Comparing Stomatal Distribution on Monocot and Eudicot Leaves

Aim

To compare the number (density) of stomata on the upper and lower epidermis of a monocotyledonous leaf (e.g. maize or grass) and a eudicotyledonous leaf (e.g. hibiscus).

Variables

• Manipulated variables: the leaf surface examined (upper epidermis or lower epidermis) and the type of leaf examined (monocotyledon or eudicotyledon).
• Responding variable: the number of stomata counted per field of view under the microscope (stomatal density).
• Controlled variables: the area and magnification of the field of view used for counting, the age and position of the leaf on the plant, and the technique used to obtain each epidermal impression.

Materials and apparatus

• A monocotyledonous leaf, such as maize or grass
• A eudicotyledonous leaf, such as hibiscus or Ixora
• Clear nail varnish (or petroleum jelly)
• Clear adhesive tape
• Microscope slides and cover slips
• A light microscope
• Forceps

Procedure

1. Apply a thin, even layer of clear nail varnish to a small area of the upper surface of the leaf.
2. Allow the layer of nail varnish to dry completely.
3. Peel off the dried layer carefully using a piece of clear adhesive tape and stick it onto a microscope slide to form an epidermal impression.
4. Repeat the same procedure for the lower surface of the same leaf.
5. Repeat the whole process for the second leaf type.
6. Observe each impression under the microscope at the same magnification.
7. Count the number of stomata visible in the field of view, repeating the count in two or three different spots on each impression and calculating an average.

Expected results

A eudicot leaf usually shows more stomata on the lower epidermis than on the upper epidermis. Fewer stomata on the upper surface, which faces direct sunlight, helps to reduce excessive water loss.

A monocot leaf, which is often held more upright or at an angle rather than horizontally, tends to have a more similar number of stomata on both its upper and lower surfaces.

Conclusion

Stomatal distribution differs between the upper and lower leaf surface and between monocot and eudicot leaves. This reflects differences in leaf orientation between the two types of plant and is an adaptation that helps to reduce excessive water loss through transpiration, particularly from surfaces exposed to direct sunlight.

Common mistakes

Paper 3-style questions

Question 1 (making a hypothesis). State a suitable hypothesis for the comparison of the two surfaces of the eudicot leaf.

Model answer. The lower epidermis of the eudicot leaf has a higher stomatal density (more stomata per field of view) than the upper epidermis. This links the manipulated variable, the leaf surface, to the responding variable, the number of stomata counted.

Question 2 (controlling variables). Name one variable that must be kept constant and explain why it matters.

Model answer. The magnification used must be kept constant, because it fixes the area of the field of view. If different magnifications were used, each count would cover a different leaf area, so the counts could not be compared as a fair test.

Question 3 (tabulating and displaying data). The student counts the stomata in three fields of view on each surface. Describe how these results should be recorded and displayed.

Model answer. Record each count and the mean per surface in a table with clear headings and no units, since a count has no unit. Display the mean stomatal density for each surface as a bar chart, because leaf surface is a discrete (categorical) variable rather than a continuous one.

Question 4 (making an inference). The monocot leaf shows a similar number of stomata on both surfaces. What can you infer about the leaf, and why?

Model answer. You can infer that the monocot leaf is held upright or nearly vertical, so both surfaces receive similar light and exposure. A similar stomatal density on both surfaces then balances gas exchange against water loss equally on each side.

Safety

• Use clear nail varnish in a well-ventilated area and keep it away from open flames, because the varnish and its fumes are flammable.
• Handle microscope slides and cover slips by their edges and place them down carefully, as the thin glass can crack and cut the skin.
• Do not touch the microscope lamp during use or just afterwards, as it can become hot.
• Wash your hands after handling the nail varnish and the plant material, and keep the varnish away from your eyes and mouth.

Source:SRC-DSKP-EN

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