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Plant Form and Function
Chapter 17
Plants
• Herbaceous (nonwoody)
• In temperate climates, aerial parts die back
• Woody
• In temperate climates, aerial parts persist
The Plant Body
Functions of:
Roots
Stem
Leaves
• Flowering plants can be divided into two
groups:
– Monocots: grasses, lilies, palms, and orchids
– Dicots: deciduous trees, bushes, and many
garden flowers
Flowers Leaves Roots Seeds Stems
Flower parts are in
threes or multiples
of three
Flower parts are in
fours or fives or multiples
of four or five
Leaves have smooth
edges, often narrow,
with parallel veins
Leaves are palmate
(handlike) or oval
with netlike veins
Vascular bundles
are scattered
throughout the stem
Monocots have a
fibrous root system
The seed has one
cotyledon (seed leaf)
The seed has
two cotyledons
(seed leaves)
Dicots have a
taproot system
Vascular bundles
are arranged in a
ring around the stem
Monocots
Dicots
embryo
cotyledon
embryo
cotyledons
Fig. 17-2
Tissue Systems
• Integrated throughout the plant body
• provide continuity from organ to organ
• Plant body has 3 tissue systems
1. ground
2. vascular
3. dermal
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Ground Tissue System
• Consists of 3 tissues, many functions
• parenchyma tissue
• collenchyma tissue
• sclerenchyma tissue
Ground Tissue: Parenchyma Tissue
• Composed of living parenchyma cells
• with thin primary cell walls
• Functions
• photosynthesis
• storage
• Secretion
Fig. 32-4a, p. 706
Vacuole
Intercellular
space
Nucleus
Cytoplasm
Cell wall Parenchyma cells
Ground Tissue: Collenchyma Tissue
• Consists of collenchyma cells
• with unevenly thickened primary cell walls
• Provides flexible structural support
• Strings of celery
Fig. 32-4b, p. 706
Thickened corner
of cell wall
Nucleus Cytoplasm Vacuole
Collenchyma cells
Ground Tissue: Sclerenchyma Tissue
• Composed of sclerenchyma cells
• sclereids or fibers
Thick cell walls
• Sclerenchyma cells often dead at maturity
• provide structural support
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Fig. 32-4c, p. 706
Lumen
Cell wall
Sclerenchyma cells
Vascular Tissue System
• Conducts materials throughout plant body
• Provides strength and support
Vascular Tissue: Xylem
• Complex tissue, conducts water and dissolved
minerals
• 2 types of cells of xylem
• tracheids
• vessel elements
Fig. 32-5ab, p. 708
Xylem
End wall with
perforations
Pits
Cell wall
Lumen
(a) Tracheid. (b) Vessel
element
Vascular Tissue: Phloem
• Complex tissue, conducts sugar in solution
• 2 types of cells of phloem
1. sieve tube elements
2. assisted by companion cells
Fig. 32-5cd, p. 708
Phloem
Sieve plate
with pores
Sieve tube
element
Phloem
parenchyma
cells
Lateral sieve
area
Plasmodesma
Companion cell
(c) Sieve tube
element.
(d) Phloem
tissue.
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Dermal Tissue System
• Outer protective covering of plant body
• Epidermis:
• complex tissue
• covers herbaceous plant body
• Periderm:
• complex tissue
• covers woody parts of plant body
Dermal Tissue: Epidermis
• Waxy cuticle reduces water loss
• secreted by epidermis covering aerial
parts
• Stomata permit gas exchange
• between shoot system and atmosphere
• outgrowths or hairs
• many sizes, shapes, and functions
Growth in Plants
• Localized in specific regions (meristems)
• Involves 3 processes:
- cell division
- cell elongation
- cell differentiation
• Primary Growth vs. Secondary Growth
Primary Growth • Increase in stem or root length
• occurs in all plants
• Apical meristems
• at tips of roots and shoots
• within buds of stems
• Responsible for primary growth
Fig. 32-7, p. 710
Root hairs
Area of cell
maturation
Area of cell elongation
Root
cap Apical meristem
(Area of cell division)
Herbaceous Stems
• Epidermis: protective layer covered by a water-
conserving cuticle
• Xylem: conducts water and dissolved minerals
• Phloem: conducts dissolved sugar
• Cortex, pith, and ground tissue:
– function primarily for storage & support
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Basic Tissues in Herbaceous Stems
• Herbaceous eudicot stems
– vascular bundles arranged in a circle (in cross
section)
– distinct cortex and pith
• Monocot stems
– vascular bundles scattered in ground tissue Pith
Cortex
Ground
tissue
Herbaceous Stems
Herbaceous Dicot Stem Monocot Stem
Fig. 34-3a, p. 734
Ground tissue
Vascular
bundles
Epidermis
500 µm
(meristematic)
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Fig. 35-3b, p. 751
Cortex cells
Endodermis
cell
Pericycle cell
Phloem cell
Xylem vessel
elements
25 µm dicot root
Monocot Root
Apical Primary Lateral Secondary
meristems tissue meristems tissues
Meristematic
cells
Primary xylem
Primary phloem
Cortex
Pith
Epidermis
Vascular
cambium
Cork
cambium
Secondary
xylem (wood)
Secondary
phloem
(inner bark)
Periderm
Secondary Growth
• Increase in stem or root girth (thickness)
• Woody plants only!
