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Helianthus annuus (Asteraceae)

Sunflowers sustain the widespread belief that their large disk-shaped heads follow the sun. While this is true of the young, the mature stay put and face eastward, eventually bending their heads weighed down by seeds.

They are a familiar sight to many who, in all likelihood, have seen only the domesticated variety. What is deemed the flower is in fact a disk-shaped inflorescence, consisting of an assembly of a large number of florets.

They are arranged in a pattern that might illustrate a class in mathematics, for instance in number theory. The disk florets are disposed spirally. Each floret faces the next with approximately the golden angle, 137.5°. This makes for interconnecting spirals, where the number of left spirals and the number of right spirals are successive Fibonacci numbers.

So-called Fibonacci numbers — for a 13th century Italian mathematicien — make up a series such that each number is the sum of the two preceding ones : 0, 1, 1, 2, 3, 5, 8, 13, 21, etc. Typically, there are 34 spirals in one direction and 55 in the other; however, in a very large sunflower head there could be 89 in one direction and 144 in the other. As wrote Darcy W.

Thomson, ‟the beautiful configuration produced by the orderly arrangement of leaves or florets on a stem have long been an object of admiration and curiosity‟ (On Growth and Form, chapter 14).

Mankind has domesticated this plant for millennia, first by Amerindians in what is now the southeastern USA, in Tennessee in particular, about five millennia ago. In 1510 early Spanish explorers encountered the sunflower in the Americas and carried its seeds back to Europe. In Spain and Mexico, the sunflower was used as an aphrodisiac.

Sadly, recent breeding of sunflowers has led to a loss of genetic diversity that was present only a century ago in Native American landraces.

I am writing these lines from Pays de cocagne, a rural French area near Toulouse. At the time of writing, the second half of July 2020, many fields are a stunningly bright yellow from sunflowers grown there. Finally, I dedicate this piece to the memory of Jeroen Brandsen, whose favorite flower it was.

Published inPlants