Cellulose, wood structure and water movement in the plant
Objectives of the lecture:
1. Describe how secondary growth takes place and results
in radial growth of stems and roots.
2. Define the principal cell components of secondary growth.
3. Describe the Transpiration-Cohesion-Adhesion Theory for
water movement in plants.
Text book pages:
87,
89-90,
152-153,
807-810,
813-828.
Stem thickening
Lateral meristems
Two lateral meristems in older stems and roots of woody plants produce
secondary growth that results in an increase in diameter (whereas apical
meristems result in an increase in length).
Vascular cambium
Cork cambium
secondary vascular tissues, xylem phloem
periderm (continually replaces the epidermis), bark tissue
secondary
xylem
Bark
cork cambium
vascular cambium
thickening
secondary
phloem
Secondary growth
L4 S12
Figure 36-28
Lateral meristems increase the width
of stems and roots.
Lateral meristems (cork cambium and vascular cambium) produce bark and wood.
Cork
Cross section of young
Linden tree
Cork cambium
Periderm
Phelloderm
3 year
Fig 36.28
Cork cambium
adds cells
primarily to
the outside
Bark
Secondary phloem
2 year
Vascular cambium
Secondary xylem
Wood
1 year
Rays of parechyma
cells
Vascular
cambium adds
many cells to
the inside and
some cells to
the outside
Cambium cells in yellow
outer surface
of stem or root
differentiation
differentiation
division
differentiation
Phloem
production
division
division
division
division
differentiation
Further cell
differentiation
differentiation
Xylem
production
The cambium is usually a few cells thick during
periods of active growth
After division then cells differentiate into specialist
cells of the tissue
direction of radial growth
The vascular cambium produces xylem and phloem
pit in cell wall
Phloem
Xylem
one vessel member
sieve plate
Conifers
Angiosperms
Sugar
transport
cytoplasm
absent
(cells
dead at
maturity
sieve-tube
member
companion
cell (living)
Water
conduction
TRACHEIDS
VESSEL
PHLOEM SIEVE
TUBE
Direction of growth
Cambium
Late wood
Cambium and secondary xylem of a conifer
Ray initials
Early wood
Cambium and secondary xylem of a conifer
Rays
Tracheids with
bordered pits
Cambium and secondary
xylem of a conifer
Recall how the
cambium
produces xylem
cells. This
drawing shows
what those cells
differentiate to.
Taking sections through a stem
Transverse
Section
Radial Longitudinal
Section
Tangential Longitudinal
Section
L4 S13
Transverse section of Quercus rubra wood
vessel
EARLY WOOD
LATE WOOD
toward stem
surface
Secondary xylem is a complex tissue
It conducts water, stores water, and provides support
In conifers it consists of:
tracheids that are long in the vertical direction, and have bordered pits that
can shut when a tracheid collapses under low water potential. Tracheids can
be re-saturated with water
ray cells running horizontally through the xylem and are composed
mainly of parenchyma and some tracheids.
In angiosperms vessel elements are an additional cell type:
Vessel elements are short but much wider compared to tracheids and are
stacked on top of each other to form a vessel
Fig. 36-26a
Tracheids are spindle shaped and have pits.
Longitudinal section
Face view
Side view
Pits
Pits
The torus at the center of the
bordered pit moves and seals the pit
when a tracheid aspirates
Tracheids and rays, pine
Tangential
Longitudinal
Section
Pits
Figure 36-26b
Vessel elements are short and wide
and have perforations as well as pits.
This vessel element has a
completely perforated end
wall
Perforations
Pits
Longitudinal section
Elongated vessel element
provides moderate support but
superior fluid conduction
compared to a tracheid
Figure 36-27
Cross section
Longitudinal section
Sieve-tube members
Sieve-tube members
Companion cells
Companion cells
Cross section
Sieve
plate
Sieve
plate
Sieve-tube members and companion cells develop from the same progenitor cell.
Sieve-tube members unite vertically to form a sieve tube.
Sieve-tube members have no nucleus at maturity and depend on companion cells
to regulate physiological processes.
Table 36-2
Water movement in plants
Water moves along potential energy gradients—from areas of high water
potential to areas of low water potential.
What happens between living cells and their environment?
Water’s potential energy in plants is a combination of
1. its tendency to move in response to differences in solute
concentration
Osmosis
2. the pressure exerted on it.
Turgor pressure
What happens outside of the living cell?
Plants do not expend energy to replace water lost in transpiration.
Water moves from soil and roots to leaves along a water potential gradient.
The gradient exists because water at the air-water surface in leaves is under
negative pressure (tension) great enough to pull water up from the roots
through xylem.
The Cohesion-Tension Theory
What happens between living cells and their environment?
Solute potential is the tendency of water to move by osmosis.