• Mitosis of meristematic at leteral
meristems (not apical meristems)
• throughout length of older stems and
roots
• Two Lateral Meristems responsible for
secondary growth
1. vascular cambium
2. cork cambium
Fig. 32-9, p. 712
Cork cambium: outer = cork cells; inner = cork parenchyma
cork cells & parenchyma = PERIDERM
Inner bark (secondary phloem)
Bark
Wood
(secondary xylem)
Vascular cambium
Secondary Growth
• Production of secondary tissues, wood, bark
– occurs in some flowering plants (woody
dicots) and all cone-bearing trees
• Vascular cambium divides in two directions
– secondary xylem (to the inside)
– secondary phloem (to the outside)
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Vascular
Cambium
Fig. 34-4a, p. 735
Primary
xylem Epidermis
Cortex Primary
phloem
Vascular
cambium
Pith
Fig. 34-4b, p. 735
Remnant
of cortex
Remnant of
epidermis Remnant
of primary
phloem Secondary phloem
(inner bark)
Secondary xylem
(wood)
Periderm
(outer bark)
Remnant of
primary xylem
Remnant of
pith
Vascular
cambium
Fig. 34-4c, p. 735
Secondary xylem
(wood)
Periderm (outer bark; remnants of primary phloem, cortex and epidermis are gradually crushed or turn apart and sloughed off)
Secondary phloem
(inner bark)
Remnant of
primary xylem
Remnant of
pith Vascular
cambium
Fig. 34-5, p. 736
1X2X3X4X 2P1P
1X2X3X 2P1P
1X2X 2P1P
Secondary xylem Secondary phloem
1X 2X 1P
Second division of vascular cambium forms a phloem cell.
1X 1P
Division of vascular cambium forms two cells, one xylem cell and one vascular cambium cell.
1X
Vascular cambium cell when secondary growth begins.
Vascular cambium cell
Tim
e
Cork Cambium
• Lateral meristem that produces “bark”
– cork parenchyma and cork cells
• Cork cells (cork)
– to outside of cork cambium
• Cork parenchyma
– to inside of cork cambium
– primarily for storage in a woody stem
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Fig. 34-6, p. 737
Primary xylem Pith
Annual ring of
secondary xylem
Secondary
xylem (wood)
Vascular
cambium
Secondary
phloem
Periderm and
remnants of primary
phloem, cortex, and
epidermis
Expanded
phloem ray
Xylem ray
0.5 mm
Fig. 34-8, p. 739
Heartwood
Sapwood
Fig. 34-9, p. 739
Cross section of
3-year-old Tilia
stem
Secondary
phloem
Vascular cambium
Summerwood
Annual
ring of
xylem Springwood
Summerwood of
preceding year 100 µm
Fig. 33-3, p. 718
Palisade
mesophyll
Vein
(vascular
bundle) Cuticle
Spongy
mesophyll
Upper
epidermis
Bundle
sheath
Xylem
Phloem Stoma
Airspace
Lower
epidermis Stoma
Guard cells
Fig. 33-7a, p. 722
Open Closed
Guard
cells Subsidiary
cells
Stoma
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Fig. 34-10, p. 740
Most water that plant
absorbs is transpired
into atmosphere.
Sugar molecules from photosynthesis are transported in phloem throughout plant, including into roots.
Once inside roots, water and minerals are transported upward in xylem to stems, leaves, flowers, fruits, and seeds.
Roots obtain water
and dissolved
minerals from soil.
Stepped Art
Transport Water Movement
• Water and dissolved minerals move from soil into
root tissues (epidermis, cortex)
• Water and minerals move upward, from root xylem to
stem xylem to leaf xylem
• Water entering leaf exits leaf veins and passes into
atmosphere (Transpiration)
Tension–Cohesion Model
• Explains rise of water
– even in the tallest plants!
• Transpiration
– evaporative pull causes tension at top of plant
• Column of water pulled up through the plant remains
unbroken
– due to cohesive (together) and adhesive (others)
properties of water
Sugar Translocation
• Dissolved sugar is moved upward or
downward in phloem
– from source area of excess sugar (usually
a leaf)
– to a sink (area of storage or sugar use:
roots, apical meristems, fruits, seeds)
• Sucrose is predominant sugar transported in
phloem
Pressure–Flow Hypothesis
• Explains movement of materials in phloem
• Companion cells actively load sugar into
sieve tubes at source
– requires ATP
• ATP energy pumps protons out of sieve tube
elements
ATP
Source
Sink
Pressure-flow theory
Sucrose loaded and
unloaded requires ATP
Water moves osmotically