Growing a Thicker Skin: A Borrowed Lab Activity

Skimming through the August 2015 issue of The American Biology TeacherI found a lab activity that I am eager to try. It’s by Troy R. Nash, Suann Yang, and John C. Inman of Presbyterian College, and it’s called “Growing a Thicker Skin: An Exercise for Measuring Organismal Adaptations to Terrestrial Habitats.” It caught my attention for several reasons: It asks students to apply the scientific method, use microscopes, and create graphs to evaluate how different environments select for adaptive traits. Also, the main part of the activity focuses on plants, and I’m always looking for ways to make my lab less “animal-centric.”

Here’s the basic idea. Each group receives microscope slides containing five tissue samples: three leaf cross sections and two animal skin cross sections. Each plant and each animal is adapted to a different environment (dry, wet, or temperate).

Students use their microscopes to observe the specimens. They sketch how each specimen appears at different magnifications and compare the appearance of the three plant specimens and two animal specimens. Each group then formulates a testable hypothesis about how the environment has selected for adaptations in plant and animal morphology.

In the Nash et al. article, students first view the slides with their microscopes but then test their hypotheses by measuring pre-printed photomicrographs with a ruler. Each group receives six photographs of each leaf type, at both 100x and 400x magnification. They measure the thickness of the “outer layer” (this vague instruction encourages creativity) of four leaves for each type and use Excel to graph their data. Finally, groups test for statistical significance using an online ANOVA calculator, and then they draw their conclusions. The article even provides helpful worksheets, a rubric, and suggestions for extending the activity.

I haven’t used this activity in my lab yet, but I had my student assistant, Lauren, try it out as a substitute for our current dull-as-dishwater “How to Use the Microscope” lab. Students are already learning to view cells and measure a field of view in that lab; why not have them test hypotheses and learn about natural selection at the same time? The existing lab even has them look at cross sections of human skin (e.g., Carolina Biological Supply Item #314522), so I already had some of the slides I needed.

For Lauren to try out the lab, I first purchased frog skin slides (Carolina Biological Supply Item #314486) and the mysteriously named “Dicot leaf types c.s.” slides (Carolina Biological Supply Item #303520). The latter slide has cross sections of leaves from three unnamed species (a hydromorph [water lily?], a mesomorph [privet?], and a xeromorph [rubber?]). I also bought stage micrometers from amazon.com for about $10 apiece. Instead of using printed micrographs, I had Lauren use her field-of-view measurements to estimate the thickness of each specimen’s “outer layer.” She did not report any problems, so I am confident that we can make it work in our class next fall.

When we do, each group will make its own measurements. We will then compile the class data and develop a homework assignment in which our students graph the average measurement for each type of organism. I look forward to reporting our results as an update to this post in the fall.

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Reference: Yang, Troy R., Suann Yang, and John C. Inman. 2015. Growing a Thicker Skin: An Exercise for Measuring Organismal Adaptations to Terrestrial Habitats. The American Biology Teacher, 77 (6): 426-431.

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