The phloem's structure is highly adapted for efficient transport. Key adaptations include:

• Sieve Tubes: These are the main conducting elements. They are long, cylindrical cells with sieve plates at their ends. The sieve plates have pores that allow for the passage of phloem sap. The lack of a nucleus and other organelles in sieve tube elements reduces obstruction to flow.
• Companion Cells: These cells are closely associated with sieve tube elements and provide metabolic support. They load and unload sugars into and out of the sieve tubes, maintaining the pressure gradient. They also provide energy and structural support.
• Phloem Parenchyma: These cells provide storage and structural support within the phloem tissue.
• Sieve Elements: These are specialized sieve tube elements that are more efficient at transport. They have fewer internal structures and more extensive plasmodesmata (connections between cells) to facilitate flow.

Method: Potted plants of the same species and size are required. The light intensity can be varied by placing the plants at different distances from a light source (e.g., lamps with varying wattage). Temperature and humidity should be kept constant. Plants should be watered regularly. Transpiration rate can be measured as described above (weight change or potometer). It is important to ensure that the light source provides consistent light quality (e.g., using lamps with a similar spectrum).

Variables:

• Independent Variable: Light intensity (e.g., low, medium, high – measured in lux or similar units).
• Dependent Variable: Transpiration rate (measured as change in mass or water uptake by potometer).
• Controlled Variables:

  • Plant species and size
  • Temperature
  • Humidity
  • Soil type
  • Pot size
  • Watering regime

Explanation: Light intensity affects transpiration rate because light stimulates the opening of stomata. When stomata are open, water vapour can escape from the leaf. Higher light intensity generally leads to a greater rate of stomatal opening and therefore a higher transpiration rate. However, very high light intensities can sometimes lead to stomatal closure due to water stress, which would then reduce transpiration.

The student would observe that the leaves of the plant remain turgid (firm) and the plant does not wilt over the 24 hours. This is because water is continuously being absorbed by the root hair cells and transported up the xylem to the rest of the plant.

This observation supports the statement that water is absorbed by root hair cells because the root hair cells are the primary site of water uptake from the soil. The water absorbed by the root hair cells then travels through the root cortex and into the xylem vessels, facilitating the movement of water throughout the plant. Without the ability of root hair cells to absorb water, the plant would quickly become dehydrated and wilt.