A Mysterious Twist

Explanation: During my last two trips to tropical forests of the southern hemisphere, I was struck by something I hadn't fully noticed previously. Nearly all of the lianas (vines that twine on other plants) and the star of this week's episode seemed to be twisting in same direction as they grew -- counter-clockwise -- as viewed from above.

I thought, this must be coincidence, or I was just being selective in what I was noticing, but site after site, trail after trail, the vast majority of the plants that showed twining behavior were twisting counter-clockwise ("anti-clockwise" for you Brits).

Let's begin with this week's star, a marvelous plant called screw pine.

It is actually not a pine (nor a screw, for that matter), and another common name for it -- screw palm -- also is not quite correct, as it is not at all closely related to palms.

It is a pandanus, which are, yes, palm-like, constituting about 600 known species. I found one once growing in a reserve in Hawaii.

Some species of pandanus have stilt roots, but this one has a single main stem with a shield of leaves that grow in a spiral -- thus its wonderfully appropriate scientific name of Pandanus spiralis.

Look closely at the spiral arrangement of the leaves: yup, they spiral counter-clockwise.

I encountered the specimens in the photo above and in this week's main photo at Yellow Waters Billabong, near Mardugal, in Kakadu National Park, which is in the tropical "Top End" portion of Northern Territory, Australia. Spiral they all did, and all in the same direction.

OK, now let's take a look at some other plants from the tropics of Australia -- true lianas (also called "climbers").

Here is a true climber wrapped around a small tree. I don't know the species, but notice its spiral orientation: counter-clockwise.
I took this along the Woodland Track at Howard Springs Reserve in the Palmerston area of Northern Territory, Australia.

Here is another interesting situation: two climbers entwined with themselves ... and yes, again, counter-clockwise.
This is from Lamington National Park in Queensland, in eastern Australia.

Another striking example from
Howard Springs Reserve.
So what is going on here?
Climbing plants tend to grow rapidly and put out "searcher shoots" or tendrils that flail about as they grow, trying to get a hold onto some surface. And they may flail in a circular pattern -- clockwise or counter-clockwise.
Others have also noticed consistent spiral growth patterns of not just climbers, but also of conifer trees (see this essay) which spiral clockwise in the northern hemisphere as consistently as lianas and screw pines seem to spiral counter-clockwise in the southern hemisphere.
A number of writers suggest that climber plant species are "right-handed" or "left-handed," implying some genetic predisposition to the angle of spiral.

My hypothesis, however, is that the searcher shoot tendrils are basically following the pattern and arc of the sun in its daily circuit across the sky.

In the northern hemisphere during spring and summer, the sun rises in the east and traces an arc across the sky remaining mostly toward the south of the meridian. A tendril that is at once flailing about to gain a foothold (so to speak) on another plant, and also trying to orient to the sun for photosynthesis, would follow the sun's path in a clockwise direction.

In the southern hemisphere, however, during that region's spring and summer, the sun still rises in the east but traces an arc across the sky mostly to the north of the meridian. For a searcher shoot tendril to track it, it would have to flail counter-clockwise.

Still, there must be many exceptions to this tendency, and perhaps plant genetics do take precedence such as with the well-known "right-handed honeysuckle" and "left-handed bindweed."

One test of my hypothesis, however, would be to inspect the growth patterns of climbers right on the equator, where the sun does not arc in a consistently northern or southern portion of the sky. I have visited many equatorial tropical forests and ... noticed that climbers there seem to have no favorite particular direction when they spiral, and even many lianas seem to grow in a confused tangle with no specific spiraling direction. More on this in a future EPOW.

But none of this explains the rather consistent counter-clockwise twist of the screw pine!

EPOW - Ecology Picture of the Week