The discussion section allows for a complete evaluation of the experimental results presented in the previous section. The evaluation should include any deductions and conclusions drawn from the data. The hypotheses stated in the introduction should be reiterated with explanation of which hypothesis is supported (and which is rejected) and why. Interpretations of patterns or trends shown in the results should be proposed and justified. Any aberrations or unexpected results should be discussed with possible sources for the causes suggested. Comparisons of the results with those from the literature should be developed when possible.
Be careful about drawing conclusions which are not fully supported by the data. If there are other possible explanations for the results, at least acknowledge them. Any weaknesses in the experiment design (not procedural errors) should be related in this section. In addition, if there are obvious suggestions for changes to make the experiment better, those should be included in the discussion section.
A certain amount of restricted creativity is used in writing the discussion section. Implications of the results in light of the literature and other experiments may be delved into in this section. At the same time, the discussion should not extend too far and certainly not beyond the framework of the introduction. The challenge is to show how the results of this experiment relate to the bigger picture.
The discussion section is written in paragraph form and in past tense. An example of the discussion section is shown below:
Plants need light to grow. The light intensity available for both terrestrial and aquatic plants is variable (Mazumder, 1998). Water clarity is one factor determining the light intensity which reaches Elodea canadensis. In this study we have shown that water clarity varies between lakes in the University lake system and also between seasons for University Lake. We have also shown that photosynthesis, as measured by oxygen bubble production, in E. canadensis occurs in a range of light intensities. The hypothesis that reduced light levels will reduce photosynthesis is true at the more reduced light intensity tested (30 and 45 cm distances).
Elodea canadensis was able to photosynthesize at all light intensities studied. Higher intensities, where the lamp was closer, did not always lead to significant differences in the rates of photosynthesis. This may be because the rate of photosynthesis was limited by other factors than light once the lamp was within 15 cm. Other factors could include temperature or nutrient availability (McKee et al., 2002). Light becomes a limiting factor when the light source is moved further from the plants, as photosynthesis is decreased. Reduction in photosynthesis would make it more difficult for the plant to grow. Comparisons of rates of oxygen production and growth of E. canadensis would also tell us if oxygen production is a good predictor of growth. More replicates might reduce the standard deviation for all of the treatments. This may result in a significant difference being observed between the 0 and 15 cm distances.
Elodea canadensis could grow at deeper depths in the Campus Lake than in University Lake based on the water clarity. Also, it could grow in a larger range of depths in University Lake in the spring than in the fall. Many factors can influence water clarity, including plants. Invasive species of aquatic plants can so dominate an ecosystem that they can block most of the light (Masser, 2007). Some surface plants extend above the water, such as water lettuce or water hyacinth. These plants are aggressive and can grow to cover the surface and block light from even reaching the surface of the lakes (Masser, 2009). Both of these plants are present in the University Lake system. Large aquatic plants were not seen at the test sites, but algae were present as indicated by the green tint to the water and chlorophyll tests (data not shown).
Further tests could compare the light intensities in the photosynthesis experiment with light intensities at various water depths of the lakes. These measurements would help determine if light intensity in the lakes is limiting for photosynthesis by E. canadensis. This would determine if water clarity changes in the University Lake system would cause changes in light intensity that could limit E. canadensis photosynthesis and therefore limit growth.