Monday, June 4, 2012

Celery & Transpiration Lab

I volunteer as the science laboratory coordinator at an all boys Christian boarding school. What this means is that I go in two mornings a week, one morning to tutor in science, math, and writing-and the other morning to do a lab with they boys. One of the labs in the SOS (Switched on Schoolhouse) General Science 1 course is about celery. But I was unimpressed with the purpose of the lab. It seemed to me, that it was being used as a demo in which it reinforced the concept that water moves up the stem, into the leaves. But to me, this is such an elementary idea, one in which my 5 year old understands.  

Its all in how you approach a topic right? Sometimes the simplest concepts are quite abstract and can strike a sense of wonder in us, if we just stop to reflect on what we think we know. Yeah, water goes up from the soil, up the stems of plants, and into its leaves...big deal! Well it is. Remember gravity? How does the water go against gravity? What if I told you, it is a passive process and doesn't take energy to do? Impressed yet? If you're thinking to yourself, "Well how does it go up?," then I've got you in the right frame of mind to do an experiment. With your students, before handing out this lab, you might have them do a reflective journal entry trying to determine how they think water moves up stems. Then maybe they'll have a higher motivation to try and understand it!   

In this lab, I provide a brief introduction to capillary action, surface tension, adhesion, all leading up to a discussion about transpiration. Ah....transpiration. This is the key to getting that water up the stem! Excess water evaporates off the leaves, and the water below it is PULLED up the xylem (tubes that cary the water in plants). And, this takes no energy!  

Before I forget, here is a copy of the student handout. On my scale of inquiry, I would consider this lab a level three, which means that the teacher poses the question, but the student plans the procedure and formulates the results. The question students address is, "Can we speed up the rate of transpiration?" They pick what variable they want to test and then the methods in which will help them answer their question. Really allow students to pick their variable. Its more fun this way! I had fans, lights, thermometers, rulers, and protractors all sitting out as they thought about what they wanted to do.  

While you listen to students talk about what they think will speed up transpiration, see if you can guide them to the understanding that sometimes by testing one variable, we're introducing another. For example, the group that tested intensity of light was inadvertently also adding a variable of heat, if they used an incandescent  light bulb.  They key here is not to tell them what to do, but to ask questions at the right time in their process thinking. You must also refrain from answering every question they ask. Refer them back to their group, and see if they can figure things out. I found myself asking questions such as
  • How might you set up the plant to make it as "fair" as possible? 
  • If you want the results to be drastically different, where will you place the stalks of celery?
  • How can you control for extraneous variables (heat, people around the experiment, time of day)
  • How will you measure the rate of transpiration? (For rate you need distance and time!)

What I love about inquiry, is that I really try to go into the class session with only general ideas of how it will turn out. Then we spend a lot of class time "Jerry-rigging" contraptions to make it work. Like our light set-up, we duck-taped a shampoo bottle to the light at an angle, and then used a binder to focus the light on the celery (and decreased the glare in the classroom). I think since both my high school classroom is all boys, and my classroom at home (my own kids) are all boys, this engineering attitude is a healthy one. However, it really is a challenge for me, to not provide detailed methods for them to follow. But I'm learning how to hang back, listen to them talk, and ask questions when I think it will help them focus. 

The students must come up with what they will measure in order to determine the rate of transpiration. The most obvious is to measure the colored water moving up the stalk at different intervals throughout the day. I will warn you, even with dark water, the movement up the stem is only slightly noticeable. Students need good lighting and keen observation skills to notice the change. I would suggest using dark red food coloring, not green or blue. You'll notice that we had several stalks in each mason jar. If we had it to do over again, we would have put 4-5 stalks in each, and each hour we would have cut  one stalk of celery every centimeter or so to look for the color in the xylem, and taken our measurements that way. I estimated it took about 5 hours for the colored water to get to the leaves. So it depends on what your students pick as their variable as to how long they may need to collect data. I understand that this may not work in the traditional educational setting. It worked for us, because the boys are all in one classroom the entire day, and they were able to take measurements throughout the afternoon, even after I left. Homeschooling families won't have any issues either, they can continually go back to take measurements. 

In these photos, you can see how colored the bottom of the celery stalk is after 24 hours. But after we cut the bottom off, you see the color only in the small dots, which is the xylem, where the water is headed up to the leaves. 

As part of the post-lab I have students describe what they see, draw some cross sections of the celery, comparing sections from the top to the bottom. They should also construct a graph to help them analyze their results. Then of course, asking to see if their predictions of which celery would move water up the fastest was correct. Remember, it is key, not to praise only those whose experiments "worked" because we learn as much from ones that don't provide the data we were expecting. If this happens, be sure to ask students "why?" Was it that the variable you tested doesn't in fact speed up transpiration? If so, that is HUGELY important! Or is it possible we could/should retest the variable in a different way and see if we get different results? Praise all types of data and methods. Students need to feel science is a safe place to fail, try things and then tweak them and try again! It is up to us--as the teacher--to create this type of environment! 


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