Solute potential inside cell
and in surrounding solution
is the same. No net
movement of water.
Cell
Cell is placed in pure water.
Its solute potential is low
relative to its surroundings.
Water moves into cell via osmosis.
Pure water
Solute
Fig 37.1a
Water
movement
Isotonic solution
Hypotonic solution
Osmosis is the spontaneous net movement of water across a semi-permeable membrane from
a region of high solute concentration to a solution with a low solute concentration, down a
solute concentration gradient.
Osmosis is a selective diffusion process driven by the internal energy of the solvent molecules.
Pressure potential is the tendency of water to move in
response to pressure.
Turgor pressure is
an important source
of pressure on
water in cells
Inside of cell
Expanding volume of cell
pushes membrane out.
Turgor pressure
Plasma membrane
Cell wall
Wall pressure
Stiff cell wall pushes back with
equal and opposite force.
Outside of cell
Fig 37.1b
What happens outside of the living cell?
The text book refers to “The Cohesion-Tension Theory”
A more descriptive term is: TRANSPIRATION-COHESION-ADHESION THEORY:
WATER POTENTIAL:
Xylem sap rises against gravity, driven by a gradient of water potential.
Water flows from an area of high potential to an area of low potential.
Water Potential is expressed in units of pressure: 1 bar is the pressure
needed to push up a column of water 10 meters. 1 megapascal = 10 bars.
Pure water has a potential of 0.
Addition of pressure increases water potential.
Placing water under tension decreases water potential.
Addition of solutes decreases water potential.
The crucial question is how is a gradient water potential maintained?
It is maintained by creation and maintenance of a gradient of tension.
Hydrogen Bonds and Cohesion
Water molecules have weak negative charges at the
oxygen atom and positive charges at the hydrogen
atoms.
H
H
O
+
Positive and negative regions are attracted. The force of
attraction, dotted line, is called a hydrogen bond. Each
water molecule is hydrogen bonded to four other water
molecules – the force of Cohesion.
H
O
H
H
H
O
H
O
H
O
H
H
The hydrogen bond has ~ 5% of the strength of a covalent bond. However, when many
hydrogen bonds form, the resulting Cohesion is sufficiently strong as to be quite
stable.
Adhesion is the tendency of molecules of different kinds to stick together –
by a similar process. Water sticks to cellulose molecules in the walls of the xylem,
counteracting the force of gravity. It is the fine structure of cellulose in tracheids and
vessels that enables tension to be maintained.
http://www.ultranet.com/~jkimball/BiologyPages/H/HydrogenBonds.html
Table 5-1
A water potential gradient exists between the soil,
plants, and the atmosphere. Plants tend to gain water
from the soil and lose it to the atmosphere.
Low water potential
Atmosphere : –95.2 MPa
(Changes with humidity;
usually very low)
Leaf : –0.8 MPa
(Depends on
transpiration rate;
low when stomata
are open)
Fig 37.4
a). water exits leaf through stomata.
b). this water is replaced by evaporation from mesophyll
cells, lowering their water potential, causing them to
take water from neighboring cells.
c). the process connects back to the tracheids and/or
vessels causing water to be taken from the xylem.
d).Water travels from the tracheids to the air following a
water potential gradient.
Root : –0.6 MPa
(Medium-high)
Soil : –0.3 MPa
(High if moist;
low if extremely dry)
High water potential
e). The cohesive and adhesive properties of water and
the small diameter of xylem aid in its vertical movement.
f). This pull decreases water potential in the xylem
causing the roots to take water from the soil.
Measuring water potential
The pressure bomb!
See
Fig 37.14
Compressed air
Field measurements of shoot
Forest laboratory in south west Scotland
Measurement every hour for 7 days
Diurnal pattern of shoot water potential
Midnight
Midday
500
Transpiration
measured by
eddy correlation
mg/sec/tree

Shoot water
potential
MPa
400
300
200
100
0
-1
-2
30 Jul 31 Jul 1 Aug 2 Aug 3 Aug 4 Aug 5 Aug 6 Aug
During daylight water loss from foliage exceeds water gain from soil so shoot water
potential decreases. On sunny days  reaches –2 Mpa
Cessation of physiological processes:
Cell growth and wall synthesis are very sensitive
and may stop at -0.5 MPa
Photosynthesis, respiration and sugar
accumulation are less sensitive. They may be
affected between -1 and -2 MPa
L11 S10
Stomatal control clip
Things you need to know ...
1. How secondary cambia produce wood and bark.
2. The cell types found in conifer and angiosperm wood and how
differences between them reflect the ecology of these two
groups of plants.
3. The structure of phloem.
4. How water moves in plants from roots to leaves including typical
water potential values: the transpiration-cohesion-adhesion
theory.