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.

We All Love Our Hypotheses: A Story of Charles Darwin

On this day in 1809, both Abraham Lincoln and Charles Darwin were born. Here’s to the next 211 years of freedom and science!

And here is a little story about Charles Darwin.

All of us, especially scientists, love our hypotheses. But what scientists try to do, and often succeed at doing, is to test the hypotheses and then let go of those that do not pass the test. This is why we consider ourselves “objective” (rather than subjective), and this is one of the major ways in which scientific thinking differs from non-scientific thinking.

But, as individuals, scientists often find it difficult to let go of cherished ideas. This is why science is a community; one scientist might cling to a useless hypothesis, but the other scientists can challenge him or her.

One good example of a scientist clinging to a cherished hypothesis was Charles Darwin. I do not refer to evolution; he was right about that. But at the time Darwin wrote the Origin of Species, and for the rest of his life, Darwin did not know how traits could be passed on from one generation to another. That is, neither he nor anybody else understood heredity. Well, one man did: Gregor Mendel. But Mendel did not know that he knew. He died, not knowing that every biology student in the world would learn his name.

Here is why this was such a problem for Darwin. A scholar named Fleeming Jenkin wrote a criticism of natural selection. Jenkin invited us to imagine a situation where a new, superior, but rare hereditary variation was introduced into a population. No matter how good it was, it would get swamped out by the rest of the population before natural selection could save it. Jenkin used a violently racist example which was, we now regret, common in his day.

If heredity acts like a paint pot, then Jenkin’s argument cannot be answered. A drop of white paint will disappear in red paint and is lost forever. But Mendel discovered, and Darwin believed, that traits were not like paint. They were more like (to use my analogy) marbles. White marbles can mix in with red marbles, but retain their individuality, and can someday show up again. Rare traits retain their individuality and can become common. But how?

Darwin, in the course of writing a big book Variation in Domesticated Plants and Animals, came up with an explanation. He believed that all parts of the body produced what he called gemmules, and these gemmules found their way, presumably through the blood, to the reproductive organs, where they were passed on to the offspring. The circumstances of life can cause organs to change the kinds of gemmules they release. This is how acquired traits can be inherited, in Darwin’s theory of pangenesis.

Darwin was not the only person who was excited about pangenesis. So was his younger cousin Francis Galton, often called the childless father of eugenics. Galton knew more about heredity than Darwin; in fact, Galton gathered lots of inheritance data for his book Hereditary Genius. He also contributed to the early development of statistics. He invented the correlation coefficient in statistical regression. And he was ready to subject pangenesis to an experimental test, which he was sure would confirm the theory.

Galton used different breeds of rabbits, with recognizably different coat colors. He took blood from one kind of rabbit and injected it into the bloodstream of a different kind. He was careful to not harm the rabbits, even though he transfused up to one-third of their blood volume. Galton expected that this blood, full of gemmules, would cause the rabbits to have offspring with the characteristics not of their parents but of their blood donors, at least sometimes.

All of the rabbit-blood experiments failed. Galton was not sure what to do next. Darwin was, however, sure. Darwin simply made a post-hoc rationalization. Triumphantly, Darwin said that the gemmules must find their way to the generative organs through some medium other than blood. This assertion led to no further experiments. Pangenesis theory declined into oblivion, and today we all learn about Mendelian genetics.

Darwin was more objective than almost every other scientist of or since his day. But, like any human, he was imperfect. Objectivity failed him in the matter of his beloved pangenesis theory.