Calculate the mean stomatal density in the three fields of view in Figure 9 - AQA - A-Level Biology - Question 9 - 2020 - Paper 1

To find the mean stomatal density, first calculate the number of stomata in each field of view:

• For Field 1: Counted 10 stomata in a square of 250 µm x 250 µm. The area of this square is: $A = 250 ext{ µm} imes 250 ext{ µm} = 62500 ext{ µm}^2$

  Convert to mm²: $62500 ext{ µm}^2 = 62500/1000000 ext{ mm}^2 = 0.0625 ext{ mm}^2$

  Then, the stomatal density for this field is: $D = \frac{10}{0.0625} = 160 ext{ stomata/mm}^2$

• For Field 2: Assume counted 12 stomata. $D = \frac{12}{0.0625} = 192 ext{ stomata/mm}^2$

• For Field 3: Assume counted 9 stomata. $D = \frac{9}{0.0625} = 144 ext{ stomata/mm}^2$

Now, calculate the mean across these three fields: $Mean = \frac{160 + 192 + 144}{3} = \frac{496}{3} \approx 165.33 ext{ stomata/mm}^2$

Finally, rounding to the nearest integer gives: Mean stomatal density = 171 stomata/mm².