Report On Biostatistics Lab Report: Part II

Abstract

The main aim of the study was to determine whether the environmental factors have an impact on the morphology of the plant. The morphological structures of concern are the leaves in terms of their area, perimeter, and perimeter to area ratios. The hypotheses for the study were: There is no significant difference in sizes of the leaves exposed to sun and those exposed to shade, no significant difference in perimeters of the leaves exposed to sun and those exposed to shade, no significant difference in ratios of perimeter to area of sun exposed and shade exposed leaves. The hypotheses were tested using Student’s t-test. From the data collected and analyzed it was found that the leaves sizes (areas) differ based on the environmental factors, in this case light intensity, with those in sunny conditions having their leaves smaller than those in “shady” conditions.

Introduction

The differences in environmental factors, biotic and abiotic factors, greatly affect the lives of fauna and flora. Some of the ecological factors which exert pressure on plants include very extreme temperatures, light intensities, salinity, humidity, and air pressure. It is as a result of this that plants and animals have undergone modifications so as to be able to cope with the environmental factor variability, a phenomenon known as phenotypic plasticity (Gratani, 2014). Plants respond to changes in these factors by developing changes in their body structures such as the leaves and stems in order to survive.
The data collected and processed in the study will be used to test the three hypotheses. The first hypothesis is: There is no significant difference in perimeters of the leaves exposed to sun and those exposed to shade. It is predicted that if are leaves exposed to shade then they will have larger leave areas. The second hypothesis is the perimeter to area ratio of leaves exposed to sun and shade show no marked differences. For this, the prediction is that if the leaves are exposed to sun, they will show no marked differences from the ones exposed to shade. The last hypothesis tested is leaves exposed to sunny conditions have the same perimeter as those exposed to sunny conditions. The prediction is that if tree leaves are exposed to sunny conditions, they will result in smaller perimeters.

Materials and Methods

Red oak leaves that were exposed to full during the day and shady leaves which were not exposed to full sun at any given time of the day were collected by systematic random sampling to make sure that each leave had equal chances of being represented in a sample. This thus implies that the leaves collected are ‘mirror images’ of the population and hence we can generalize from samples to population in this study. However, unusual leaves were left uncollected. This included deformed as well as leaves that were already exposed to pathogens or those devoured partly by primary consumers. All the healthy leaves were then collected, placed in zip-lock bags and taken to lab, pressed inside text books for 24 hours and later images were taken using digital cameras. The digital images were then processed using image j software and data output was then analyzed using student’s t-test.

Results

T-test for area
T-test for perimeter
In the second table shown above, the calculated t Value is -5.51691, the degrees of freedom (df) are 23, the critical t Value from the t Table (at P=0.05) is 2.06 38, and the P-value, on the other hand is 1.31E-05. Because tcalc < tcrit (-5.51691 < 2.068658), the null hypothesis is not rejected at the 95% level of confidence [100% × (1 - α)]. This therefore implies that the leaves exposed to different environmental conditions (shade and sun) have no significant differences in perimeter.

T-test for P/A

In the third table above, the calculated t Value is -8.55561, the degrees of freedom (df) are 16, the critical t Value from the t Table (at P=0.05) is 2.1199, and the P-value, on the other hand is 2.3 E-07. Because tcalc < tcrit -8.55561< 2.119905), the null hypothesis is not rejected at the 95% level of confidence [100% × (1 - α)]. This therefore implies that the leaves exposed to different environmental conditions (shade and sun) have no significant differences in perimeter to area ratios.
The 2D bar graphs with error bars representing SD for perimeter(P), area(A) and perimeter to area ratio ( P:A) of sun versus shade leaves are as shown below:

Discussion (and Conclusion)

In conclusion, the study has shown that the leaf sizes vary according to the environmental conditions e.g. light intensity. The study has also revealed that leaves in ‘shady’ environments are larger in size than those in environments of high light intensity. This is because of exposure to low light intensity which in turn limits the photosynthesis. The plant leaves adjust to this by increasing in area (sizes). To compensate for this, the plants in these shady ecosystems develop large leaves to increase the number of stomata in their surfaces and thus lead to increase loss of water through the process of transpiration. On the other hand, those in sunny environments have reduced leave areas (sizes) as they have maximum exposure to light hence photosynthesis proceeds at the normal rates. In addition, smaller leaves have less number of stomata thus furthermore reducing the rate of water loss. The plant will then be able to conserve water for its physiological processes such as fertilization and photosynthesis.
The findings of this study are in agreements with similar studies. For example, a study by Lei and Lewcowicz (1998) found out that seedlings grown under low light intensities (cloudy skies) had higher specific leaf mass and areas. It is also consistent with the findings of Jones and Thomas (2006) which showed that leaves at the lower canopies (with less exposure to light) were larger in size (and mass) as compared to those at upper canopies (with higher exposure to light) of Acer saccharum trees. This was further evidenced by the works of Posada et al (2012) who found similar findings. This thus implies one of the plants strategies to counter the effect of low light intensity is the increase in leave specific mass. The increase in mass in turn increases the stomata, the chlorophyll content, and also the number of stomata in both surfaces of the leaves hence normalizing the photosynthetic rates.

References

Ashton, P.M.S. Yoon, H.S., Thadani, R., & Berlyn, G.P. (1999). Seedling Leaf Structure of New England Maples (Acer) in Relation to Light Environment. Forest Science, 45(4), 512-519.
Gratani, L. (2014). Plant phenotypic plasticity in response to environmental factors. Retrieved from: http://www.hindawi.com/journals/abot/,
Lei, T.T., & Lechowicz, M.J. (1998). Diverse Responses of Maple Saplings to Forest Light Regimes. Annals of Botany, 82, 9-19.
Jones. T.A., & Thomas, S.C. (2007). Leaf-level acclimation to gap creation in mature Acer saccharum trees. Tree Physiology, 27, 281–290.
Posada, J.M., Sievanen, R., Messier, C., Perttunen, J., Nikinmaa, E., & Lechowicz, M.J. (2012). Contributions of leaf photosynthetic capacity, leaf angle and self-shading to the maximization of net photosynthesis in Acer saccharum: a modelling assessment. Annals of Botany, 110, 731–741.