Leaves from Chinese Holly (Ilex cornuta) bushes were examined for their
stomata characteristics. The factor that was chosen to study was sunlight and
its effect on the stomatal density of the leaves. Leaves were observed under a microscope
and then the density of stomata was calculated based on the field of view. Data
was observed in SPSS and results were found. Leaves that were exposed to more
sunlight had higher stomatal density than leaves exposed to more shade. The
differences between these two treatments suggests that exposure to sunlight
causes more stomata and therefore the hypothesis was supported.
Throughout nature, different organisms have certain
adaptations that help them survive or be more suited to their environment. Plants
are included in these organisms and have a structure that is vital to
photosynthesis and maintaining homeostasis. Stomata, which are small pores on
leaves, are structures that control the different exchange of gases, including
water and carbon dioxide, between the atmosphere and the inside of the leaf
(Hetherington, 2003). Many factors can influence the stomatal density of leaves
including climate, elevation, and carbon dioxide levels (Casson, 2008). The
factor that was observed in this experiment was the effect of sunlight on
stomatal density. This experiment studied the stomatal density of the Chinese
Holly (Ilex cornuta) when exposed to
two different conditions, sunlight and shade. The question that was presented
was whether or not sunlight affected the stomatal density of leaves in the
Chinese Holly. After pondering this question, I was able to form two hypotheses.
The first was the null hypothesis or Ho and stated that sunlight had
no effect on the stomatal density in the bushes. The second was the alternate
hypothesis or Ha which stated that sunlight did have an effect on stomatal
density in the bushes. The prediction that I came up with is that when there is
an increase in sunlight, there will be an increase in stomatal density. Because
there is an increase in sunlight, there would be more of a need to exchange
gases at a faster rate and therefore there would be more stomata.
To do this lab, I went to East Campus at James Madison
University and collected 120 leaves from a Chinese Holly, 60 of which were in
the shade and 60 of which were in the sun. After I had collected my samples, I
prepared the leaves for the next step by coating them in clear nail polish.
This allowed me to get an imprint of the leaves, which I then transferred over
to a blank microscope slide using a piece of tape. Next, I used a microscope to
accurately count the number of stomata within my calculated field of view. Once
I calculated my density using my field of view as an area, I used SPSS to
calculate a t-value and a p-value, which showed that my data was normal, and to
create a graph.
Once the data was collected, I
entered it in to SPSS in order to graph a comparison between the different
treatments. The average density for the sun treatment was 207.04 and the
average density for the shade treatment was 184.35. Once the data was entered
in to SPSS, a Shapiro-Wilk test was run to test for normality. The data was
found to be normal and therefore an independent samples t-test was performed
which calculated a p-value of p<0.001 and a t-value of 4.518. Figure 1: histograms show the distribution of data between the two treatments. The data were normal (Shapiro-Wilk p=0.547 shade, p=0.107 sun), so an independent sample t-test was performed. Figure 2: the comparison of mean stomatal density of leaves in sun vs. shade. The data were normal (Shapiro-Wilk p=0.547 shade, p=0.107 sun), so an independent sample t-test was performed. The means test resulted in a t-value of 4.518 and a p-value of p<0.001. Discussion: After analyzing the results, it can be seen that there is a clear difference between the stomatal density of leaves exposed to greater amounts of sunlight and leaves that spend more time in the shade. As shown in figures 1 and 2, the data supports my hypothesis that sunlight effects the stomatal density. The data also supports my prediction that when there is an increase in sunlight, there will be an increase in stomatal density. The t and p-values allow me to reject the null hypothesis, that sunlight does not have an effect on stomatal density, because the p-value was less than 0.05. My results are conclusive with other studies done on this topic. When a study was done in Wisconsin, it was found that sun leaves of hardwoods had a greater stomatal density than the shade leaves (Abrams, 1990). For future research on this topic, further environmental conditions should be accounted for such as elevation, temperature and carbon dioxide levels. It has been observed in other studies that an increase in carbon dioxide levels creates a trend towards lower stomatal density (Casson, 2008). By taking this factor in to account, another study could be done to find if there is a correlation between carbon dioxide levels and temperature on stomatal density. This study went well overall with no errors. To conclude, after the study was over and all data was observed, the data is supportive of the alternate hypothesis and had a p-value that allowed for the rejection of the null hypothesis.