Pollen tube growth rate from graticule calibration

Steps:

• Observe initial pollen grain size in Diagram 1 (no tube, length 0 units) and final tube length in Diagram 2 (20 graticule units after 4 hours).
• Use eyepiece graticule calibration: 1 unit = 1 μm.
• Calculate total growth: 20 units × 1 μm/unit = 20 μm.
• Determine rate: 20 μm / 4 hours = 5 μm/hour (denoted as 5 μm⁻¹).

Why A is correct:

• Matches the formula for growth rate (change in length / time) using calibrated units in μm per hour.

Why the others are wrong:

• B: Overestimates rate (would require 40 μm total growth).
• C: Uses mm scale, but pollen tubes are microscopic (μm), not macroscopic (mm).
• D: Uses mm scale with overestimated rate.

Final answer: A