letter on the behaviour of vines

Chrestomather | November 25, 2017 in Uncategorized | Comments (0)

 

Dear Mr. Ghosh,

I just read your notes about vine behavior in your masterful “The Great Derangement” and thought you might be interested in the following. I study vines, but over the past decades my research has become focused on tropical forest conservation and rural development through sustainable management for timber, principally in Southeast Asia. As a side project I study sea level rise in Florida (see the attached popular account of this work).

VINE BEHAVIOR
Sequential expansion of cells around the perimeter of terminal buds of most plants causes elongating shoots to circulate. Because the sequence progresses in an anti-clockwise direction, the circumnutation spirals of most plants are also anti-clockwise. In most species, the radius of the arcs of a circumnutating shoot tips are usually less than a few centimeters, but in many vines, the paths follow by ‘”searcher” shoots and tendrils can be exaggerated by an order-of-magnitude. In under an hour in completely still air, the top 10-20 cm of a rapidly growing vine searcher shoot might follow a circular path 20-40 cm diameter in under an hour. Vine shoots and tendrils that encounter an obstacle within their circumnutation spiral continue to revolve, which is how they attach to trellises. If the obstacle is too large, the angle of ascent is too shallow for the stem or trellis tissue to support against the pull of gravity, and the attempt at climbing fails and the plant falls. (Note that contrary to the old song about the morning glory and the woodbine, it is NOT because she twines to the left and he twines to the right.)
Naturalists have long noted the phenomenon of circumnutation, and I recall that Darwin wrote about it in his book on climbing plants, but I found a series of papers on this topc by researchers from the University of Besancon in France to be particularly intriguing. I do not currently have access to my files, but as I recall, back in the early 60s, Professor Baillaud wrote a tome about the behavior of climbing plants. In that monumental work, he described how when circumnutating vine shoots and tendrils detect the presence of a potential trellis in the vicinity, they switch from following a circular path to an oval, with the long axis oriented toward to the potential support. In a very French manner, he was apparently comfortable describing the support-foraging shoot tip as having detected the “essence” of a support, and change their behavior so as to increase the chance of making contact.
You should note that I never had the pleasure of meeting in person any of the people about whom I am writing, and presume that they are no longer living. In any case, I will take the liberty of “connecting some dots” so as to make sense of the drama that unfolded over the decade after Baillaud published his seminal work.
Continuing the work on vine behavior under Baillaud’s tutelage was a young man by the name of Tronchet. In a paper published in the same journal, which was presumably based on his dissertation research, Monsieur Tronchet described the results of an elegant experiment in which he explored the “essence” detected by circumnutating shoot tips. He demonstrated with a column covered by wet cloth soaked in bark extract that the shoots detected the presence of potential supports chemically.
A scant year or two later, Madam Tronchet, presumably another student of Baillaud and the wife, or soon-to-be ex-wife of Monsieur Tronchet, refuted the latter’s findings. When she presented circumnutating shoots with clear glass columns that were dry, they also elongated their path towards the potential support.
I am not aware of any really definitive follow-up to this research, but by piecing together what is known from related more mechanistic studies, I believe I can explain this phenomenon through invocation of the gaseous hormone, ethylene. In response to even very low concentrations of ethylene, the cellulose microfibrils that strengthen cell walls in plants become arranged randomly, rather than like hoops of a barrel. Growing plants release ethylene, which accumulates where there is some obstacle to its diffusion. Cells with randomly arranged microfibrils expand equally in all directions whereas cells wrapped with parallel (horizontal) microfibrils elongate more than they increase in girth. This differential expansion causes the shoot to bend towards whatever it is that causes ethylene gas to accumulate.

Beautiful, no? I am thrilled by the way that when science reveals some underlying physical explanation for an observed phenomenon, it becomes even more fascinating. I hope you agree.

Looking forward to your next book.
Jack


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