(a) With the aid of annotated diagrams only, outline the water cycle. (b) Describe an experiment to show that water is conducted in the xylem tissue of flow...

Answer Details

(a) The water cycle (annotated diagram required)

The diagram should show a continuous cycle with labelled arrows and brief annotations:

• Evaporation: heat from the Sun evaporates water from oceans, lakes, rivers and moist soil, forming water vapour.
• Transpiration: plants give off water vapour from their leaves into the air.
• Condensation: the rising water vapour cools at high altitude and condenses into tiny droplets forming clouds.
• Precipitation: the droplets join and fall back to the earth as rain (or hail/snow).
• Run-off and drainage: the water flows over the land into streams, rivers and back into the sea, and some sinks into the ground (infiltration) to form ground water, completing the cycle.

The arrows should form a closed loop: sea/land \( \rightarrow \) evaporation/transpiration \( \rightarrow \) condensation (cloud) \( \rightarrow \) precipitation \( \rightarrow \) run-off \( \rightarrow \) back to sea.

(b) Experiment to show that water is conducted in the xylem tissue

• Materials: a fresh leafy herbaceous shoot (for example balsam or Impatiens) with a soft translucent stem, a beaker, water and red ink (eosin), a sharp blade.
• Method: Put water in the beaker and add enough red ink (eosin) to colour it deeply. Cut the base of the shoot cleanly under water and stand it in the coloured water. Leave the set-up in a warm, well-lit place for a few hours.
• Observation: the veins of the leaves and the upper parts of the stem gradually become stained red.
• Now cut a thin transverse (cross) section of the stem and examine it. It is seen that only the xylem tissue is stained red; the other tissues are not stained.
• Conclusion: since the red dye rose only through the xylem, this shows that water (with dissolved dye) is conducted upward through the xylem tissue of the plant.