You're probably familiar with sunflowers, a member of the Asteraceae family of flowering plants. But the biology of the sunflower is a bit different than the common perception of a big flower composed of small yellow petals surrounding a large black center.
"Actually, that structure is the flower head, or the capitulum, which may be composed of hundreds of flowers, also known as florets. The surrounding 'petals' are florets different in structure and function to those closer to the center," explained Professor of Horticulture Paula Elomaa from the Faculty of Agriculture and Forestry at the University of Helsinki.
It's good for a plant to have a big flower head because that means more it's easier for them to attract pollinators. As pollinators move around the flower head, hundreds of individual florets are pollinated.
These florets are not randomly distributed. They follow a pattern that's known in mathematics as the Fibonacci sequence, where the next number is the additive of the two that precede it: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144 and so on. In a flower head, the florets wind in left- and right-handed spirals that are two consecutive Fibonacci numbers. In sunflowers, there can be as many as 89 right-winding and 144 left-winding spirals. In the gerbera, another flower in the Asteraceae family, there aren't as many spirals.
"The gerbera is a favorable study subject because we can use transgenic plants grown in a greenhouse as tools when investigating the functions of individual genes, for example, during the development of the plant. In the case of sunflowers, gene transfer is not yet a routine procedure. The gerbera genome, which is close to the size of the human genome, is currently being sequenced. Experience has shown that the gerbera is an excellent model plant," said Elomaa.
While scientists, mathematicians, and artists have long been aware of patterns and geometry in nature, this work has revealed more about the molecular mechanisms that underpin spiral patterns - in this case, in the meristem or growing point of gerberas.
In this study, which was reported in the Proceedings of the National Academy of Sciences, the researchers were able to use a tool called X-ray tomography to scan meristem development in different stages. Confocal microscopy was used to follow the location of a plant hormone called auxin. The data were analyzed with computational tools to create a model of the patterning of a flower head.
"During growth, auxin levels rise to the maximum simultaneously in several locations of the meristem. The number of these clustered spots, which are called auxin maxima, increases rapidly as the diameter of the meristem grows, following the Fibonacci numbers. A new auxin maximum is always formed between two neighboring maxima and moves so that it is always closer to the older of the neighbors. This is why the spirals are regular even in meristems that are not entirely symmetric."