Structure, Function, and Density of Stomata

Background

All plant leaves need to “breathe”. The exchange of atmospheric gases is essential to photosynthesis, the process by which plants use sunlight to convert carbon dioxide and water into oxygen and fuel (carbohydrates). Leaves have special pores called stomata that make gas exchange possible while helping to control the loss of water. The stomata operates through the use of two tiny jellybean shaped cells called guard cells located in the outer layer of tissue called the epidermal layer. Most stomata are on the lower epidermis of the leaves on plants (bottom of the leaf). Unlike other plant epidermal cells, the guard cells contain chlorophyll to perform photosynthesis. This allows the cells to expand/ contract to open or close the stomata. Guard cells swell, through the process of osmosis, to allow opening of the stomata (for CO₂ to enter and excess O₂ and H₂O to leave), and they shrink in order to force the stomata shut (either partially or completely) to prevent dehydration. Similarly, plants must also permit the movement of water from the roots to the leaves through the process of evaporation/transpiration in order to make water available to the cells for photosynthesis. However, a constant concern for terrestrial plants is controlling the rate of transpiration to prevent dehydration (desiccation).

To conserve water during dry times, the stomata remain closed to reduce the loss of water vapour. Due to the requirement for carbon dioxide, it is possible for the lack of moisture that forces the stomata to stay closed to prevent the process of photosynthesis from occurring. The number of stomata on the epidermal surface can tell you a lot about a plant. Usually, a high concentration of stomata indicates fast growth and a wet climate. Lower concentrations of stomata indicate lower rates of photosynthesis and growth or adaptations for dry weather. Stomata are useful to drought-threatened plants because they can close to prevent dehydration.

When stomata close:

1. Guard cells actively pump out K⁺ ions
2. This raises the water potential inside the cells
3. Water exits the guard cells by osmosis (moves into adjacent cells)
4. Vacuoles shrink, guard cells become flaccid and stoma close

In general, having these openings located on the underside of leaves helps to prevent further loss of moisture. However, this is not always an issue for plants growing in high moisture environments, and can result in some unusual stomata configurations. For example, water lilies grow in ponds where their leaves generally float directly on the surface of the water or slightly above it. To thrive in this environment, they have developed stomata on the water lily's large leaf surface (rather than underside) making gas exchange more efficient. Another adaptation that helps to keep lily leaves above the water where stomata can function, are the large number of spongy internal cells that promote flotation.

Other adaptations plants use to reduce evaporation and infestation of insects is to use surface hairs. It is likely that in many cases, hairs interfere with the feeding of at least some small herbivores and, depending upon stiffness and irritability to the "palate", large herbivores as well. Hairs on plants growing in areas subject to frost keep the frost away from the living surface cells. In windy locations, hairs break-up the flow of air across the plant surface, reducing evaporation. Dense coatings of hairs reflect solar radiation, protecting the more delicate tissues underneath in hot, dry, open habitats. And in locations where much of the available moisture comes from cloud drip, hairs appear to enhance this process.