Biology, Seventh Edition
Solomon • Berg • Martin
Chapter 33
Stems and Plant
Transport
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• External features of a woody
twig
• Buds (undeveloped embryonic
shoots)
–Terminal bud at tip of stem
–Axillary buds (lateral buds) in leaf axils
–Dormant bud covered and protected by
bud scales which leave bud scale scars
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• External features of a woody
twig, cont.
• Node is area on a stem where
leaf is attached
• Internode is region between two
successive nodes
• Leaf scar remains when leaf is
detached from stem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• External features of a woody
twig, cont.
• Bundle scars are areas within a
leaf scar where vascular tissue
extended from stem to leaf
• Lenticels are sites of looselyarranged cells allowing oxygen to
diffuse into interior of woody stem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
External
structure of a
woody twig
in its winter
condition
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Herbaceous stems possess
• Epidermis
• Vascular tissue
• Either
–Ground tissue or
–Cortex and pith
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Epidermis
• Protective layer covered by a
water-conserving cuticle
• Stomata permit gas exchange
• Xylem conducts water and
dissolved nutrient minerals
• Phloem conducts dissolved sugar
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Epidermis, cont.
• Storage functions carried out by
–Cortex
–Pith
–Ground tissue
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• All herbaceous stems have same
basic tissues, but arrangement
thereof varies
• Herbaceous dicot stems have circular
arrangement of vascular bundles and
distinct cortex and pith
• Monocot stems have vascular
bundles scattered in ground tissue
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Cross section
of a
Helianthus
annuus stem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Closeup of two vascular bundles
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Cross section of a Zea mays stem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Closeup of a vascular bundle
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Lateral meristems
• Vascular cambium produces
–Secondary xylem (wood)
–Secondary phloem (inner bark)
• Cork cambium produces
periderm
–Cork parenchyma
–Cork cells
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Periderm, cont.
• Cork parenchyma functions
primarily for storage in a woody
stem
• Cork cells are the functional
replacement for epidermis in a
woody stem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Secondary growth occurs in
• Some flowering plants (woody
dicots)
• All cone-bearing gymnosperms
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Transition from primary growth to
secondary growth in a woody stem
• Vascular cambium, which develops
between primary xylem and primary
phloem divides in two directions,
forming
–Secondary xylem (to the inside)
–Secondary phloem (to the outside)
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Development of
secondary
xylem and
secondary
phloem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Transition from primary growth
to secondary growth in a woody
stem, cont.
• As secondary growth proceeds,
in the original vascular bundles,
two elements become separated
–Primary xylem
–Primary phloem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Onset of secondary growth
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Beginning of division of vascular cambium
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
A young woody stem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Pathway of water movement
• Water and dissolved nutrient
minerals move from soil into
–Epidermis
–Cortex, etc.
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Pathway of water movement,
cont.
• Once in root xylem, water and
dissolved minerals move upward from
–Root xylem to stem xylem
–Stem xylem to leaf xylem
• Most water entering leaf exits leaf
veins and passes into atmosphere
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Water potential is a measure of
the free energy of water
• Pure water has a water potential
of
–0 megapascals
• Water with dissolved solutes has
–Negative water potential
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Water potential, cont.
• Water moves from an area of
higher (less negative) water
potential to an area of lower
(more negative) water potential
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• The tension-cohesion model
explains the rise of water and
dissolved nutrient minerals in
xylem
• Transpiration causes tension at
top of plant
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Transpiration, cont.
• Tension at top of plant results
from water potential gradient
ranging
–From slightly negative water
potentials in soil and roots
–To very negative water potentials in
atmosphere
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Transpiration, cont.
• Column of water pulled up
through plant remains unbroken
due to properties of water
–Cohesive
–Adhesive
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
The
tensioncohesion
model
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Root pressure
• Caused by movement of water into
roots from soil as a result of active
absorption of nutrient mineral ions
from soil
• Helps explain rise of water in smaller
plants (especially when soil is wet)
• Pushes water up through xylem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Pathway of sugar translocation
• Dissolved sugar is translocated
up or down in phloem
–From a source (area of excess
sugar, usually a leaf)
–To a sink (area of storage or of
sugar use)
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Pathway of sugar translocation,
cont.
• Area of storage or of sugar use
–Roots
–Apical meristems (fruits and seeds)
• Sucrose is predominant sugar
translocated in phloem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Aphids used to study translocation in plants
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Pressure-flow hypothesis
explains the movement of
materials in phloem
• Companion cells actively load
sugar into sieve tubes at source
• ATP required for this process
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Pressure-flow hypothesis, cont.
• ATP supplies energy to pump
protons out of sieve tube
elements
• Proton gradient drives uptake of
sugar by cotransport of protons
back into sieve tube elements
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Pressure-flow hypothesis, cont.
• Sugar therefore accumulates in
sieve tube element
• This causes movement of water
into sieve tubes by osmosis
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Pressure-flow hypothesis, cont.
• Companion cells unload sugar
from sieve tubes at sink
–Actively (requiring ATP)
–Passively (not requiring ATP)
• As a result, water leaves sieve
tubes by osmosis
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Pressure-flow hypothesis, cont.
• Unloading of sugar causes decrease
in turgor pressure inside sieve tubes
• Flow of materials between source and
sink is driven by turgar pressure
gradient produced by
–Water entering phloem at source
–Water leaving phloem at sink
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
The pressure-flow hypothesis
(diagram divided in two)
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