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Plant Form
Chapter 36
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
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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
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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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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 à Epidermis
-Procambium à 1o vascular tissue
-Ground meristem à Ground tissue
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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 à Outer bark
-Vascular cambium à 2o vascular tissue
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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
Roots hairs are tubular extensions of
individual epidermal cells
-Greatly increase
the root’s surface
area and efficiency
of absorption
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Vascular Tissue
Xylem
-Constitutes the main water- and mineralconducting 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 11
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 13
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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Roots
Root cap
-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
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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
-Pericycle: Multiple-layered cylinder
-Gives rise to lateral (branch) roots
or the two lateral meristems
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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
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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
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 nodes
Blade = Flattened part of leaf
Petiole = Stalk of leaf
Axil = Angle between petiole/blade and stem
Axillary bud = Develops into branches with
leaves or may form flowers
Terminal 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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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
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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
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
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
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 43
Leaves
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Leaves (Cont.)
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