermisPrimary xylem Primary phloem
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
55
Internal Stem Structure
Rings in the stump of a tree reveal annual patterns of vascular cambium growth
-Cell size depends on growth conditions
In woody eudicots and gymnosperms, the cork cambium arises in the outer cortex
-Produces boxlike cork cells on outside and parenchyma-like phelloderm cells on inside
-Collectively called the periderm
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Internal Stem Structure
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Internal Stem Structure
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Internal Stem Structure
Cork tissue cells get impregnated with suberin shortly after they are formed
-They then die and constitute the outer bark
The cork cambium also produces unsuberized cells called lenticels
-Permit gas exchange to continue
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Internal Stem Structure
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Modified Stems
Bulbs = Swollen underground stems, consisting of fleshy leaves
Corms = Superficially resemble bulbs, but have no fleshy leaves
Rhizomes = Horizontal underground stems, with adventitious roots
Runners and stolons = Horizontal stems with long internodes that grow along the surface of the ground
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Modified Stems
Tubers = Swollen tips of rhizomes that contain carbohydrates
Tendrils = Twine around supports and aid in climbing
Cladophylls = Flattened photosynthetic stems resembling leaves
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Modified Stems
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Leaves
Leaves are the main site of photosynthesis
-They are determinate structures whose growth stops at maturity
Exist in two morphologies
-Microphyll = Have one vein which does not extend the full length of the leaf
-Found mainly in the phylum Lycophyta
-Megaphylls = Have several to many veins
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Leaves
The flattening of the leaf blade reflects a shift from radial to dorsal-ventral symmetry
-It increases the photosynthetic surface
The mechanism of this shift is becoming clearer through the analysis of mutants that lack distinct tops and bottoms
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PHABULOSA
KANADI
PHAVOLUTAYABBY
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Leaves
Veins consist of both xylem and phloem and are distributed throughout the leaf blades
-Monocot leaves have parallel veins
-Eudicot leaves have netted or reticulate veins
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Leaves
Leaf blades come in a variety of forms
-Simple leaves contain undivided blades
-May have teeth, indentations or lobes
-Compound leaves have blades that are divided into leaflets
-Pinnate = Leaflets in pairs along an axis
-Palmate = Leaflets radiate out from a common point
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Leaves
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Leaves
The leaf’s surface is covered by transparent epidermal cells, most having no chloroplasts
Epidermis has a waxy cuticle
-The lower epidermis contains numerous mouth-shaped stomata flanked by guard cells
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Leaves
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Leaves
The mesophyll is the tissue between the upper and lower epidermis
-Most eudicot leaves have two types
-Palisade mesophyll = Usually two rows of tightly packed chlorenchyma cells
-Spongy mesophyll = Loosely arranged cells with many air spaces in between
-Monocot leaves mesophyll is usually not differentiated into palisade/spongy layers
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Leaves
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Modified Leaves
Floral leaves (bracts) = Surround true flowers and behave as showy petals
Spines = Reduce water loss and may deter predators
Reproductive leaves = Plantlets capable of growing independently into full-sized plant
Window leaves = Succulent, cone-shaped leaves that allow photosynthesis underground
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Modified Leaves
Shade leaves = Larger in surface area but with less mesophyll than sun-lit leaves
Insectivorous leaves = Trap insects
-Pitcher plants have cone-shaped leaves that accumulate rainwater
-Sundews have glands that secrete sticky mucilage
-Venus flytrap have hinged leaves that snap shut