CourseCourseQuestionQuestionDocumentDocument

B3.1.10—Stomatal density

Stomatal density measures stomata per unit leaf area and helps compare potential gas exchange and water loss between leaf surfaces.

Syllabus
First assessment 2025
Objective
B3.1.10
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1B
Typical marks1

Common command terms

  • Outline

Recent exam appearances

May 2025Paper1B ["SL"] · TZ11(c)(i)[ 1 ]B3.1.10—Stomatal density

Coverage 2025–2025 · Updated 15 Jul 2026

Stomatal Density Measures Pores per Leaf Area

Stomatal density = number of stomata counted ÷ sampled epidermal area. It estimates potential pore abundance, while aperture determines how open those pores are at a given moment.

A hand applies clear nail varnish with a brush to a small area of a green leaf beside an open bottle labeled clear nail varnish.

clear nail-varnish impression → peel and mount → calibrate field area → count stomata in random fields → calculate each density → repeat and report variation

SL Summary: One Diffusion Model

Feature Lung Leaf
area many alveoli broad blade + mesophyll walls
path thin barrier thin leaf + air spaces
gradient air + blood flow CO₂ use + stomata

Spirometry measures ventilation; a potometer estimates water uptake; a leaf cast measures stomatal density. Match each method to its direct measurement.

Stomatal density

[ 1 ]

Outline how stomatal density in busy Lizzie leaves can be estimated within a known field of view.

Concept essentials

  • Stomatal density is calculated as the number of stomata divided by leaf area observed.
  • More stomata on a surface can indicate greater potential transpiration from that surface.
  • A known field of view or calibrated image is needed for a valid density estimate.
  • Density comparisons are meaningful only when the same area basis is used.