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.

ExamineExperiment set up on Capillarity.

Drawdiagram and write explanation.

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 Activity Sheet 2 Transpiration Pull / Cohesion Tension theory

Activity Sheet 3 Reduced Circumference of Tree.

Uses of Water in the Plant - Importance of transpiration stream.

Read P177 and write a note.

Stomatal Mechanism

Complete:Activity Sheet 2.14. 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?
  1. Stomata close in the dark, therefore water saved. Why do plants still lose water even if their stomata are closed?

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

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.

  1. 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.15 marks

Hydrophytesplants living partly submerged e.g. water lily

plants living completely submerged e.g. water milfoil.

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.

  1. 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.
  2. 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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