Evolutionary Mysteries 2: Why Some Young Leaves Are Red

This week’s evolution video is about why some young leaves are red. You may have noticed that the unfolding leaves on plants as diverse as redbuds and blackjack oaks are reddish—in some cases, as with the blackjack, a brilliant scarlet. This is not true of all plants, but true of enough of them to invite an evolutionary answer.

The leading contender for an explanation is that young leaves are pretending to be dead. A young leaf has its whole life before it. Its entire contribution to the growth of the plant can be threatened by a herbivore eating it. When a herbivore eats an older leaf, however, the plant has less to lose in terms of future growth. The reddish color of a young leaf is a camouflage that makes a healthy young leaf look dead, thus diverting herbivores to the older leaves. This becomes more believable when you consider that many insects would not distinguish between red and brown, the color of plant death. The red color is caused by anthocyanins. They will reduce the amount of sunlight that gets to the chloroplasts, but by the time the leaf is fully grown and ready to produce food, the anthocyanins fade.

While this explanation is the most likely, it is not the only possibility. It could be that the young leaf is vulnerable to harmful ultraviolet radiation, which can cause damage but is outside the range of photosynthetically useful light. The red color, therefore, might be protection from ultraviolet damage.

And there is no reason why both answers could not be true at the same time.

To prove which of these explanations, or any other, might be true would be difficult. It might require breeding plants that produce more or less anthocyanin: young leaves without it, older leaves with it. Actually, some horticultural variants (such as red-leaved redbud trees) have adult leaves that are red (and green at the same time, resulting in a purplish color). Do these reddish adult leaves have less herbivore damage than green leaves? I do not know, but it is possible. Another way would be to compare herbivore damage in red vs. green patches on the same leaf; one study demonstrated that herbivores avoided the red areas ofvariegated leaves.

Keep your eyes open and there are mysteries of nature all around you that cry out for an evolutionary explanation.

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.

And He Became a Living Soul

I do not write in this blog very much about abortion, since I do not fully understand the issues. But I heard an interesting news report from South Carolina. Anti-abortion protestors stand at the top of the escalator in front of state senators’ offices and accost democratic senators who did not support the total ban on abortion that has just passed. Women senators have had objects slapped onto their backs, and protestors have sent children to run up to them and beg them to stop killing kids. If progressive protestors did this to Republican senators, for example regarding the death of the environment on which our lives depend, they would be jailed.

In one case, a Democratic senator knew more about the Bible than did the protestor. The protestor screamed that the Bible says abortion is murder. The senator, a woman, quoted the Bible back to him. Genesis 2:7 says that Adam became a living soul after God had breathed into him the breath of life. This implies that before a newborn breathes, it is not yet a living soul. Of course, nobody believes abortion is ethical right up to the moment of birth.

Genesis implies, but does not prove, that a fetus is NOT alive. The point is that this interpretation is just as valid as the other interpretations that the anti-abortion activists put upon it. The so-called Christian activists insist that their interpretation, and theirs alone, is correct. They will not permit anyone else to interpret scripture. Apparently God speaks through the Republican party, not through the Bible.

Only Republicans think they have the right to interpret the Bible. The Bible is their weapon, and nobody else can lay a finger on it. God is their God, not the God of the rest of us. Puh-raise Trump!

Uncommon Happiness

How many people are really happy? Assuming we can even define happiness. The one thing we can be sure of is this: far fewer people are happy than appear happy.

Among the countless examples I could cite, here are two that I found when looking through the works of Robert Ripley (see previous essay) (Believe It Or Not). These two examples are of men whom the world thought must surely be rapturously happy. But they were not.

The first is the story of Grimaldi the clown. A man came to a psychologist back in the early Freudian days and said he was depressed. The psychologist said the best cure would be for him to go watch a performance by the funniest man in the world, a clown named Grimaldi. The man said, “I am Grimaldi.”

The second is the story of Caliph Abdurahman III, who ruled for 49 years and had 6,321 wives. This sounds like the ultimate fantasy of many men. But in his will he said that he had, during those years, only fourteen days of complete happiness.

