Stomata
Short answer: Stomata are microscopic pores, each bordered by a pair of guard cells, that control a plant's intake of carbon dioxide and its loss of water vapor.
Guard cells open the pore by taking up ions and water and swelling, and they close it when they lose turgor, for example in response to the drought hormone abscisic acid (ABA). Open stomata cover only about 5% of the leaf surface, but they're the main route of water loss (transpiration). Stomatal behavior shapes the global water and carbon cycles, and stomata appear in fossils from about 400 million years ago.
What should you know about Stomata?
Quick facts. Each one stands alone and has a stable ID you can cite.
Open stomatal pores cover only about 5% of a leaf's surface.
The drought hormone ABA makes stomata close.
Video transcript: Stomata: The Tiny Pores That Let Leaves Trade Gases
DID YOU KNOW? A plant stoma is a tiny pore found primarily on the surface of leaves and stems that regulates gas exchange between the plant and its environment. Each stoma is flanked by two specialized guard cells that can open or close the pore in response to light, humidity, and carbon dioxide levels. When open, stomata allow carbon dioxide to enter for photosynthesis and release oxygen as a byproduct. NOW YOU KNOW!
Burned-in captions, read frame by frame (see transcript.md) · 22.4s Reel ·
The write-up
The tiny pores that let leaves trade gases. Stomata cover only a few percent of a leaf, yet most of its water loss goes through them.
- Exhibit A
A two-cell valve
Each stoma is a pore between two guard cells. When they take up potassium ions and water, they swell and bow apart to open the pore, and when they lose turgor the pore closes.
- Exhibit B
The trade-off
Opening the pore lets in CO2 for photosynthesis but also lets water vapor out. Plants constantly balance carbon gain against water loss.
- Exhibit C
Small area, big effect
Stomatal pores cover only about 5% of the leaf surface when open. Even so, water loss through them can reach up to about 70% of what a leaf with no waxy cuticle would lose.
- Exhibit D
Signals
Guard cells respond to light, CO2, humidity and the drought hormone abscisic acid (ABA), which triggers closure when water is short.
- Exhibit E
Planet-scale
Transpiration through stomata moves huge amounts of water from soil to air and ties plant physiology to climate. Stomata appear in fossil plants from about 400 million years ago.
Questions people ask about Stomata
How many guard cells surround a stoma?
Two guard cells.
Source: Write-up · Exhibit A (A two-cell valve) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature; Wikipedia: Stoma
Besides CO2 and O2, what else passes through stomata?
Water vapor leaves through them, which is transpiration.
Source: Write-up · Exhibit B (The trade-off) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature; Schroeder et al. / Plant Physiology (2006), The Biology of Transpiration. From Guard Cells to Globe
Which hormone makes stomata close during drought?
ABA.
Source: Write-up · Exhibit D (Signals) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature; Schroeder et al. / Plant Physiology (2006), The Biology of Transpiration. From Guard Cells to Globe
About how much of a leaf's surface do open stomatal pores cover?
About 5%.
Source: Write-up · Exhibit C (Small area, big effect) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature
What makes guard cells swell to open the pore?
Taking up ions and water raises their turgor.
Source: Write-up · Exhibit C (Small area, big effect) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature
Test Your Knowledge
All quizzes →5 questions, all from this case file. Answers are revealed instantly.
How many guard cells surround a stoma?
- One
- Two
- Four
- Ten
Show the answer
Answer: B. Two Two guard cells. Source: Write-up · Exhibit A (A two-cell valve) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature; Wikipedia: Stoma
Besides CO2 and O2, what else passes through stomata?
- Water vapor
- Nitrogen gas only
- Sugar
- Soil
Show the answer
Answer: A. Water vapor Water vapor leaves through them, which is transpiration. Source: Write-up · Exhibit B (The trade-off) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature; Schroeder et al. / Plant Physiology (2006), The Biology of Transpiration. From Guard Cells to Globe
Which hormone makes stomata close during drought?
- Abscisic acid (ABA)
- Insulin
- Adrenaline
- Melatonin
Show the answer
Answer: A. Abscisic acid (ABA) ABA. Source: Write-up · Exhibit D (Signals) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature; Schroeder et al. / Plant Physiology (2006), The Biology of Transpiration. From Guard Cells to Globe
About how much of a leaf's surface do open stomatal pores cover?
- About 5%
- About 50%
- About 90%
- 100%
Show the answer
Answer: A. About 5% About 5%. Source: Write-up · Exhibit C (Small area, big effect) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature
What makes guard cells swell to open the pore?
- Taking up ions and water
- Sunburn
- Freezing
- Losing water
Show the answer
Answer: A. Taking up ions and water Taking up ions and water raises their turgor. Source: Write-up · Exhibit C (Small area, big effect) · Hetherington & Woodward (2003), The role of stomata in sensing and driving environmental change, Nature
Cite this case file
Easterling, Marcus. "Stomata." The Detail Department (DetailEnjoyer), case file DD-715, last updated Apr 29, 2026. https://detailenjoyer.com/case/stomata/
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