Chapter 23
The Transpiration Stream
The flow of water from a plant's roots via the stem to the leaves is called
the Transpiration Stream.
1) Entry of Water.
Absorbing surface is the root hair. Explained in terms of Osmosis and
Diffusion. Since the cell sap inside a root hair is a region of lower water
concentration than the soil solution outside, water passes into a root hair
by Osmosis through the selectively permeable plasma membrane. Water
now passes across root cells along a water concentration gradient i.e.
High Water concentration ------- Low Water Concentration. Much
water also diffuses across the root via the permeable cell walls and inter
cell spaces without entering the living cells. (However the flow is
interrupted by the endodermis, so the osmotic route through the cells is
needed to be taken.
The flow of water through the root has a measurable pressure ---Root
Pressure. This pressure value is not great enough to explain the rise of
water in the stems of plants and trees.
Examine
The experiment set up on root pressure.
Draw diagram. Text Book P176
2) Rise of water in Stem
Adhesion is the force of attraction between unlike particles.
Cohesion is the force of attraction between like particles.
Examine
Experiment set up on Capillarity.
Draw
diagram and write explanation.
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Chapter 23
Water moves through the stem as a series of thin continuous columns of
water in the Xylem Vessels. The columns are maintained unbroken by
the forces of cohesion and adhesion. The molecules of water are held
together by cohesion. Adhesion, the force involved in capillarity, helps
support the columns as the water molecules adhere in the narrow xylem
vessels to the cell walls. PITS in the walls of the xylem vessels allow
lateral transport of water to the tissues all the way up the plant.
3) Exit of Water
Transpiration is the evaporation of water from the aerial parts of a plant.
Water leaves a plant as vapour and although the transpiration occurs via
Stomata, the Mesophyll cells are the evaporating surface of the plant.
As water evaporates, these cells have a lower water concentration than
cells nearer the xylem vessels, therefore.
A water concentration gradient also exists in a leaf. As water is
evaporating from the mesophyll cells, water is drawn form neighbouring
cells and leaf xylem to replace their losses. This sets up the
transpiration pull, the major force which pulls water up through the
stem.
Complete
theory
Activity Sheet 2 Transpiration Pull / Cohesion Tension
Activity Sheet 3 Reduced Circumference of Tree.
Uses of Water in the Plant - Importance of transpiration stream.
Read P177 and write a note.
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Chapter 23
Stomatal Mechanism
Complete: Activity Sheet 2.1 4. Booklet
Collect:
Diagram sheets of stomata. 5.
Complete: Structure of the stomata and make a short note on the
distribution of stomata.
Complete: labelling of guard cell diagrams to explain opening and
closing mechanism. Write a short note.
Plant Osmoregulation
1. Stomata sometimes close during very hot weather. Why?
2. Stomata close in the dark, therefore water saved. Why do plants
still lose water even if their stomata are closed?
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Chapter 23
Mineral (Nutrient) Ion Uptake
Read:
page 209.
Write:
a note.
Why do plants die in water logged soil?
Make up a table to summarise the importance of the more common
mineral ions to plants.
Measuring the Transpiration Rate
Complete: Activity sheet 2.2.
Make a note on the following factors which have an effect on
transpiration rate.
Temperature
Light Intensity
Humidity
Air Pollution
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Chapter 23
Currents
Complete: Weight potometer worksheet
Compulsory Experiment worksheets.
Structural Adaptations which reduce Transpiration Rate
Read:
pp212 - 214
Mesophytes Live in areas where water is abundant and excessive
transpiration is not a problem.
Xerophytes Live in areas where mesophytes would not survive due to
transpiration rate being excessively high.
e.g. 1. Desert with not dry conditions and lack of soil water.
3. Exposed windy conditions.
Adaptations
1. Reduced number of stomata.
2. Thick cuticle.
3. Leaf rolled and hairy.
4. sunken stomata in pits. these pits trap pockets of moist air.
5. Some leaves are small and circular e.g. pine needles therefore
reducing the relative surface area of transpiring leaves exposed to
the atmosphere.
6. Many cacti have long roots allowing absorption of subterranean.
7. Presence of surface roots in order to absorb water in times of flash
floods.
8. Stored water in succulent stem.
9. Stem folded as in cacti.
10.Reversed stomatal rhythm resulting in stomata closed in hottest
periods of the day.
Write an Essay
Give an account of water balance in xerophytes.
Hydrophytes
15 marks
plants living partly submerged e.g. water lily
plants living completely submerged e.g. water milfoil.
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Chapter 23
Adaptations
1.
Airspaces Plant possess problem of getting enough oxygen
which is only sparingly dissolved in water. Hydrophytes adapt
by possessing an extensive system of inter communicating air
spaces. Much of the oxygen produced in photosynthesis is
stored in these air spaces instead of escaping from the plant.
This aeration tissue also gives the plant buoyancy to keep it's
leaves near the surface for photosynthesis.
2.
3.
Reduction in Xylem. Water is now available for absorption all
over it's surface. Water is now available for support. A
hydrophyte is found to possess little strengthening, water
conducting xylem. It's xylem is found in the centre of the stem
to allow for maximum flexibility in response to water
movements and yet still be able to resist pulling strains.
Specialised Leaves. Submerged leaves are narrow in shape and
finely divided. this adaptation helps to prevent them from being
torn by water currents.
Submerged Leaves - no stomata.
Floating Leaves - stomata on upper surface. Possess long leaf
stalks (petioles) which prevent the stomata from being flooded
when the water level rises.
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