Challenges to Anticipate and Solve:

       We have identified 5 challenges that commonly arise:

1. correlation and causation: Many students do not understand the fundamental difference between correlation and causation. In this lab, stomata density variation likely results from interacting environmental factors (e.g. CO₂, temperature, water, etc.); therefore, higher stomata density might be consistent with a student's hypothesis about causation, but students need to understand that this does not allow one to say anything like "the data prove our hypothesis." We suggest that instructors discuss inductive/ deductive reasoning and hypothesis testing in science.



2. falling behind in the first 15 minutes: Many students seem to stumble into class without much of a clue about what is happening in lab on the day the stomata lab begins. The instructor must engage these people immediately - which is why we begin the lab with a "call and response" activity (see below).



3. little sense of direction: Many students do not really understand what an environmental difference is from a plant's perspective - also, they have very little conception of which way is "NORTH" and the degree to which a particular shrub on campus is "sunny" or "shaded" over the extended daylight hours other than the instant of their observation. In other words, few students really understand the path of the sun in the sky across their campus. Instruction about this should be done outdoors - not in class.



4. statistics literacy: Most students have not seen the basic concepts of statistics we expect them to use in this experiment. The instructor must attend to these needs. For some non-majors or pre-college settings, it might be better to have students assess their differences in stomatal averages visually, perhaps using a more simplified short-cut, such as the two standard deviation rule.



5. presentation effects: Most students have never made a scientific presentation before. The instructor must attend to these needs, and in particular encourage supportive behavior by the peer learning community.

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Comments On the Lab Description:

Introducing the Lab to Your Students.

       We introduce this lab using a "call and response" activity in which we ask the students to list and explain factors that affect the photosynthetic rate of a leaf. Two categories quickly emerge - characteristics of the environment (sunlight, air temperature, humidity, water and CO₂ availability) and characteristics of the leaf (CO₂ uptake capacity through stomata, water balance, and leaf size, shape, color, orientation, evaporative water loss rate as these affect leaf temperature).

       We take an important moment to review a basic misconception found in many students. We point to a wooden table in the classroom and ask the students "where does most of the mass of this wooden table come from - the ground or the air?" Many say the ground, which then opens a discussion of what wood is (cellulose and lignin), what photosynthesis does (basic chemical equation to make sugar), and we remind them that the carbon that forms a huge component of the mass of wood, in fact comes from the air as CO₂.

       Next, we specifically guide the discussion to an explanation of the means of plant uptake of CO₂ through leaf stomata - what stomata are, briefly how they work, and we review their roles in gas exchange with their environment (CO₂ and O₂ exchange, and H₂0 loss). Stop and poll your students to recall from where the water comes that exits via the stomata (i.e., uptake at the roots). Next, we talk about principles of leaf design, and we project images of photos of different types of leaves (desert shrubs, tropical understory plants, etc.) to reinforce a few basic morphological themes in biophysical adaptation. Students are now thinking about leaf design and are ready to think about hypotheses that they can test with the plants on our campus.

Comments On the Activities in the Lab.

       Try to move the students toward a novel project, but advise against taking on too ambitious a variation since the rate limiting step is always the drudgery of making the slides and counting stomata. The key consideration is to not let anyone out the door to collect their plants who does not have a very clear idea of what they are doing. We take the students outside as the next thing after the lab introduction, and we (1) show them which way is north, (2) we walk to and examine plants on the north and south side of the biology building, (3) we walk to and examine a few leaves from the upper south and lower north sides of a thick leafy shrub nearby (to demonstrate "sun" and "shade" leaves), and we show them that in fact you can easily see the morphologial differences among plants in different environments on campus. Next, we leave the students outside with the explicit understanding that they will return to class in 15-30 minutes for a second consult session to finalize their hypothesis, sampling sites, and study species BEFORE collecting plants.

            DO NOT SUGGEST A PROJECT FOR THEM TO DO.

       Remind everybody which way is north - it never ceases to amaze us how disconnected many students are from basic compass directions; however, in our students' defense, we are all in an urban environment where there is little need to know this kind of thing (and in fact, a major interstate highway, I-95, officially runs "north" and "south" despite that it runs due east and west as it comprises the southern border of our campus).

       Another consideration is about the plants on campus. Some students may not know that it is less than a good idea to cut daffodil leaves from beneath the window of the President's office for their sample. Again, this reinforces the importance of the consult session with the instructor PRIOR to collecting plant samples.

       Advise the students to collect a few extra leaves for trials to see if the nail polish works - in our experience, some of the brands of nail polish chemically react with some of the epicuticular waxes of leaves (especially so for holly leaves). If so, the nail polish will not harden. We suggest having several brands of clear nail polish on hand, one of which will undoubtedly harden to make the impressions.

       Open the classroom windows, because nail polishes often contain some nasty smelling volatiles that should be ventilated. Circulate your Departmental policy on exposure to potentially toxic chemicals (this is especially important for students who have allergic sensitivities or who may be pregnant). If you do not have a written policy on this - get one!

       Instructors should review basic microscope techniques. Assume very little retention about the details of handling, focussing, etc., with the scopes from previous courses.

       Advise the students to select the magnification of choice based upon how many stomata they see. Flip to a higher or lower power so as to see about 20-60 stomata per field of view. More than 60 is too many to count, and fewer than 20 introduces too much variation.

