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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

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Introduction

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. ...read more.

Middle

This was done to enable the area of the leaf and the total number of stomata to be calculated. A graticule slide was used to measure the diameter of the field of view. This allowed the area of the field of view to be calculated using the formula: Area of a circle = ? r � Where ? is 3.142 (3dp) and r is the radius of the circle The number of stomata per mm� was then calculated. Finally the stomatal densities on the upper and lower epidermis were compared. Table 1: The time taken for the cobalt chloride paper to become fully hydrolysed on the upper epidermis. Rate of transpiration on the upper epidermis calculated using the formula: Rate of transpiration = 1/Time Co-ordinate Time (seconds) Average time (seconds) Rate of transpiration (sec��) Trial 1 Trial 2 1 3846 3922 3884 2.57 x 10� 2 3928 4100 4014 2.49 x 10� 3 3675 3615 3645 2.74 x 10� 4 3832 3682 3757 2.66 x 10� 5 3629 3793 3711 2.69 x 10� Average 3802 2.63 x 10� Table 2: The time taken for the cobalt chloride paper to become fully hydrolysed on the lower epidermis. Rate of transpiration on the lower epidermis calculated using the formula: Rate of transpiration = 1/Time Co-ordinate Time (seconds) Average time (seconds) Rate of transpiration (sec��) Trial 1 Trial 2 1 2940 3133 3037 3.29 x 10� 2 2620 2904 2762 3.62 x 10� 3 2961 2798 2880 3.47 x 10� 4 2866 2936 2901 3.45 x 10� 5 2983 2964 2794 3.36 x 10� Average 2875 3.44 x 10� Table 3: The number of stomata counted at five different fields of view at each of the five co-ordinates on the upper epidermis of the leaf Co-ordinate Readings Average 1 2 3 4 5 1 10 21 18 12 14 15 2 11 14 19 14 20 16 3 21 12 16 13 25 17 4 10 19 12 12 16 14 5 15 15 14 18 12 ...read more.

Conclusion

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 control the rate at which transpiration occurs. 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. 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. Xerophytes such as evergreens have a thick cuticle to reduce cuticular transpiration and are often shiny so they reflect sunlight causing leaf temperature and therefore transpiration to fall. Xerophytes like Ammophilia and Calluna can roll their leaves, which lengthens the path for diffusion and traps moist air. Ammophilia also have protective hairs on the surface of the leaf, which traps moist air. These results show that the plant is adapted to its environment because the upper epidermis has less stomata, which minimises water loss from the plant. The upper epidermis receives more sunlight, than the lower epidermis, during the day. This means that the upper epidermis is exposed to more light and higher temperatures than the lower epidermis. With the upper epidermis having less stomata, water loss is minimised. This shows that this xerophyte is well adapted to its environment. ...read more.

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