9.2 Applications and skills

9.2.1 Counting stomata

When stomata are open, the air spaces in the spongy mesophyll layer of a leaf become continuous with the atmosphere. This means that photosynthetic gases are free to diffuse in and out of the plant. In general, carbon dioxide diffuses in through stomata while oxygen and water diffuse out.

Stomata are generally open during the day to allow the free exchange of photosynthetic gases, and closed at night to prevent water loss when photosynthesis is not taking place.

 As a consequence, water loss is highest during the day. This is not a problem for well-watered mesophytes. However, plants that live arid conditions with saline soils – xerophytes – have special adaptations to reduce water loss by transpiration. These include:

  • a thick cuticle, giving the leaf a waxy or leathery appearance, or leaves covered in small hairs to prevent water loss through evaporation
  • stomata concentrated on lower surfaces or in deep pits protected from the wind
  • fleshy stems that store water – in the case of cacti, stems are photosynthetic and leaves are reduced to short spines
  • stomata closed during the day and open at night.

Controlling the opening and closing of stomata

Even if kept constantly in the dark, stomata will continue to open and close on a daily cycle. Control of stomatal aperture is an example of a circadian rhythm, which is controlled by changes in chemical signals. The most important of these are:

  • Turgor pressure – when water is abundant, the guard cells surrounding the stomatal pore take up water by osmosis, become turgid and swell, thereby increasing the diameter of the pore.
  • K+ concentration – when potassium ions are released from the guard cells, water potential goes up and the cells become flaccid, closing the stomatal pore.
  • Abscisic acid – in low water conditions, abscisic acid acts as a stress hormone, preventing further wilting of leaves by closing the stomatal pore.

There are a number of easy ways to observe stomata using a light microscope.

Procedure 1: Observing stomata using varnish casts

In this activity, you will use a cast method to observe stomata.

You will need:

  • clear nail varnish
  • tweezers
  • clear adhesive tape (not shiny)
  • microscope slides and cover slips
  • leaves from two or three types of plants

Instructions

  1. Paint the underside of a leaf with clear nail varnish and allow it to dry completely.
  2. Use the tweezers to gently pull the dry varnish off the leaf. Place it on a slide, being careful not to bend the cast.
  3. Cover the varnish with a piece of adhesive tape. Be careful not to leave fingerprints.
  4. Observe stomata on low, medium and high power. Identify the guard cells and pore. Determine if the stomata are open are closed.

Procedure 2: Observing stomata in living tissue

In this activity, you will observe the lower and upper epidermis of a leek in order to determine the patterns of distribution of stomata. Leeks are very good plants to use for this activity, as the epidermis can be easily removed. You may also wish to try this method with other plants.

You will need:

  • one large leek, Alium porrum
  • scalpel and tweezers
  • microscope slides and cover slips.

Instructions

  1. Using the scalpel, score a 10 x 10mm square on the surface of the leek. Carefully use the tweezers to remove the top layer of tissue (epidermis) from the square.
  2. Mount the epidermis on a slide with a drop of water and cover slip.
  3. Observe stomata on low, medium and high power.
  4. Draw conclusions on the size and distribution of stomata in leeks.

cuticle of lavandula plant

Figure 9.2.1a – A thick cuticle and fine hairs give the leaves of the Lavandula plant a whitish appearance

stomata in pit

Figure 9.2.1b – Micrograph of xerophytic adaptationstomata in pit

cactus

Figure 9.2.1c – Succulent stems provide a site of photosynthesis

Discussion

  • How well do models represent real-world phenomena?
  • To what extent does scientific research depend on improvements in apparatus/technique?
  • By which measurable criteria should an environmental protection plan be evaluated?

Key question

Why do horticulturalists spray water on the leaves of plants?