Comparing number of stomata on the upper and lower epidermis of a xerophyte and its effect on the rate of transpiration

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Comparing number of stomata on the upper and lower epidermis of a xerophyte and its effect on the rate of transpiration

Introduction: 

This coursework aims to compare the number of stomata on the upper and lower epidermis of a xerophyte and its effect on the rate of transpiration.

Transpiration is the loss of water from a plant. This loss of water can occur through the cuticle and lenticels, but mainly through the stomata. Transpiration is necessary for transporting nutrients, cooling the plant, moving sugar and chemicals, and keeping upward water pressure. Water is pulled into the roots because of evaporation and hydrogen bonding. Evaporation then pulls on this chain of molecules.

The rate of transpiration is dependent on the size of the stomatal aperture and the diffusion gradient between the leaf and the atmosphere.

The internal factors of the plant, which affect the rate of transpiration, include surface area of the leaf, thickness of the cuticle and stomatal density. The larger surface area, the higher the rate of transpiration. A thick cuticle reduces the rate of cuticular transpiration. The greater the number of stomata per unit area of leaf, the greater the rate of transpiration.

Stomata are found in leaves and herbaceous stems. Stomata control the rate at which transpiration occurs. Specialised epidermal cells called guard cells gain and lose water, which close and open the stomata respectively. When the water pressure in the guard cells becomes greater than in the surrounding cells the stomata open allowing transpiration. Almost 90% of water contained in a plant is lost through transpiration.

The frequency of stomata varies with environment and species. They are generally more numerous on the underside of leaves than on the upper side. This is because the waxy cuticle of the Casparian strip on the upper areas of the leaf prevents water loss easily. Whereas the lower parts of the leaf consists of the spongy mesophyll layer containing air spaces, through which water can pass through easily and be lost as water vapour. Stomata usually open in the light and close in the dark.  

Xerophytes are plants adapted to survive in dry conditions of unfavourable water balance. They have the effect of reducing the rate of transpiration to conserve water. Xeromorphic adaptations include thicker cuticles on leaves and stems, reduction in the size of leaves, curling or rolling of the leaves into a cylindrical shape, presence of epidermal hairs, the number and distribution of the stomata in pits or grooves.

Hypothesis: 

        

Incorporating all the background information, there will be a positive correlation between the stomatal density and the rate of transpiration. Therefore, a greater stomatal density will have a greater rate of transpiration. The area in which the stomatal density is low will be the area in which the rate of transpiration will be at its lowest.

Null Hypothesis:

There will no correlation between the number of stomata and the rate of transpiration.

Key Variables:

        

Factors affecting the rate of transpiration include humidity, wind speed, temperature and light. These four factors must be controlled in order to gain accurate results.

The lower humidity outside the plant, the steeper the diffusion gradient between the leaf and the atmosphere. Transpiration is faster the lower the humidity of the atmosphere. Keeping all windows and doors closed throughout the duration of the experiment will control the humidity.

An increase in wind speed normally increases the rate of transpiration since saturated air is blown away from the stomatal pore and a diffusion gradient is maintained. This is partially counteracted by the cooling effect of wind. Again, all windows and doors will be kept closed.

A simultaneous increase in the temperature of the air and the leaf results in an increase in vapour pressure of the leaf increases so the diffusion gradient and transpiration increases. The temperature will be kept constant by carrying out experiments in the same room at room temperature.

The size of the stomatal aperture is controlled by light. As stomata open during the day, the rate of transpiration increases, decreasing at night when the stomata close. The experiments will be done in one place only, where the same amount of light will reach the plant at all times.

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The size of the cobalt chloride paper will be the same for each of the co-ordinates, 2cm long by 1cm wide. This will make a fair test for observing a colour change from blue to pink.

The co-ordinates for sampling will be chosen at random using the random button on the calculator. To further make this a fair test, the same co-ordinates used on the upper epidermis will be used on the lower epidermis.

Proposed Method: 

        

The rate of transpiration will be measured first using a strip of anhydrous cobalt chloride paper. This ...

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