Evolutionary Mysteries: The Sensitive Plant

 

One of the common weeds in open areas of Oklahoma is the sensitive plant, genus Mimosa. Its leaves are divided into leaflets, and each leaflet into pinnules. It is called a sensitive plant because the leaves, when touched, will close up. The most famous sensitive plant (available from retailers) is Mimosa pudica, a now-widespread weed of tropical and other warm areas, although the Oklahoma plants are probably M. quadrivalvis. I have just posted a Darwin video about this plant if you would like to see it at work.

The reason I call it an evolutionary mystery is that nobody is quite sure why the leaves close up in response to touch. The leading explanation, which I dutifully included in my botany classes, is that it is a defense against herbivorous insects. An insect such as a grasshopper might land on what looks like a big juicy leaf, only to see it disappear, becoming what looks to an insect like an inedible stem.

This leaf response is a response to touch, not necessarily to an insect’s touch. The leaves also close up in response to raindrops. This opens the possibility that the leaves close up not so much to escape the attention of a herbivore but to keep from being weighed down my raindrops.

A related genus within the subfamily Mimosoideae, the genus Albizzia, is the famous mimosa tree. Its leaves do not close in response to touch. Instead, they close up each night and open each morning. The leading explanation for this, one first proposed by Charles Darwin and his son Francis (widely considered the founder of modern plant physiology), is that by closing up the leaf avoids frost damage caused by the clear night sky.

Of course, more than one explanation can be correct at the same time. The mimosa tree, by closing its leaves at night, might avoid frost damage and at the same time damage from raindrops. I have frequently seen mimosa trees which, on rainy mornings, have delayed re-opening their leaves.

The biggest problem with all evolutionary stories, this one and others to follow, is that they leave unanswered the question of my other plants do not have this adaptation. All leaves would benefit from closing up in response to an insect, but almost none of them do, except those of sensitive plants.

In this case, the herbivore and raindrop explanations may be valid within this evolutionary lineage. For whatever reason, the genus Mimosa evolved bags of pressurized water known as pulvini that quickly push the leaves closed in response to touch. Other leaves, without pulvini, cannot close up. The leaves benefit from closing up if they happen to have, for this or for some other reason, pulvini. Other plants, such as the velvetleaf Abutilon theophrasti, lower their leaves each night and raise them each morning, just like Mimosa. Abutilon has pulvini. I do not know why pulvini are not more widespread in the plant kingdom. Abutilon is in the Malvaceae family. Pulvini are also found in the Marantaceae and Oxalidaceae. Most plants do not have them. It is not that the other plant families would receive no benefit from having their leaves open and close, or raise and lower; they, perhaps by chance, do not have pulvini.

The ”adaptive stories” of evolution, such as touch-sensitive leaves, are usually very good at explaining the benefit that an organism may get from having the adaptation, but are usually not very good at explaining why other organisms do not have the adaptation.

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.