Sunflower
In 1597, John Gerard appeared disappointed the sunflowers he grew in his Holborn garden were only 14 feet (4.3 m) tall; those of his European competitors reached 24 feet (7.3 m)! Sunflowers, with their massive flower heads surmounting a single stem, have attracted the attentions of gardeners, and mathematicians, since they were introduced to Europe from the Americas in the sixteenth century.
Sunflower heads comprise hundreds of tiny flowers. Those around the outside of the head (ray florets) have large, bilaterally symmetrical corollas (group of petals) and are sterile. Those on the inside of the head (tube florets) have symmetrical corollas and are fertile. Each tube floret produces one, single-seeded, dry fruit. Gerard commented that sunflower fruits were 'set as though a cunning workeman had of purpose placed them in very good order'. Look carefully at the tightly packed arrangement and there appears to be two series of spirals. Mathematically, the numbers of these spirals (often 34 clockwise and 55 counter-clockwise) follow the Fibonacci sequence. Fibonacci sequences are frequently found in the ways that plants arrange their organs to fill limited spaces. In the sunflower head, fruit packing is maximised by arranging adjacent fruits at precisely 137.5 degrees to each other; the golden angle.
DNA evidence shows that the sunflower was domesticated once, about five millennia ago, in eastern North America. Native Americans had many uses for sunflower including oil, flour, medicine, fibre and dye. It is only in the last two centuries that sunflowers have become an international commodity; one of the few globally significant food plant domesticated in North America.
In the mid nineteenth century, when the Swiss botanist Alphonse de Candolle wrote his classic book on cultivated plant origins, there was no mention of the sunflower, just its close relative the Jerusalem artichoke. For most of their time in Europe, sunflowers have been primarily horticultural novelties. However, in early nineteenth-century Russia, sunflowers began to be commercially cultivated for their oil. The sunflower selections made by Russian plant breeders significantly increased oil content and seed yield above those in European and North American cultivars. By the late nineteenth century, highly productive sunflower cultivars, e.g., 'Mammoth Russian', taken back to North America, probably by Russian immigrants, became the basis of today's global sunflower oil industry. In 2012, more than 37 million tonnes of sunflower seeds were produced, of which over 40% were grown in Ukraine and the Russian Federation.
Further reading
Adam JA 2009. A mathematical nature walk. Princeton University Press, pp.31-42.
Blackman BK et al. 2011. Sunflower domestication alleles support single domestication center in eastern North America. Proceedings of the National Academy of Sciences USA 108: 14360-14365.
Heiser CB 1998. The domesticated sun?ower in old Mexico? Genetic Resources and Crop Evolution 45, 447-449.
Stephen Harris