BIO 344 – Stomatal Density InvestigationPage 1

BIO / GES 344 – Ecology

Lab Exercise 1 – Control of Stomatal Density by Environmental Factors

During Tuesday’s lab, you collected data to answer the question about whether leaves exposed to full sun show a different stomatal density than those that develop in the shade. The data have been posted to the course website, and are available for you to download.

You are being asked to download the data, analyze the information, and prepare a report that you will submit for grading Monday, 6 October. The report will be worth 25 points.

To do:

Download the Excel workbook containing the data posted to the Internet. While a link has been provided on the course webpage (now at the spreadsheet can be found directly at

You will notice that the Excel workbook contains five worksheets, each containing data for a single species. Each sheet contains eight columns of data. The first four pertain to leaves that developed in the sun, while the last four pertain to those that developed in the shade. In each set, the columns contain information about the number of stomata, the length in cm, the width in cm, and the area in squared cm. Most of the columns contain 36 values. No statistics are provided on the table; that will be for you to do.

You are to analyze the data collected to determine whether: (1) sun leaves have different surface area than do shade leaves – and if so which is higher, (2) sun leaves have different stomatal density than do shade leaves – and if so which is higher, and (3) you can discern any relationship between surface area and stomatal density.

Note that your analysis can be approached in a few different ways.

  1. You can compare all sun plants to all shade plants.
  1. You can compare data for each species.

In your analysis, you will need to draw upon your knowledge of statistics and skills in spreadsheet manipulations. At the very least, you will be expected to calculate means for each columns. You can then compare various populations using appropriate statistical tests (t-test or even analysis of variance).

Report:

Provide a report that consists of:

  1. An Introduction that explains the concept of stomatal density, and how light intensity might affect it. Outline the two hypotheses.
  1. A Methods section that reviews the procedure that the class performed.
  1. A Results section that contains two or more tables that denote the means and standard deviations for each species on each site (i.e., each column), the results of any statistical tests you performed, and a narrative (in sentence / paragraph form) that describes the main trends that you observed.
  1. A Discussion that relates the data and statistical tests to the hypotheses. Do the data support either hypothesis? What assumptions did you make to carry out this investigation (hint, think about the manner in which plants were collected and made available to you)? Also, reflect on the methods used to test the hypotheses. What changes would you suggest to improve this assessment?

As you prepare your analyses and report, please keep the following in mind:

  1. Previous classes of BIO 344 students did this investigation, and the results are posted to the course webpage (though the formats might be different). You might want to include the results of those investigations to provide some comparison. Are the results for this year more or less clear-cut than those for previous years?
  1. As noted, this investigation has been published on the Ecological Society of America’s Teaching Issues and Experiments in Ecology (TIEE) website ( It has thus been performed at other universities. Data may exist elsewhere on the Internet.
  1. Since everybody in the class is working with the same dataset, it is important that each student do their work as independently as possible. I will be especially alert to unique data analyses to show your ability to think independently.