First watch this:

La lenta belleza de las plantas (nuestroviaje-alive.blogspot.com) from tanavoltan on Vimeo.

This gorgeous video by Maurice Maeterlinck shows the movement of plants – movements we don’t normally notice since they happen on a time scale too slow for our perceptions and patience. It inspired me to look into the science of how plants move.

For most plants simply growing upwards is their strategy for reaching sunlight, competing with their neighbors for the highest positions. Climbing plants have a more complex strategy since they don’t simply seek out light but also other plants and objects to attach themselves to. They are the star performers in Maeterlinck’s video, twining in circles like someone reaching out for a handhold as they grow upwards. Some plant species always twine clockwise, in left-hand helices while others twine counterclockwise. Will Edwards, a researcher from James Cook University in Cairns, Australia and others (2007) wondered whether what, if anything, influenced the direction of these movements. For instance, if they were to track the sun from east to west while on the sunny side of an object (the south side here in the northern hemisphere) they would twine in left-handed helices here in the north and right-handed ones down in Edwards country. To find out they tracked the twining movements of plants all over the world.

The verdict? Maybe there is a method to the plants madness but it has nothing to do with the sun. Edwards found no difference in twining direction between the hemispheres. He did find, however, that almost all the plants, 92% of them, twined in the same direction – a right-handed helix. Why this is so remains a mystery. Perhaps it owes simply to some quirk in the cellular controls of twining. Heck maybe, like us humans, the lefties tend to be different in other ways too. Maybe they’re artists.

The full moon is a powerful symbol. It speaks to us of fertility, cycles, pregnancy, and wildness. The moon itself has been shown to influence animal behavior, particularly in the seas. But what of plants, who lack a central nervous system? Can they too be influenced by the pull of the moon?

Such texts as ‘The Farmer’s Almanac’ suggest planting should be done during certain phases of the moon. It’s such a beautiful metaphor of working with and connecting to nature I want such influences to exist. So far science has been unable to prove such an influence though several possible, albeit questionable, mechanisms have been proposed for such and influence. Someone prove it for us.

Plants are amazingly sensitive to light such that full moon intensities are easily enough to disturb their time measurement via photoperiodic cycles. This could be quite catastrophic for plants that then flower at inappropriate times. Thankfully, evolution has come the rescue. Plants have adapted night time leaf movements that reduce the moonlight striking their upper leaf surface by as much as 20 times, well below the level that might confuse them.

What about the gravitational pull of the moon? It effects tides, can it’s influence be felt on as small a scale as a single tree? Yes, recent evidence seems to confirm this. Dr. Peter Barlow and others looked at measurements of tree diameter in six species and found that they were correlated with tidal variation. As the gravitational pull of the moon was greater, the trees became minutely but significantly fatter!

Fattening trees and curling leaves. Our revolving sister rock invisibly reaches out to the world of plants. Who knows what else is left to be discovered.

Paul Magelsdorf, distinguished professor at Harvard’s Botanical Museum, stormed out on a lecture at the “Origin of Corn Conference” at U. of Illinois in 1969. He wasn’t happy, I imagine. The findings presented at this particular lecture contested much of the research he had spent his life investigating, unraveling the mystery of the origin of domestic corn. Corn’s origin was clouded in mystery because it seemed to appear out of nowhere in the fossil record with no obvious direct ancestor. In numerous papers Magelsdorf had outlined his hypothesis that corn’s ancestor was a now extinct thick seeded popcorn that gradually, through selection over many years by early inhabitants of the Americas, became the grain that we know today. Teosinte, the closest wild relative of corn, and which can be hybridized with corn, was considered by Magelsdorf to be a mutant of this same common ancestor. The fact that no intermediaries could be found between this ancient ancestor and modern day corn was troubling however.

The speaker at that conference was Hugh Iltis who contested Magelsdorf’s theory on several accounts. It was not until 1983, however, that he posed a theory of his own. This theory, which met with some skepticism at first, posed a more radical evolutionary shift – Catastrophic Sexual Transmutation. Unlike the typical Darwinian stepwise model of evolution, Iltis posed a sudden and dramatic evolutionary shift for Teosinte. To explain this I’ll start by pointing out a few differences between Teosinte and corn. First off, Teosinte is a many-branched plant with each of the primary lateral branches terminating in a male flower tassel. Corn, on the other hand, has a single stalk with the male tassels only at the top of the plant. In addition, Teosinte produces but a few, hard, kernels with nut-like casings in each of its ears, unattractive to anyone seeking food.
Iltis hypothesized that several mutations led to a reorganization of the plant, eliminating the lateral branches, bringing the male flowers that terminated these branches tight against the stalk where they became feminized. The original female ears of Teosinte where suppressed and a grand reallocation of resources towards the new female flowers resulted in larger, and more tempting to humans, ears of corn. It was only after this catastrophic evolutionary shift that humans took over and, through selection, caused additional changes in the plant such as husking and larger grain size. In fact the sudden transmutation of Teosinte may have been lethal had not people interfered by preserving this anomaly.

Recent genetic research has found that several of the differences in morphology (branching pattern, kernel covering) between teosinte and corn are, in fact, due to minor changes in single genes. It is an important evolutionary lesson that only a small number of mutations can, at times, lead to such dramatic changes in an organism.

A number of times this summer I was visited by a creature that appeared something like a hummingbird but smaller and more insect like in it’s appearance, usually hovering around a phlox or similar tubular flower. This was the Hummingbird Clearwing Moth (Hemaris thysbe) and I was lucky and patient enough to snap a few decent shots of it with my phone’s camera.

Pollinating a list of flowers similar to a hummingbird, I marveled at it’s likewise similar aerial abilities. I wondered about the mechanics of this flight and promised myself that I’d look into it at some point. I was excited today to stumble on a broadcast by[highlight] Science Friday[/highlight] on just this topic. The following video, in which researches blast tiny cannonballs at these hovering creatures (though not the same species as visited me), explains the evolutionary insurance plan these moths have developed in their flight.

Okay, let’s talk about sex again. Plants are so adventurous in this realm, having none of the hang-ups or intolerances that we humans often do. Plants can be male, female, hermaphroditic, or non-sexed; they might be self-sexing or fertile only when with partners, often having many partners; they regularly entice other species into their escapades; many switch sexes throughout their lives. It’s this last trait, the floral sex-change, that I’ll explore today.

An example should help illustrate: the Jack-in-the-pulpit (Arisaema triphyla) usually spends the first years of its life sexless as a small, single-leaved pre-adolescent. After several years the plant will begin to flower – always male flowers only. Given time, the once single leaved plant will return with a second leaf and suddenly blossom a female flower (or sometimes both male and female ones). Scientists call this bisexual potentiality – the potential to be either sex.

Now let’s get into the evolution part of this. Bisexual potentiality makes sense in at least one
particular evolutionary case – when the functioning as a male or female is done best by individuals of different ages. In the case of our Jack-in –the-pulpit, younger plants do not have the energy reserves yet to produce seeds but are capable of producing pollen without exhausting themselves. It is only when they are older and have built up underground energy reserves that they are capable of producing seeds (and the female flowers that precede them). At this age being female makes them more capable of passing on their genes to further generations.

Only a small number of plant species are known to go through this sex-change operation. Striped maple (Acer pensylvanicum) is another, exhibiting a similar strategy. This maple is an understory tree in the Northeastern American forests. It generally remains small but if a light gap opens in the forest canopy from the loss of a mature tree it takes advantage and can grow to 30 feet or more. Only once it has this light advantage will it change from male to female and begin producing the familiar two-winged seeds called samaras.
Next time: Catastrophic Sexual Transmutation