Symmetry

Most animals seem to be attuned to recognizing another individual’s symmetry as a means of judging its fitness as a potential mate. Among most human cultures, both men and women will usually choose the most bilaterally (equal halves) symmetrical face as the most beautiful. Thus, it’s no surprise that we humans prize the beauty of a highly symmetrical flower as well:  e.g., orchids, with their perfect bilateral symmetry, or dahlias, with their fascinating radial symmetry.

dahlia

dahlia

While plant breeders may have selected for the perfect radial symmetry of a dahlia, which we love to show off in our gardens, other forces were at work in the production of the spiral pattern of the seeds in a sunflower head, or disc flowers of the Black-eyed Susan, Purple Coneflower, and Sunflowers.

bumblebee-on-sunflower

Only one ring of yellow disc flowers are open for pollination at any one time.

Only one ring of yellow disc flowers are open for pollination at any one time.

purple coneflower

The arrangement of the emerging disc flowers is positioned so as to maximize their number in the available space.  When fertilized by pollinators, this will produce the maximum number of seeds per flower head — i.e., maximizing the fitness of that particular plant in its ability to pass on its genes.  To achieve this packing density, the disc flowers must lie at an angle of exactly 137.5 degrees from its neighbor, resulting in a series of spirals, rather than distinct horizontal or vertical rows. The same pattern is established in the growth of basal leaves of some plants (e.g., agave) and flower petals.

Chrysanthemum petals emerge in a spiral pattern, which reduces the overlap of petals in adjacent layers.

Chrysanthemum petals emerge in a spiral pattern, which reduces the overlap of petals in adjacent layers.

The spiral pattern is laid out with mathematical precision  — nature’s symmetrical design.  You can read more about the mathematical basis of the design in an earlier post.

8 thoughts on “Symmetry

    • Radial symmetry came first in the evolution of flowers, then later more species with bilaterally symmetrical flowers appeared. I wonder which type is more appealing to most humans — or perhaps it is a tie.

    • Thanks, Mike — I never really noticed the positions of petals on a flower, but it does make sense that the plant would want ALL of its advertisement to show, so it should arrange the petals so they don’t overlap, and the best way to do that is to position them at a 137 degree angle to one another (the “golden angle”).

  1. I reckon that whatever we humans come up with if we look hard enough we’ll find a design precedent out there in nature somewhere. The awesome power of evolution!

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