Stoma

Lore & Background

Stomata are bordered by a pair of specialized parenchyma cells called guard cells, which regulate the size of the stomatal opening. The term is often used collectively to refer to the entire stomatal complex, including the guard cells and the pore itself (the stomatal aperture). Air containing oxygen and carbon dioxide passes through stomata by gaseous diffusion, while water vapor diffuses out during transpiration. Stomata are present in the sporophyte generation of most land plants, except liverworts and some mosses and hornworts. Their number, size, and distribution vary widely among vascular plants. Dicotyledons typically have more stomata on the lower leaf surface, while monocotyledons like onion, oat, and maize may have similar numbers on both surfaces. Floating leaves may have stomata only on the upper epidermis, and submerged leaves may lack them entirely. Guard cells open and close stomata in response to environmental conditions such as light intensity, humidity, and carbon dioxide concentration. The opening mechanism involves a proton pump, potassium ion uptake, and water influx via osmosis, causing the guard cells to bow apart. Closing occurs when abscisic acid triggers solute loss and water efflux, leading to plasmolysis.

Reader's Guide

Stomata are fundamental to plant physiology, balancing the need for carbon dioxide uptake with the inevitable loss of water vapor. This trade-off is central to photosynthesis and transpiration. The article notes that most plants require stomata to be open during daytime to acquire carbon dioxide, but this simultaneously allows water loss. Alternative strategies, such as the use of phosphoenolpyruvate carboxylase (PEPcase) or crassulacean acid metabolism (CAM), help mitigate water loss under arid conditions by fixing carbon dioxide at night or using intermediary molecules. Stomatal behavior can be inferred from gas exchange measurements, allowing scientists to calculate stomatal resistance and conductance. This has practical applications in understanding plant responses to environmental changes like light intensity, humidity, and gas concentrations. The diversity of stomatal distribution across plant groups—amphistomatous, hypostomatous, or epistomatous leaves—reflects adaptations to different habitats. The article also notes that guard cells contain more chloroplasts than other epidermal cells, though their function remains controversial. Overall, stomata are a key evolutionary innovation that enabled land plants to regulate gas exchange while minimizing water loss.

Did You Know?

• Stomata are present in the sporophyte generation of most land plants, except liverworts and some mosses and hornworts.
• Dicotyledons usually have more stomata on the lower leaf surface, while monocotyledons like onion and maize may have similar numbers on both surfaces.
• Guard cells open stomata by taking up potassium ions and water, causing them to bow apart; closing involves abscisic acid triggering solute loss.
• CAM plants open their stomata at night to fix carbon dioxide and close them during the day to reduce water loss.