I immediately thought that I should never be jealous of the prosperity and conquests of other men. I wrote this on the day of the coronation of King Charles III. I do not believe that he is nearly as happy as I am. Can you say the same? Statistically, at least among my readers, there is a good chance that you can.

 

 

I recently retired from being a professor. Press a button and out comes a lecture or an essay. I will spare you both. I just urge you to think about happiness, about your happiness, about the happiness of others, and about the happiness that you can create in others.

What I Learned from Robert Ripley

Do you remember the cartoonist Robert Ripley, who ran a series called Believe It Or Not! in newspapers across the country (and probably the world)? He proclaimed random amazing facts (amazing to him, at least) accompanied by his very realistic drawings. Do you remember? If you are younger than me, as you probably are, you don’t. Anyway. I ended up with six paperback books with his cartoons. It is impossible for me to summarize these random pieces of information, but I can see some overall themes. In particular, Ripley’s cartoons give us a glimpse into how our brains work: how we gather information about the world and draw conclusions from it, a topic about which I wrote an entire book, Scientifically Thinking.

Many of the items in Ripley’s cartoons were completely subjective to the imagination of the observer. This is a trick our brains play on us. For example, a dog had the shape of a three-leaved clover on its back. Of all the dogs in the world, with splotches of color on their coats, it is nearly inevitable that you will find one splotch that looks, to you, like a cloverleaf. Another example was the Canadian map which, when held upside down, looks like a profile of the explorer Jacques Cartier. How close does the resemblance need to be? It depends on how credulous you are. Lots of rock formations looked like lots of different things, the most famous of which was the Great Stone Face in New England. (It recently collapsed.) The list is long, but I could add one more. Along a state highway to the west of Rapid City, South Dakota, there is a cliff that looks like a caricatured face.

 

It would be a waste of time to read a book that consisted of nothing except such random coincidences of appearance. Fortunately, Ripley’s work consisted of a lot more than this.

Many of the facts Ripley cited were statistical anomalies, things that are almost but not quite impossible. Any one of them is almost impossible, but collectively, there are lots of statistical anomalies happening all around us. He noted that on one day in 1950 there were 78 babies born in Washington, D.C., and they were all female. The odds against this happening are one in two to the 78^th power. Enough to make you say gee-whiz but not enough to change your view of the world. What about all the other days and all the other places?

Some of these statistical improbabilities involved human birth defects. Like the man who had two irises in each eyeball, and another who had three tongues, another who had two hearts, and a woman whose breasts were on her back. One man had mirror-image organs inside his body. Ripley’s major example was a child who had a single eye, in the middle of her forehead. This is known as a cyclops mutation. These bits of information are rather creepy, since they show us how the genetic health we take for granted can be so easily disrupted by a little mutation, or a mistake during fetal development.

Some of the examples Ripley cited were things that were only surprising to him but not to anyone else. He reported tree seedlings growing out of gutters on houses. This always happens unless you don’t have trees, or don’t have gutters. He was also amazed that violets would bloom in October.

Other examples were of almost unbelievable human cruelty or stupidity or just strangeness. For example, there was a man who locked his wife inside their house every day for 52 years. There was a Japanese priest who said “Butsu” (their word for Buddha) sixty thousand times every day for thirty years. Voltaire, who could never be accused of being ordinary, drank up to 72 cups of coffee a day. Hawthorne’s story “The Minister’s Black Veil” was apparently based on a real event. Juliette Drouet wrote Victor Hugo twenty thousand love letters.

Some of the supposed facts must have been based on misinformation. A 160-year-old man who fathered a child? When he died, his oldest son was 106, his youngest 3. There are no verified accounts of people living, certainly of being virile, this long. He also said goatsucker birds actually sucked goats.

I will always remember, from the first time I read them until now, some really amazing stories. For example, a Zulu king ordered thousands of his soldiers to march into the sea and drown, just to prove how much power he had over them. Then there is the story of Grimaldi the clown, which I write about briefly in the next essay, which I will post soon.

Life throws at us lots of disconnected bits of information. Our minds seek, or should seek, some generalities in them, for example, that happiness is found in creating a good life, not in power or possessions; and that there is almost no limit to human cruelty.