Gardening glossary
Stomata
Stomata (singular: stoma) are the plant equivalent of regulated breathing pores. Each stoma is a tiny gap in the leaf epidermis flanked by two specialised guard cells. When the guard cells are turgid, they bow apart and the pore opens. When they lose turgor, the pore closes. A typical leaf has 100–1,000 stomata per square millimetre, mostly on the underside (the abaxial surface), which is shaded and humid.
What stomata do:
- **Let carbon dioxide in.** Photosynthesis is impossible without CO₂, and CO₂ enters the leaf almost exclusively through open stomata. - **Let water vapour out.** As water evaporates from the wet inner surfaces of the leaf into the open stoma, the resulting suction (the [transpiration](/glossary/transpiration) stream) pulls water and dissolved nutrients up from the roots. - **Let oxygen out.** The O₂ produced by photosynthesis exits the leaf through stomata.
When stomata open and close:
- **Open during the day** in most plants — the trade-off between losing water and gaining CO₂ is worth it while light is available. - **Close at night** in most plants. No light means no photosynthesis, so there is no reason to lose water. - **Close in heat and drought.** Above roughly 35 °C or under water stress, stomata close to prevent fatal water loss, even at the cost of pausing photosynthesis. This is why tomato plants stop fruiting in heatwaves and why houseplants in dry air look stressed. - **CAM plants invert the schedule.** Cacti, agaves, sansevierias, and many succulents open their stomata at night, store the absorbed CO₂ as malic acid, and then close them by day. This minimises water loss in arid environments.
Practical relevance for gardeners:
- **Spray timing.** Foliar feeds, insecticidal soap, and neem oil work better when applied in cool morning or evening light, when stomata are open and absorption is highest. - **Dirty leaves.** Dust and grime physically block stomata. Wipe leaves of broad-leaved houseplants every month or two. - **Heat stress symptoms.** Closed stomata mean the leaf cannot transpire to cool itself, leading to leaf-edge scorch on hot afternoons. - **Humidity tweaks.** Higher humidity reduces the vapour-pressure gradient that pulls water out of open stomata, which is why misted tropicals look happier.