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Investigation06 en

  1. 1. Biology teacher support material Investigation 6 Comparing leaf adaptations in two groups of Ilex aquifolium in terms of number of prickles per leaf Background A leaf's form and function reflects the environment in which that plant lives in, for plants tend to adapt to their habitat. Leaf adaptations happen in terms of competition and environmental conditions. Temperature, light intensity, carbon dioxide availability, humidity, water availability among others can act as limiting factors in the growth and development of plants by influencing the rate of cell division, cell metabolism and photosynthesis. It is for this reason that the same plants are not found in all geographical ranges. Plants tend to modify their features to guarantee survival; such modifications can be carried through genes leading to successful evolution. Surface area has a great impact on a leaf's activities because the main substances needed to carry out important reactions such as photosynthesis and cellular respiration. Substances need to diffuse into and out of the cell The larger the surface area, therefore, the greater the amounts of substances that enter and leave the cell, and at a higher rate: diffusion is more effective. Similarly, the surface area has an impact on capturing energy from the sun, which is vital for cell functioning. Aim To compare the number of prickles per leaf on branches of Ilex aquifolium coming from the east side and west side of a holly tree as an indication of leaf adaptations. Hypotheses The leaves on the branches on the eastern side of the Ilex aquifolium bush receive sunlight from the time when the sun rises until noon; the leaves on the branches on the western side of the tree receive sunlight from noon until the sun sets. Since the time period for receiving sunlight is almost the same for both sides of the tree there would be no need for special adaptations in either the east or the west. Thus, the average number of prickles in both wood sectors of the tree would be the same. If there are no limitations for either the west or east leaves then no special surface area adaptations are required and so the average number of prickles for each should be equal HO: There is no significant difference between the number of prickles in the leaves from the east and the west sides of the Ilex aquifolium tree. HA: There is a significant difference between the number of prickles per leaf in Ihe east and west sides of the tree. Variables -Independent: location of the branch when attached to the bush: east or west. -Dependent: Number of prickles per leaf. -Controlled: species of the tree and the tree itself. © International Baccalaureate Organization 2007
  2. 2. Biology teacher support material Investigation 6 Materials 5 branches of Ilex aquifolium from the east side of the tree 5 branches of Ilex aquifolium from the west side of the tree Paper Pen Calculator Procedure 1.Separate the branches from the east side from those from the west side to make sure they do not get mixed. 2. Label each type 'east' and 'west' to identify them. 3. Notice any striking feature that may differentiate one side form the other and record the observations. 4. Draw a label on the piece of paper to collect the data, separating the east side from the west side. 5. Collect the data: count the number of prickles per leaf for each branch and record the information in the table. Do this for the five branches for each side. 6. Study the average number of prickles per leaf on each side. 7. Study if there is a significant difference between the number of prickles of the two populations using the t-test. © International Baccalaureate Organization 2007
  3. 3. Biology teacher support material Investigation 6 Raw data Table 1 © International Baccalaureate Organization 2007
  4. 4. Biology teacher support material Investigation 6 Graphs Graph 1: Frequency distribution of the number of prickles per leaf in the west side. Frequency distribution of the number of prickles per leaf in the west side 0 10 20 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Number of prickles per leaf Frequency Graph 2: Frequency distribution of the number of prickles per leaf in the east side. Frequency distribution of the number of prickles per leaf in the east side 0 10 20 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Number of prickles per leaf Frequency © International Baccalaureate Organization 2007
  5. 5. Biology teacher support material Investigation 6 Data Processing Table 2: The student's t-test Student's t-test Frequency X-Mean (X - Mean2 ) Prickles West East West East West East 1 3 0 -7.8 60.84 2 1 1 -6.8 -8.65 46.24 74.82 3 7 1 -5.8 -7.65 33.64 58.52 4 5 4 -4.8 -6.65 23.04 44.22 5 10 3 -3.8 -5.65 14.44 31.92 6 6 9 -2.8 -4.65 7.84 21.62 7 10 8 -1.8 ·3.65 3.24 13.32 8 15 5 -0.8 -2.65 0.64 7.02 9 8 16 0.2 -1.65 0.04 2.72 10 20 15 1.2 -0.65 1.44 0.42 11 15 25 2.2 0.35 4.84 0.12 12 13 16 3.2 1.35 10.24 1.82 13 4 10 4.2 2.35 17.64 5.52 14 7 11 5.2 3.35 27.04 11.22 15 2 9 6.2 4.35 38.44 18.92 16 2 7 7.2 5.35 51.84 28.62 17 0 1 6.35 40.32 18 0 1 7.35 54.02 Total 128 142 Mean 8.80 10.65 © International Baccalaureate Organization 2007
