Q.63 The continuous and dashed lines in the following graph represent the velocity of sap flow in two
different parts of a plant at different times of a day. Which of the following statements is most
appropriate based on this graph?
(A) The continuous line represents the trunk, and the dotted line a twig
(B) The continuous line represents a twig, and the dotted line the trunk
(C) The continuous line represents a root, and the dotted line a twig
(D) The continuous line represents a root, and the dotted line the trunk

Introduction

Sap flow velocity in plants varies across different organs such as roots, trunks, and twigs, and also changes with the time of day. These variations are mainly driven by transpiration rates, stomatal activity, and hydraulic resistance.

The given graph shows two curves (continuous and dotted) representing sap flow velocity in two different plant parts over a 24-hour cycle (day and night). The question asks which plant parts these curves most appropriately represent.

Understanding the Graph

Key observations from the graph:

• Continuous line

  • Shows a sharp increase during the day

  • Peaks around midday

  • Declines steeply during the evening

  • Drops significantly at night

• Dotted line

  • Shows lower overall velocity

  • Changes more gradually

  • Declines slowly at night

  • Exhibits less diurnal fluctuation

These patterns directly reflect how different plant parts respond to transpiration demand.

Correct Answer

✅ Option (B): The continuous line represents a twig, and the dotted line represents the trunk

Explanation of the Correct Option

Why does the continuous line represent a twig?

• Twigs are closest to leaves, where transpiration occurs.

• During the day, high transpiration causes rapid and strong sap flow.

• At night, stomata close, causing a sharp decline in sap flow.

• Hence, twigs show high diurnal variability, exactly as seen in the continuous line.

Why does the dotted line represent the trunk?

• The trunk acts as a buffer and storage conduit.

• Sap flow in the trunk is more stable and less sensitive to immediate changes in transpiration.

• Therefore, trunk flow shows lower velocity and smoother variation, matching the dotted line.

Explanation of All Options

❌ Option (A): Continuous = trunk, dotted = twig

Incorrect. The trunk does not show such sharp midday peaks. Twigs, not trunks, respond rapidly to changes in transpiration.

✅ Option (B): Continuous = twig, dotted = trunk

Correct. Matches physiological behavior and diurnal sap flow dynamics.

❌ Option (C): Continuous = root, dotted = twig

Incorrect. Root sap flow is relatively stable and does not show strong daytime peaks like the continuous line.

❌ Option (D): Continuous = root, dotted = trunk

Incorrect. Roots show minimal diurnal fluctuation compared to twigs and do not exhibit sharp daytime maxima.

Conceptual Takeaway

• Twigs → Highest diurnal variation, sharp daytime peak

• Trunk → Moderate, buffered flow

• Roots → Most stable, least fluctuation

Understanding these differences is crucial for plant physiology, eco-hydrology, and competitive exam questions.

Final Answer (One-Line)

✔️ The continuous line represents a twig, and the dotted line represents the trunk (Option B).