Testing a Hypothesis: Sometimes It's Your Only Choice

Plants are dynamic beings that respond continuously to their environments. (Continuous means without interruption, while continual means at recurring intervals.) That is, plants adjust their growth, and even their movements, all the time, day and night, all year.

One fascinating example is called nyctinasty. This is where a plant raises its leaves up, or opens its leaflets, during the day, and lowers the leaves or closes the leaflets at night. Makes sense. During the day they need the light for photosynthesis, and at night they don’t.

Problem is, most plants don’t bother opening and closing, or raising and lowering, their green surfaces. The leaves face the sky in the day, but at night, most plants just leave them where they are. Only a few plants raise and lower, or open and close, their leaves. Examples include the velvetleaf Abutilon theophrasti and many members of the bean family, including the mimosa tree Albizzia julibrissin. This photo shows mimosa leaves folding up for the night, even before sunset. Each mimosa leaf consists of leaflets, each of which has lots of little pinnules. It is, as you can see, the pinnules that close up, and they do so by moving upward.

Why should a plant fold up its leaves at night? Nobody knows, but there is plenty of speculation. Some think it protects the leaves from nighttime rain; others say it hides them from bugs. The most likely reason is that the night sky can be very cold, any time of year, even if the air is not. This could cause the leaves to get too cold during the spring (or fall, but autumn leaves have little value to a deciduous plant that is going to drop them anyway). This was Darwin’s idea, and (working with his son Francis) he made the observations to support (but not to prove) it. Of course, this doesn’t explain why the other plants don’t do this.

This is what mimosas do during under regular conditions: they raise their pinnules each sunset and lower them each sunrise. They do so by the alternate swelling and shriveling of little sacs called pulvini (singular pulvinus). But what I wanted to know was, is the pulvinus on the top side of the pinnule stalk, in which case the pulvinus swells to push the pinnule down, and the pinnule moves up when the pulvinus shrivels; or is it on the bottom of the stalk, in which case it pushes the pinnule down when it swells, and the pinnule moves up when the pulvinus shrivels. The swelling of the pulvinus requires energy; the shriveling does not. Therefore, I wondered, does the pulvinus push the pinnule up at dawn, or does it push it down at sunset?

I looked in the scientific literature and all over the web, and (at least in 2018) could not find an answer to this simple question. So, in desperation, I decided to test a hypothesis myself. And I did so without any fancy equipment or a research grant.

Nyctinasty is not the only process that can make a pulvinus shrivel. Drought can also make a pulvinus shrivel, just as it can make every other cell or structure in a plant shrivel.

There happened to be a drought going on in Tulsa, where I live, in summer 2018. So I went looking for a mimosa tree that was experiencing drought. I had to go up on Turkey Mountain (which is a hill and has no turkeys) to find one. As you can see from the photo, a mimosa leaf experiencing drought during the day raises its pinnules before they start to curl up. This means that the pulvini must be on the top of the pinnule stalks. The pinnules lift up because of tension in the cellulose fibers in the cell walls, unless pulvini push them down.

I wanted to know the answer to the question, but I found enjoyment in answering the question in a way that anyone can do, even without a laboratory.

Another question that you might wonder about is this. After you exercise, of course, you breathe faster, and this helps your body to get rid of carbon dioxide that has built up in your blood. But does each breath have more carbon dioxide? To answer this question, all you need to do is to blow bubbles in slightly basic water. The water should contain some phenolphthalein, which turns pink under basic conditions, and turns clear in acid conditions. If each breath has more carbon dioxide (which becomes carbonic acid in water), then the phenolphthalein should turn clear sooner when you blow bubbles in the water right after exercising than when you blow bubbles in the water while rested. You can answer this physiological question, about lungs and carbon dioxide, without any equipment other than two glass jars, a couple of straws, and a little phenolphthalein. (You can’t just walk into the drug store for phenolphthalein, but it is not a controlled substance. You get it from science education supply companies.) If you can’t afford a carbon dioxide measuring device, you can test the hypothesis in a cheap and easy fashion.

These are actually experiments. In the case of the mimosa, it is a natural experiment: nature imposed the drought on the plants. In the case of breathing, the experiment compares the treatment (breath after exercise) against the control (breath while resting).

And it’s fun, too. Students like it. If you are an elementary school teacher, you can do the bubble experiment in your class.

There are probably lots and lots of questions that you wonder about and for which you cannot find an answer. Rather than to give up, try finding the answer by some simple observation or experiment.