Why Are Some Flowers So Symmetric?

“And then my heart with pleasure fills, and dances with the daffodils.”

Although William Wordsworth named this poem ‘I wandered lonely as a cloud’, it is popularly known as ‘Daffodils’. From Wordsworth’s Daffodils to Van Gogh’s Sunflowers, flowers have spread their scent and wonder across human history, culture, and literature.

But what is so special about flowers? Sure, they’re pretty to look at, but what makes them so pleasing to the eye? The colors? Yes. The patterns? Absolutely. But what truly catches the eye is the symmetry.

Symmetry is a sense of proportion and balance. When two or more objects with the exact same structure face each other or are placed around an axis, they become symmetrical to each other.  For instance, picture a butterfly. If you cut a butterfly in two vertical halves, each side will have one antenna, three legs, and one pair of wings. Each half has the exact same structure and is therefore balanced.   

What are the types of floral symmetry?

Floral symmetry refers to how many mirror image parts a flower can be divided into. Typically, the type of symmetry is decided by how many divisions can be made of a flower’s perianth—the parts of the flower that aren’t involved in reproduction—like the petals and sepals. Primarily, there are three types of symmetry.

Actinomorphic flowers (actino – ray-like, morphic – shape) are those that have radial symmetry. These flowers can be divided into three or more identical mirror parts. Some radially symmetric flowers can be divided in more than one way, that is, they have two planes of symmetry and are biradially symmetric. Hibiscus flowers are radially symmetric, while mustard flowers are biradially symmetric.

Zygomorphic flowers (zygote – yoke-like) can be divided in only one way to give two mirror images, and are therefore considered bilateral. Orchids display this type of symmetry.

Some flowers lack any specific symmetry and are asymmetric in nature. For example, the flower of the ginger plant.

Floral symmetries

Floral symmetries

Why should flowers be symmetrical?

The function of a flower is to attract pollinators, so these flowers have altered their appearance and menu according to their customers. Evolutionary biologists have observed that floral symmetries have changed over many generations from being radial or bilateral to even asymmetric! Scientists have also found that bilateral flowers facilitated pollinator landing.

When scientists genetically manipulated the lower petals of the Fireweed plant to be more symmetric, it produced more nectar. In another study performed on bumblebees, the results showed that bees preferred these more symmetric flowers, owing to their greater nectar-producing capacity. Insects’ preference for such symmetry must have resulted in the sexual selection of symmetric flowers. A diverse genome is nature’s most favored pick.

On a plant branch, the shoot that bears the flower is called the mother axis. The side of the flower towards the mother axis is dorsal and the side away from the axis is ventral. A flower attains a certain symmetry type when cells on any of these sides elongate. When scientists studied Snapdragons, they found four genes—CYCLOIDEA (CYC), DICHOTOMA (DICH), RADIALIS (RAD), and DIVARICATA (DIV)—that contributed to symmetry. The CYC, DICH, and RAD genes regulate cell division and elongation on the dorsal side. When CYC and DICH express themselves, they activate RAD. The DIV gene functions in a similar fashion on the ventral side, but also inhibits the expression of RAD and changes the degree of elongation on the dorsal side. An interesting study done on sunflowers showed that when a CYC-like gene was added to its genome, the sunflower developed its second whorl of petals. Due to this second whorl, the flower cannot be radially divided, but it can be divided bilaterally. Surprisingly, this phenomenon is seen in Van Gogh’s famous nineteenth-century sunflower paintings.

sunflowers

Double-whorled sunflower & Van Gogh’s double whorled sunflowers. (Photo Credit: Natthawut Raruen & Vovalis/Shutterstock)

Naturally Asymmetric

Plants have an obvious advantage by generating symmetrical flowers. Pollinator preferences influence the plant’s choice to shift to a symmetric flower, and such prolonged positive selections have caused many plants to permanently shift to that desired trait. Given that, all plants dependent on insect pollination should have symmetrical flowers, certain flowers can be asymmetric. This leads us to believe that perhaps asymmetry, if not completely undesirable, doesn’t necessarily hamper pollination.
One example of an aymmetric flower is Gorteria diffusa. G. diffusa produces variable flowers that are only somewhat radially symmetrical. Since the plant can’t solely rely on symmetry to attract pollinators, they can fortunately use other tactics. The flower has spots on its petals that attract bee flies and some beetles. When scientists studied G. diffusa for pollination visitation, they observed that male flies visited these flowers more than female flies.
These spots mimicked resting flies and triggered a mate-seeking or aggression response in the males. Upon closer examination, the scientists found that these spots reflect the same amount of UV as a bee fly’s back would reflect. Scientists then bred flowers without these spots, but replaced some spots on flowers with simple ink. They observed a notable reduction in visits by pollinators in both cases. Such dark spots have evolved in other South African flowers like Gazania, Dimorphotheca, and Pelargonium species—all of which are pollinated by bee flies.
A Gorteria diffusa wildflower in the Namaqualand, South Africa(Cathy Withers-Clarke)s

Gorteria diffusa with spots that mimic beeflies. (Photo Credit : Cathy Withers-Clarke/Shutterstock)

Flowers, symmetric or otherwise, are beautiful and mesmerizing, but once you understand the underlying shape strategies and aesthetic evolution, you can appreciate them even more!

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About the Author

Swanandee Nulkar has done her Bachelor’s in Microbiology and Biochemistry. She is currently perusing her Master’s in Environmental Science. Her natural habitat is a library nook. She’ll be up for a few pints of science jibber jabber if you ever run into her!

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