  6. 6. Biology teacher support material Investigation 6 West Side (X - mean)2 =1488.12 Variance = 1488.12 / n - 1, where n is the number of observations, 128 Variance = 11.72 Variance/n =0.092 East Side (X - mean)2 =1467.74 Variance = 1467.74 / n – 1, where n is the number of observations. 142 Variance =10.41 Variance/n = 0.073 Difference between the means = 10.65 - 8.80 = 1.85 Calculating the t value west west east east n Variance n Variance meansbetweenthedifference t + = 073.0092.0 85.1 + =t t = 4.58 Degrees of freedom = neast + nwest - 2 = 268 The critical t value at 268 degrees of freedom and when P = 0.05 = 1.64 © International Baccalaureate Organization 2007
  7. 7. Biology teacher support material Investigation 6 Results The critical t value for 268 degrees of freedom at P = 0.05 is less than the t value found 1.64 4.58. As t is greater than the critical value, the null hypothesis is rejected and it is accepted that there is a significant difference between the number of prickles per leaf at the east and west sides of an Ilex aquifolium tree. The mean number of prickles per leaf on the west side of the tree (8.80) was less than the mean for the east side (10.65). The t value measures the amount of overlap between sets of data -east and west-; a t value greater than the critical value proves that the difference between the means is significant. Conclusion and Evaluation The t value measures the amount of overlap between two sets of data. The larger the t value, compared to the critical value, the more certain it is that there is a significant difference between the two populations. In this case, the t-test shows that the number of prickles per leaf on the east side of the Ilex aquifolium tree is significantly different to the number of prickles per leaf on the west side of the same tree. Considering that prickles are a leaf's adaptation to the conditions of the environment and that the eastern and western leaves seem to adapt in the different way. It can be concluded that the conditions are different. This, however, can be linked to the availability of many abiotic factors and it is hard to define precisely. In general, a higher number of prickles increases surface area, and facilitates diffusion of carbon dioxide and oxygen and, above all, the capturing of solar energy for photosynthesis. The eastern side leaves showed a greater number of prickles, and thus a greater surface area. This can be explained as follows: during day time there is more sunlight available. Plants need to take advantage of the conditions to carry out their metabolic processes. A higher surface area allows the plants to be fully efficient because it allows them to use as much energy from the sun as if is available. Having a greater number of prickles is a modification that shows how plants adapt to their environment to guarantee survival. Similarly, the leaves on the western side of the tree have a smaller number of prickles because the conditions do not require them to increase surface area. Yet this experiment limits itself to compare the number of prickles per leaf on each side of the tree; it does not provide an explanation for the observed data. Its biggest limitation, therefore, is that it does not directly account for the effect abiotic factors have on the adaptations of the tree. Further investigations could provide an insight and confirmation to the results here obtained. Nonetheless, it is important to consider the amount of error involved in this experiment. Five branches per side were taken as representatives of the whole tree, a larger sample would have been needed to give more accurate results and reach more asserted conclusions. Also, collection error should be accounted for: the data was collected by counting each prickle and there is a chance that miscounting occurred. This could have affected the results in some way. In addition the error introduced by the t-test has to be taken into consideration. Figures need to be rounded up when worked with and this leads to a loss in accuracy. Leaf adaptations were considered mainly in relation to light availability, but other factors that play a role in leaf adaptations, such as carbon dioxide availability, were not taken into consideration. The study of the effect of other abiotic factors and limitations in leaf distribution of the same species would complement this investigation. For example, the rate of photosynthesis could be measured for both sides to compare their efficiency. Also, for further investigations it is suggested that a more precise sample is studied; it could be either a larger sample or one that is a representative as possible of the whole tree. Also, the results found could © International Baccalaureate Organization 2007
  8. 8. Biology teacher support material Investigation 6 be compared to those of other trees of the same species, to determine if the modification in the leaves is an adaptation of this tree or of the species in general. Sources Green, Stout Taylor, Biological Science 2: Systems, Maintenance and Change. Great Britain: Cambridge University Press, 1989. The Leaf. Published at: www.encarta.msn.com. Last modification: 2003. © International Baccalaureate Organization 2007

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