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Investigation of the factors affecting the rate of transpiration from a shoot of Privet.

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Biology planning exercise: investigation of the factors affecting the rate of transpiration from a shoot of Privet Introduction Transpiration is the evaporation of water from a plant, primarily from the leaves. This is because the undersides of the leaves have many small holes, called stomata, which are necessary for the absorption of carbon dioxide. Inside each stoma is an air space, and the surface of all the cells bordering that are coated with a thin film of water, in order to maximise the absorption of carbon dioxide. This inevitably means that some of the water evaporates - this is called transpiration. I intend to measure the rate of transpiration in a privet shoot (see preliminary work). However, it would not be very easy to directly measure the amount of water lost directly - the only way would be to measure relative humidity with the plant in an enclosed space, but humidity affects the rate of transpiration. Therefore, I will have to measure the rate indirectly. To do this, I could measure the weight loss of a plant over a period of time, but then I would also have to take into account the amount of gasses exchanged, so the best way would be to measure the amount of water taken up. This can be done with a potometer. Key Factors Transpiration is affected by the following factors, which will have to be taken into account during the experiment: Relative Humidity - if the air if humid, it has more water potential, so there is less evaporation. ...read more.


I found that almost all of the stomata were on the underside of the leaf - could not see any on the top, and also that privet leaves had a stomatal density more than three times greater than that of laurel. That is why I decided to do this experiment with a privet shoot, because more stomata will mean more transpiration, and so the results can be more accurate (it is easier to accurately measure a large amount of anything than a small amount). I also considered that a high density of stomata suggests that privet has adapted to live in conditions where water is plentiful, this might suggest that a privet shoot would be less likely to close its stomata than a laurel shoot, however I have no evidence for this. Prediction I predict that as I increase the wind speed, the rate of transpiration will increase, until the rate becomes so great that the stomata close, (and the leaves blow off). This is because an increase in the wind speed, means faster removal of any moisture from just below the stomata, and so means that the gradient of water from inside the stomata to the outside is steeper, and therefore the rate of transpiration increases. However, I think I can go further than this by saying that the steepness of the water gradient is directly proportional to the wind speed, and the rate of evaporation is directly proportional to the steepness of the water gradient. ...read more.


This plan is valid as I am confident that I have taken into account all the factors that could affect the rate at which a plant takes up water. Water is used for transpiration, making the cells turgid, and for photosynthesis, it is produced by respiration. I have made sure in my plan that the plant is water stressed at no point, so all the cells would already be turgid. The factors that affect photosynthesis, light intensity, temperature and the concentration of carbon dioxide in the atmosphere should all remain constant, and the of the factors that affect transpiration, only the wind speed will change. Therefore, I am confident that any changes in the rate of transpiration can be attributed to the change in the wind speed. It is reliable only as long as you use a very similar piece of privet that had been subjected to similar conditions before the experiment. But with the same piece of privet, under the same conditions, providing the privet hasn't died, and the stomata haven't closed, then similar results should be obtained every time. 1 I know light intensity does from a previous experiment, in which I measured the rate at which oxygen was produced from Elodea when a lamp was positioned at certain distances from the plant. I found out that the rate of photosynthesis was inversely proportional to the square of the distance the lamp was from the plant, hence was directly proportional to the light intensity. I found out that temperature has an effect on photosynthesis in the Jones and Jones Biology Textbook. Hector Guinness 03/05/2007 ...read more.

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