       Remind the students to apply the nail polish to the leaf undersides where stomata are generally the most dense. Ask you students why this might be so? However, if any of your students happen to select a species of grass, advise them to first find out which side has the most stomata, since some grasses show the reverse pattern (for which during development the grass blade flips over and grows upside down). This may also prove problematic with conifers and some monocots for which no clear "leaf underside" is apparent. The rule is to check first.

       Remind the students to avoid leaf veins when selecting areas to count stomata.

       Instructors will have to demonstrate the use of a stage micrometer to estimate the size in mm² of the field of view at various magnifications. Alternatively, the conversions could be worked out beforehand and averages determined for the microscope brands in your lab.

       A great deal of very interesting and important statistical content has been omitted from the handout in Appendix 1. For example, there is no discussion of assumptions of normality, non-parametric tests, type 1 vs. type 2 errors, degrees of freedom, or anything about how the t-test critical values are calculated. Put this and other content back for some or all of your students at your discretion.

       During the Stomata Symposium it is absolutely critical that the instructors abstain from dominance; in fact, we play only observer/ discussion moderator/ and time-keeper roles. Let the students who are presenting pick who ask questions, and let them provide their answers, too. And, let the students ask the questions. If you must, ask your ONE question last. Reward your students for asking good questions and make them aware that a reward is available beforehand (see our comments on assessment below). Jump in to encourage the more silent students to engage in the discussion; especially women and minority students. Step in quickly if a pair of presenters too quickly dismisses a valid question. And, react even more quickly to gender-bias or other effects on this type of dismissal. The basic mandate to the instructor is to create a safe classroom and facilitate the interaction among students to become a community of learners, which is what scientists are supposed to do.

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Comments On Questions for Further Thought:

Comments on the Question. How exactly do stomata open and close?...

      Comment for instructors: ...this is right out of their textbook - beware of plagiarism.


Comments on the Question. Why do plants vary stomata number and not stomata size? ...

      Comment for instructors: ...what you are looking for here is not necessarily a single correct answer but rather evidence of research and good ecological and scientific thinking. This is a complicated problem, and stomata morphology and function are active areas of research in plant physiology.


Comments on the Question. Why might it be adaptive for stomata to occur mostly... on the undersides of leaves?...

      Comment for instructors: ...part of this is right out of their textbook, but they will have to seek outside information on plants with unusual stomata configurations (such as some grasses, pond lillies, etc.) to complete their answers.


Comments on the Question. Photosynthetic and stomatal adaptations of cacti...

      Comment for instructors: ...part of this is right out of their textbook, but they will have to seek outside information on CAM plants such as cacti to complete their answers.


Comments on the Questions. Leaf design and stomatal behavioral adaptations of mesic vs. xeric plants...

      Comment for instructors: ...part of this is right out of their textbook, but they will have to seek outside information to complete their answers.


Comments on the Questions. How might stomata density serve as a bioindicator... of climate change... and of paleoclimates...

      Comment for instructors: ......what you are looking for here is not necessarily a single correct answer but rather evidence of research and good ecological and scientific thinking. This is a complicated problem, and is an area of research in plant physiology. There are several very good references on stomata and climate change research in the Description: References and Links section.


Comments on the Question. In bryophytes, why might it be adaptive for only the sporophyte and not the gametophyte stage in the life cycle to possess stomata?

      Comment for instructors: ......what you are looking for here is not necessarily a single correct answer but rather evidence of research and good ecological and scientific thinking. This is a complicated problem, and frankly, we have not heard a definitive explanation of this, yet.


Comments on the Question. How might stomata density be affected along the urban - rural gradient...

      Comment for instructors: ......what you are looking for here is not necessarily a single correct answer but rather evidence of research and good ecological and scientific thinking.


Comments on the Question. What are some of the specific adaptations possessed by plant parasites to gain access to leaf tissue...

      Comment for instructors: ......what you are looking for here is not necessarily a single correct answer but rather evidence of research and good ecological and scientific thinking. This is an interesting ecological problem, and is of great economic importance to our agricultural ecosystems.


Comments on the Question. What role do stomata play in the solution to the problem of getting water up to the leaves...

      Comment for instructors: ......part of this is a classic problem right out of their textbook, but they will have to seek outside information to complete their answers.

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Comments On the Assessment of Student Learning Outcomes:

       Extensive notes and tools on Assessment of Student Outcomes are in the Description section of this lab under Tools for Assessment of Student Learning Outcomes.

       In addition, we have included extensive scoring rubrics for every assessment component of this activity in Scoring Rubrics for Assessment of Student Learning Outcomes

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Comments On the Evaluation of the Lab Activity:

       Please examine the specific suggestions for evaluation tools that are in the "Description: Tools for Formative Evaluation" page.

      In addition, extensive notes on how to conduct a formative evaluation are in the Teaching Resources sector of TIEE in an "ESSAY ON TEACHING EVALUATION."

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Comments On Translating the Activity to Other Institutional Scales:

       This activity is HIGHLY TRANSFERABLE. It could be modified to run in any undergraduate introductory biology lab course (major or non-major) at any college or university. It also could run in introductory lab courses in botany, ecology, environmental science, or upper division courses in a variety of sub-disciplines. It could also be modified to run in biology lab courses in grades 8-12.

       It would be difficult, however, to run this lab in a single three hour lab period. A key facet of this inquiry-based activity is the students' ownership of their question, sampling activity, analysis, interpretation, and presentation.

       One aspect of this activity that also is HIGHLY TRANSFERABLE is the assessment scheme - please feel free to modify the scoring rubrics presented here for use in your laboratory activities.