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Read pages 106-109, 624, 628-629, 642-649 in your text (Reece et. al., 2009. Biology concepts and connections. 7th ed. Pearson Education, San Francisco.). Answer the following questions in your personal electronic lab notebook (don't post answers to your lab section notebook) prior to attending lab during the week of 19 September.
The following experiments is derived from Grant, B.W. and Vatnick, I. 2004. Environmental correlates of leaf stomata density. Teaching Issues and Experiments in Ecology. Vol. 1.
By the end of this two-week lab, students should be able to:
|Lab 1||Introduction to stomata and methods for study|
Class defines a question to answer
Groups design a study for how to collect leaf samples
Groups prepare stomata impressions for next week
|Lab 2||Identify and count stomata
Organize data, graph, and use statistics to interpret data
|Lab 3||Stomata report due|
Your groups will define a testable question to investigate dealing with the effect of an environmental variable on the density of stomata in Mountain Laurel leaves. Each group will determine how to study the question, select leaves to collect, count stomata, and interpret the results. The methods for how to obtain leaf impressions are included in this handout.
Your group should answer the following before collecting leaves. Discuss any questions you have with your instructor:
How will you collect leaves to answer the question? The most important thing to think about is how you will try and keep all variables the same except the one that is being tested. From where on the tree will you collect leaves? Note that it is recommended that you should collect and measure at least 12 different leaves (replicates) from each treatment type. Why are replicates important?
What is your hypothesis (prediction) for the outcome of your study? Try to come up with more than one. Explain your reasoning for each. Draw one clearly labeled figure of hypothetical data that would support one of your hypotheses (i.e. what would a graph look like if one of your hypotheses were supported?).
Each lab section has been assigned to collect leaves from a specific patch of Mountain Laurel on campus. Please review the images below to determine where your lab section (e.g., A, B, C, D, F, G, or H) should obtain their samples.
Figure 1. Aerial photograph of the WSU campus with the locations of Mountain Laurel indicated by light green blocks and the assigned sampling patches for each lab section identified in red. We are currently in W215 denoted by the red X.
Figure 2. Topographic map of the WSU campus with the locations of Mountain Laurel indicated by light green blocks and the assigned sampling patches for each lab section identified in red. We are currently in W215 denoted by the red X.
Leaf stomata are the principal means of gas exchange in vascular plants. Stomata are small pores, typically on the undersides of leaves, that are opened or closed under the control of cells called guard cells. When open, stomata allow CO2 to enter the leaf for synthesis of glucose (sugar), and also allow for water, H2O, and free oxygen, O2, to escape.
Stomata viewed at 400x in nail polish impression from leaf underside. © Marc Brodkin. 2000.
In addition to opening and closing the stomata (stomata behavior), plants may exert control over gas exchange by varying stomata density in new leaves when they are produced (such as in the spring or summer). The more stomata per unit area (stomata density) the more CO2 can be taken up, and the more water can be released. Thus, higher stomata density can increase a plant’s potential for behavioral control over water loss rate and CO2 uptake.
But why, you might ask, might it be beneficial (or adaptive) for a plant to control its rates of water loss and CO2 uptake? One answer can be found in the sun. The plant cell "machinery" that is responsible for photosynthesis functions well over a narrow range of temperatures. When heated, cytochromes, pigments, and membranes critical to the process of photosynthesis denature (i.e., they cook). To avoid this, an individual plant may open its stomata, allowing water to evaporate, which lowers leaf temperature. Thus, one may hypothesize that leaves in the sun should have higher stomata density than do leaves in the shade - all else being equal.
On the other hand, if water is not available, such as under drought conditions, excessive evaporation might lead to desiccation (i.e., drying out) and an equally severe disruption of photosynthetic function. Thus, one might expect plant leaves exposed to drought conditions to have fewer stomata in sunlit environments.
The above discussion illustrates a very important concept in experimental biology – there are often alternative hypotheses to explain variation in nature. In this case, stomata density may increase or decrease in response to environmental variation in sunlight and water availability.
Note that since you will not be measuring sunlight or water availability you should use caution in how you word your acceptance or rejection of your hypothesis for your plants.
Remove a leaf from a tree/shrub and place it into a labeled paper bag. Write down as much information about your samples as you think is important. Consider the following:
* Your instructor will demonstrate the use of a stage micrometer so that you may convert your data from units of stomata number per field of view at 100x to units of stomata per mm2.
Your instructor will introduce some very basic analysis techniques to be used in the analysis of the stomatal density data. Please read the resources in the navigation bar at the left (e.g., R Commander, Graphing with Excel, and Computing with Excel) to get an overview of data analysis techniques. Also, please "Statistics with stomata data".
A title should convey to your audience both the general topic of interest that you will address as well as something specific about your particular research.
Provide a brief description of the context of the lab project. What are you studying, how do stomata work, etc. This can be about a paragraph for this project and you can cite this handout as a reference.
Be sure to include the following:
Provide a description of how you went about answering your question. NOTE: You don’t need to repeat the stomata methods handout, but you must refer to it. Provide a description of how you addressed your group's particular question.
This section should be a written description of the data that you documented. You should provide mean stomata density for each of your treatment groups as well as present the variation in the data. Include at least one table of data and one graph of your results, each having its own figure caption. Be sure to label all axes and include units on your graph. All of this should be computer generated. NOTE: This is NOT the place to interpret the meaning of your data. Save interpretation for the discussion section.
This is the place to interpret the meaning of your results. Write this in paragraph form and address the following:
Double space the text and use 12 point font. How long should it be? Enough to fully address the questions and communicate your findings thoroughly. You will have to make a judgment about what constitutes appropriate effort and quality, however 2 pages of text would be meager, 10 pages would be too much!
A = Exceptional report, all requirements of a “B” are met and:
B = Good report, all components are present.
C = Fair report, all or most components are present but errors occur in a few.
D = Incomplete or poor report, several components are missing, have substantial errors, and/or are not acceptable.
F = Absent or unacceptable report.
NOTE: Pluses and minuses are used to modify the above rubric.