3/12/2012
Bud scale
Terminal
bud
One year's
growth
Stems and Transport in
Vascular Plants
Terminal bud
scale scars
Node
Internode
Axillary bud
Leaf scar
Node
Lenticels
Chapter 34
Terminal bud
scale scars
Bundle scars
A Woody Twig
Fig. 34-1, p. 732
Herbaceous Stems
• Epidermis: protective layer covered by a
water-conserving cuticle
– Stomata: permit gas exchange
• Vascular Tissue:
– Xylem: conducts water and dissolved
minerals
– Phloem: conducts dissolved sugar
• Ground tissue:
– Pith and Cortex
– function primarily for storage
Herbaceous Stems
Basic Tissues in Herbaceous
Stems
• Herbaceous eudicot (dicot) stems
– vascular bundles arranged in a circle (in
cross section)
– distinct cortex and pith
• Monocot stems
– vascular bundles scattered in ground
tissue
Herbaceous Dicot Stem
Pith
Cortex
Ground
tissue
1
3/12/2012
Epidermis
Monocot Stem
Cortex
Phloem
Vascular
cambium
Xylem
Vascular bundle
Phloem
fiber cap
Vessel
element
Pith
250 µm
Fig. 34-2b, p. 733
Phloem
Sieve
tube
element
Companion
cell
Ground tissue
Vascular
bundles
Xylem
Vessel
element
Air space
Bundle sheath
(surrounds the
vascular bundle)
Epidermis
500 µm
100 µm
Fig. 34-3a, p. 734
• Primary tissues (epidermis, cortex, pith,
xylem, and phloem) of stems
– develop from shoot Apical meristems
• Seconday tissues (xylem, phloem, and
periderm of stems
– Develop from Lateral Meristems (Vascular
Cambium,Cork Cambium)
Fig. 34-3b, p. 734
Apical
meristems
Primary
tissue
Lateral
meristems
Primary xylem
Vascular
cambium
Meristematic
cells
Primary phloem
Secondary
tissues
Secondary
xylem (wood)
Secondary
phloem
(inner bark)
Cortex
Cork
cambium
Periderm
Pith
Epidermis
2
3/12/2012
Vascular
Cambium
Secondary Growth
• Production of secondary tissues, wood, bark
– occurs in some flowering plants (woody
dicots) and all cone-bearing gymnosperms
• Vascular cambium divides in two directions
– secondary xylem (to the inside)
– secondary phloem (to the outside)
Primary
xylem
Remnant
of primary
phloem
Epidermis
Remnant
of cortex
Remnant of
epidermis
Secondary phloem
(inner bark)
Secondary xylem
(wood)
Periderm
(outer bark)
Cortex
Primary
phloem
Vascular
cambium
Remnant of
primary xylem
Pith
Remnant of
pith
Vascular
cambium
Fig. 34-4a, p. 735
Fig. 34-4b, p. 735
2P1P
1X2X3X4X
Periderm
(outer bark; remnants
of primary phloem,
cortex and epidermis
are gradually crushed
or turn apart and
sloughed off)
Secondary xylem
(wood)
Remnant of
Remnant of
primary xylem pith
Time
1X2X
Secondary phloem
(inner bark)
2P1P
1X2X3X
2P1P
Secondary xylem
Secondary phloem
1X 2X
1P
Vascular
cambium
1X
1P
1X
Second division of vascular
cambium forms a phloem cell.
Division of vascular cambium
forms two cells, one xylem cell
and one vascular cambium cell.
Vascular cambium cell when
secondary growth begins.
Fig. 34-4c, p. 735
Vascular cambium cell
Fig. 34-5, p. 736
3
3/12/2012
Cork Cambium
Periderm
• Lateral meristem that produces periderm
– cork parenchyma and cork cells
• Cork cells
– to outside of cork cambium
– suberin and waxes make it waterproof
• Cork parenchyma
– to inside of cork cambium
– primarily for storage in a woody stem
Primary
Pith xylem
Annual ring of
secondary xylem
Secondary
xylem (wood)
Heartwood
Vascular
cambium
Secondary
phloem
Sapwood
Periderm and
remnants of primary
phloem, cortex, and
epidermis
Expanded
phloem ray
Xylem ray
0.5 mm
Fig. 34-6, p. 737
Fig. 34-8, p. 739
Water Movement
Cross section of
3-year-old Tilia
stem
• Water and dissolved minerals move from
soil into root tissues (epidermis, cortex)
Secondary
phloem
Vascular cambium
• Water and minerals move upward, from
root xylem to stem xylem to leaf xylem
Summerwood
Springwood
Annual
ring of
xylem
• Water entering leaf exits leaf veins and
passes into atmosphere
Summerwood of
preceding year
100 µm
Fig. 34-9, p. 739
4
3/12/2012
Transport
Sugar molecules from photosynthesis
are transported in phloem throughout
plant, including into roots.
Water Potential
Most water that plant
absorbs is transpired
into atmosphere.
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.
• A measure of the free energy of water
– Pure water = 0 water = lots of free energy –
no solutes
– Water with dissolved solutes has lowered
water potential (a negative number)
• Water moves
– from higher (less negative) water potential to
lower (more negative) water potential
– From area of fewer solutes to more solutes
Fig. 34-10, p. 740
Tension–Cohesion Model
Tension–Cohesion Model
• Explains rise of water
– even in the tallest plants!
• Transpiration
– evaporative pull causes tension at top of
plant
– result of water potential gradient
– (ranges from slightly negative in soil and
roots to very negative in atmosphere)
• Column of water pulled up through the
plant remains unbroken
– due to cohesive and adhesive properties
of water
Tension–Cohesion Model
Root Pressure
• Explains rise of water in smaller plants
– particularly when soil is wet
• Pushes water up through xylem
– water moves from soil into roots due to
active absorption of mineral ions from soil
5
3/12/2012
Root Pressure
(solutes)
Water enters roots by
osmosis
(No solutes)
Sugar Translocation
Soil
Root xylem
Stem xylem
Leaf xylem
atmosphere
• Dissolved sugar is translocated upward or
downward in phloem
– from 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 translocated
in phloem
Water is pulled and pushed through a plant
Pressure–Flow Hypothesis
• Explains movement of materials in phloem
• Companion cells actively load sugar into
sieve tubes at source
– requires ATP
Pressure–Flow Hypothesis
• Turgor pressure gradient
– produced by water entering phloem at
source and water leaving phloem at sink
– drives flow of materials between source
and sink
• ATP energy pumps protons out of sieve tube
elements
6
3/12/2012
Source
ATP
Pressure–Flow
Hypothesis
Pressure-flow hypothesis
• translocation
Sucrose loaded and
unloaded requires ATP
Water moves osmotically
Sink
7