Compare stomatal densities of the upper and lower epidermis of a leaf.

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Biology key skills                                                               Stephanie Adu

BIOLOGY – IT KEY SKILLS

INTRODUCTION.

The purpose of this assignment is to compare stomatal densities of the upper and lower epidermis of a leaf.

This assignment of stomata is also relevant to my AS biology course as stomata is a key factor in transpiration of plants, and transpiration and the transport of water is a major section of the syllabus. This assignment will therefore help me to understand why transpiration occurs and how the stomata affect it as stomata’s activity is related to the rate of transpiration.

It is said that in general, the greater the number of stomata per unit area, the greater the rate stomatal transpiration, however distribution and densities are also important.

Stomata are pores in the epidermis layer, which is found in the mesophyll spongy layer of the leaf. They are found mainly in leaves, but also in stems.

There are two main functions of stomata

  1. To allow gaseous exchange of carbon dioxide and oxygen between the inside of a leaf and the surrounding atmosphere - When the stomata are open, carbon dioxide diffuses into the sub-stomata air chambers and then into the intercellular spaces between mesophyll cells. When it comes into contact with the wet surface of a cell it goes into solution and diffuses into the cytoplasm. Oxygen travels via the same route but the opposite way.
  2. To permit the escape of water vapour from the leaf – this is the evaporation of water vapour from spaces in the mesophyll cells of the leaf otherwise known as transpiration.

The diagram below shows a vertical section through a stoma.

Each stoma is bordered with two semicircular guard cells whose movements due to changes in water content, control the size of the stomata by changes in their turgidity.

 If water is drawn into the guard cells by osmosis the cells expand and their turgidity is increased. . But they do not expand uniformly in all directions. The thick, inelastic inner wall makes them bend. The result is that the inner walls of the two guard cells draw apart from each other and the pore opens creating the stoma. Stomata and changes in turgidity can be seen very clearly under an electron microscope. (A diagram of stomata under an electron microscope is presented later on in the assignment)

As noted before stomata activity affects the rate of transpiration, but now more specifically, it is the turgidity of stomata determines is the main cause of transpiration. It is thought that in normal circumstances when a stoma opens the turgidity of the guard cells is increased by their taking up water from the surrounding epidermal cells but it is also know that the turgidity of the stomata is also affected by external factors of the particular environment, such as light, wind, and humidity.

During the day stomata tend to be open, this is because the guard cells of the stoma become flaccid to light. This is important as it allows gaseous change of carbon dioxide and oxygen to take place for photosynthesis of the plant. This can be investigated by means of a perometer, an instrument for measuring the resistance to the flow of air through a leaf. If you attach a perometer to a leaf and take measurements of its resistance to airflow at intervals, you will find that there is a generally less resistance during daylight hours than at night. This is because the stomata open during the day and close at night.

For wind, in still air, a highly saturated air shield builds up around the stoma. Air movement will sweep this layer away, which decreases the humidity of the stomata therefore increasing transpiration also a xeromorphic feature of some leaves is the presence of sunken stomata, the stomata grooves into the epidermis which then a high humidity can build up inside the stoma and reduce transpiration rates. An increase in temperature also has an effect as the guard cells become more flaccid thus increasing the capacity of the stoma therefore increasing transpiration rates.

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A main factor that affects the distribution and densities of stomata is the type of plant. There are two main types of plants, monocotyledonous and dicotyledonous.

Monocotyledonous (monocot) or more modern liliidae, have leaves that have parallel veins and therefore do not grow to a large size. Also their leaves are held vertically rather than horizontally, which affect where the stomata is distributed and thus the densities, leafs of a moncotylic nature therefore have equal stomatal densities on both upper and lower epidermis.

Dicotyledonous (dicot) otherwise known as magnoliidae, has branched veins and therefore can grow very large leafs. ...

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